Flood Risk & Drainage Glossary

1D modelling treats a river as a chain of cross sections and solves flow along the channel line only. It handles in-channel conveyance well. Once water leaves the bank and spreads across a floodplain, the representation weakens.

Choose it for a defined channel with steep banks and you get quick, defensible results. Pick it for a wide, flat floodplain crossed by roads and embankments and the answer flatters the site, so scope for the flood modelling gets settled before a single cross section is cut.

See also: Flood modelling, 2D modelling, Hydraulic modelling

Defra published the National Standards for Sustainable Drainage Systems in June 2025, and they set a discharge hierarchy, an interception target and seven design criteria. In England the standards bite through the planning system as guidance rather than statute.

Guidance is not statute. A council can therefore ask for more than the seven criteria or quietly accept less, and both happen, which makes the 2025 national standards a floor to argue from rather than a rule to cite.

See also: Sustainable Drainage Systems (SuDS), Discharge hierarchy, Interception

2D modelling spreads floodwater across a gridded surface and solves depth and velocity in two directions at once. Overland flow paths, ponding hollows and floodplain storage all appear. A 1D model cannot reproduce that behaviour.

Resolution is the argument you will actually have. A two metre grid smooths out the kerb line and the garden wall that decide whether water enters a doorway, so before signing off 2D flood modelling for a small infill site, check what the mesh does around the buildings.

See also: Flood modelling, Linked 1D-2D modelling, 1D modelling

Adoptable drainage is designed and constructed to the standard that lets a sewerage undertaker or authority take it over. Ownership and maintenance then pass to that body. Anything falling short stays private.

Adoption turns on gradients, cover depths, chamber positions and the sewer's route across third party land. Get those wrong at planning and the retrofit cost lands after the units are sold, which is why the split between planning permission and building control matters early.

See also: Build-over agreement, Sewer adoption, Building Regulations Part H

Afflux is the localised rise in water level immediately upstream of a constriction such as a bridge, culvert or weir. The obstruction limits how much flow the channel can pass, so depth builds behind it.

Watch for it wherever a scheme narrows a channel or drops a new access culvert in. A few centimetres of afflux at the structure can still matter a couple of hundred metres upstream, so the effect is quantified inside the hydraulic model we build for the site.

See also: Areal reduction factor (ARF), Backwater, Catchment descriptors

An allocated site is land identified in an adopted development plan as suitable for a particular type of development, with the principle of that use already established through the plan-making process rather than argued from scratch at application stage.

Allocation does not exempt a site from the flood tests. The plan may have allocated it years ago on an older Environment Agency map, and the case officer will still want the Sequential Test addressed against current evidence, which is where the NPPF flood risk advice we give tends to start.

See also: Sequential Test, Environmental Impact Assessment (EIA), Greenfield land

Alluvium is river-deposited sediment: silt, clay, sand and gravel. Floodplain alluvium is often soft and permeable, and the watercourse that formed the valley put it there in layers.

Soft ground and shallow water go together here. Alluvium looks promising for infiltration on a borehole log and then fails a soakage test because the water table is already within a metre of the invert, which is the sort of thing our ground conditions and soakaway testing is meant to catch.

See also: Superficial deposits, Infiltration, Soakaway

Amenity is the SuDS pillar concerned with recreational, visual and place-making value: what a drainage feature gives a site beyond managing water. A basin that doubles as a green people actually use scores on amenity.

Designers treat this pillar as decoration and then lose it at the first value engineering meeting. Amenity is what turns a fenced concrete tank into a space residents will defend, and the four pillars of SuDS only work as a set: strip one and the scheme is a pipe with landscaping around it.

See also: Four pillars of SuDS, Diffuse pollution, Rain garden

Annual Exceedance Probability is the percentage chance that a flood of a given size will be equalled or exceeded in any single year. A 1% AEP event is the same thing as a 1-in-100-year flood.

The 1-in-100 phrasing causes most of the trouble. People hear a hundred year gap between events, so a site flooded twice in a decade looks like proof the map is wrong, which is why the arithmetic behind return periods and annual probability is worth ten minutes with a client.

See also: Exceedance, Flood return period, Breach

Antecedent conditions are the state of a catchment before rain arrives, above all how wet the ground already is. Saturated ground sheds more of the rainfall as runoff instead of absorbing it, and flood risk rises with it.

Two identical storms, two different outcomes. The one landing on a catchment already saturated by a fortnight of rain produces the flood, and work on making landscapes hold water for longer attacks that starting point instead of the headline rainfall figure people quote.

See also: Catchment, Baseflow, Soil Moisture Deficit (SMD)

An aquiclude is a geological layer of very low permeability that effectively prevents groundwater passing through. Nothing meaningful transmits. Water arriving at the layer has to go somewhere else.

Confusion between an aquiclude and an aquitard changes a drainage strategy. Design a soakaway above a true aquiclude and it fills, holds and backs up into the sub-base, and the perched water that follows is one of the mechanisms behind groundwater flooding on development sites.

See also: Aquitard, Groundwater flooding, Permeability

An aquifer is a body of permeable rock or soil below ground that both stores groundwater and transmits it, holding enough to supply abstraction and, where levels rise close to the surface after a wet winter, enough to flood the ground above it.

Confined and unconfined aquifers behave differently. An unconfined one responds to rainfall within days, a confined one can lag by months, and that is worth understanding before you read anything into groundwater flood risk mapping.

See also: Confined aquifer, Unconfined aquifer, Chalk aquifer

An aquitard is a geological layer of low permeability that restricts groundwater movement without preventing it entirely. Water still gets through. The layer slows the exchange between what sits above it and what sits below.

Half the arguments we have with a geotechnical report come down to this distinction. A borehole log that says clay tells you nothing about whether the layer leaks slowly over a winter, and that difference decides whether a basement stays dry or becomes part of the local groundwater flooding picture.

See also: Groundwater flooding, Aquiclude, Permeability

The area of search is the geographic area across which alternative, lower-risk sites are compared for the Sequential Test. It must be defined defensibly and in proportion to the type and scale of development proposed.

Draw it too tight and the officer rejects it as self-serving. Draw it across a whole county for a four unit scheme and you have handed the officer a reason to refuse, so the boundary you choose is most of the work in a Sequential Test.

See also: Sequential Test, Call for sites, Exception Test

An areal reduction factor scales point rainfall down to an area-average value for catchment-scale design. Heavy rain rarely falls uniformly across a whole catchment, so using the gauge figure everywhere would overstate the total volume arriving.

Size matters here, and small sites do not need it. On a half hectare site the factor barely moves off one, so applying it because the software offers the box is how a design ends up under-sized, and the assumption belongs in the audit trail of any catchment model we hand over.

See also: Catchment, Afflux, Depression storage

Areas Benefiting from Defences is a mapping layer showing land that gains protection from existing flood defences. The headline flood zones are drawn as though no defences exist, so the layer is published as context alongside them.

Protected is not the same as safe. Land inside this layer still sits in Flood Zone 3 for policy purposes, and the residual risk of a breach or a defence being overtopped is exactly what an officer will ask about, which the Flood Map for Planning itself will not tell you.

See also: Flood Map for Planning, Flood defence, CIRIA Code of Practice (C790)

Artesian describes groundwater confined under pressure, so that water tapped by a borehole rises above the top of the aquifer and sometimes above ground level without pumping.

Pressure, rather than rainfall, drives the problem. Puncture a confining layer during piling or a deep drainage run and you can get flow into an excavation that no pump hire was budgeted for, which is one of the less familiar routes into groundwater flooding and its planning implications.

See also: Groundwater flooding, Aquifer, Groundwater recharge

Holding surface water runoff back on site and letting it out slowly, so that peak flows to a watercourse or sewer drop, normally designed for events up to the 1 in 100 year storm plus a climate change allowance.

The volume you need is the gap between what the developed site produces and what you are allowed to release. That single calculation sits at the centre of every surface water drainage strategy we write.

See also: Peak flow, Runoff, Surface water drainage strategy

The volume held back during heavy rainfall so that outflow can be capped at an agreed rate. Most drainage strategies stand or fall on getting this number right.

Storage can go above ground in a basin or pond, or below it in tanks and geocellular crates. Which a site ends up with usually comes down to how much space is left once the layout is fixed, how high groundwater sits beneath it, and who will still be maintaining the thing in thirty years, and that reasoning belongs in the surface water drainage strategy.

See also: Surface water drainage strategy, Attenuation, Geocellular storage

A backwater is the raised water level extending upstream of a control or obstruction, where flow depth exceeds the depth the channel would otherwise carry. The effect reaches back along the channel from the control.

Downstream boundaries decide upstream answers. Set a model's tailwater too low and the backwater vanishes, along with the flood depths at the site you were meant to be assessing, so we test the sensitivity of that boundary in every river model before the results go in a report.

See also: Normal depth, Depth-duration-frequency (DDF), Flood Estimation Handbook (FEH)

A balancing pond attenuates flows by storing surface water runoff through a storm and releasing it at a controlled rate afterwards, so the peak leaving the site is lower than the peak arriving at the pond. Most hold a permanent pool between events.

That permanent pool is the difference from a detention basin. It buys treatment and habitat, and it brings a drowning risk to design out, which is a conversation worth having early in the surface water drainage strategy.

See also: Detention basin, Pond (SuDS), Retention pond

Banded depth data presents flood depth in fixed bands for a range of flood events. The bands give planning-grade design depths, enough to set finished floor levels and size drainage without commissioning bespoke modelling.

Bands are not a substitute for a model on a complex site, and nobody should pretend otherwise. For a single dwelling or a small commercial unit they close a gap that used to need a commissioned model, and that shift is the substance of the May 2026 Flood Map for Planning update.

See also: Check Your Long Term Flood Risk, Environment Agency flood data products, BGS Susceptibility to Groundwater Flooding

Baseflow is the sustained, low-level flow a watercourse carries between rainfall events. Groundwater mainly feeds it. River levels through a long dry summer are baseflow and very little else.

High baseflow before a storm leaves less room in the channel and the ground beside it. Catchments with chalk or limestone underneath run high for weeks after a wet autumn, which is part of why how much water a landscape holds shapes what a winter storm does downstream.

See also: Catchment, Antecedent conditions, Blue-green infrastructure

Beaver dams are natural structures beavers build across small watercourses. Each one slows and stores water in the upper catchment, and the effect is studied as a low-cost natural flood management measure.

Storage upstream buys time downstream, and that is the whole argument. The catch is control: a dam moves, fails or floods land its owner did not volunteer, so anyone reading the case for beavers as a flood management tool should treat it as catchment policy rather than a site solution.

See also: Watercourse, Channel conveyance, Confluence

Bedrock is the solid, consolidated rock that underlies soil and superficial deposits, the deeper geological foundation a site sits on. Everything above it, from clay to made ground, is younger and looser.

Whether bedrock helps or hinders depends entirely on what it is. Fractured chalk will take a soakaway happily while a mudstone will not take anything at all, and the only way to settle it is to dig and test, so soakaway testing and ground conditions work earns its fee on exactly this question.

See also: Superficial deposits, Made ground, Soakaway

A bespoke permit is the tailored form of Flood Risk Activity Permit, used for higher-risk or complex work and supported by a detailed risk assessment. Most FRAP applications end up in this category.

Programme is the thing people underestimate. A bespoke determination runs longer than a standard rules registration and the Environment Agency will stop the clock the moment the risk assessment looks thin, so we start the flood risk activity permit conversation while the layout can still move.

See also: Flood Risk Activity Permit (FRAP), Standard rules permit, Environmental permit

Betterment is an improvement to flood or drainage conditions that goes further than offsetting a development's own impact. It is most often sought on redevelopment, where discharging surface water at a lower rate than the existing buildings and hardstanding already achieve leaves the drainage network better off than before.

Whether a council can insist on it is the contested part. A bare planning condition seeking betterment will not hold, because it fails the test of being necessary to make the development acceptable, though local plan policy, a section 106 agreement or a voluntary offer can each get a council to the same place lawfully.

See also: Grampian condition, Local Plan, Planning condition

The BGS Susceptibility to Groundwater Flooding dataset maps the relative susceptibility of land across Britain to groundwater flooding, and each area carries a hazard band. Bands are a national screening product, not a probability for an individual property.

Reading a red band as a prediction about one address is the error we correct most often. The dataset was built at national scale from geology, so a whole village can share a band while the risk in one garden differs, a limitation covered in our note on groundwater flood risk mapping.

See also: Groundwater flooding, Borehole record, British Geological Survey (BGS)

Bioretention is a shallow planted area that collects runoff and filters it down through engineered soils, which strip pollutants out on the way through. Water then infiltrates into the ground or is piped onwards.

Specify the soil mix or you will get topsoil and a dead feature within two winters. Bioretention earns its space because it does water quality and planting at the same time as attenuation, a combination the four pillars of SuDS reward and a plain tank cannot match.

See also: Four pillars of SuDS, Rain garden, Amenity

Block paving is modular concrete or clay paving laid in units. Set with permeable joints over an open sub-base, the same blocks form a permeable pavement that takes rainfall through the surface instead of shedding it to a gully.

The blocks are rarely the problem. Joint material gets swept clean and refilled with builder's sand, or the sub-base is compacted clay, and a permeable driveway drains like a solid one within a season, so judge permeable paving in planning applications on the build-up beneath rather than the blocks you can see.

See also: Permeable paving, Sub-base, 2025 National Standards for Sustainable Drainage Systems

A blockage scenario is a modelling assumption in which a culvert, screen or structure is treated as partly or fully blocked. Running the model that way shows where water goes when the structure stops working.

Screens blind. A supermarket trolley, a fallen willow and one flood later the trash screen is solid, which is why an LLFA will ask for a blocked case on any scheme near a culverted channel, and why knowing whether you are dealing with a main river or an ordinary watercourse decides who asks.

See also: Main river, Ordinary watercourse, Trash screen

A blue roof is a roof designed to hold rainwater temporarily where it lands and release it afterwards at a controlled rate, so attenuation happens at source and the peak flow leaving the building is capped rather than passed straight into the drainage system below.

The roof is the tank. Storage sits in the build-up and drains out through a flow restrictor, which on a tight urban site with nothing spare at ground level often makes it the last source-control option standing, so we size it inside the surface water drainage strategy rather than bolt it on later.

See also: Attenuation, Peak flow, Surface water drainage strategy

Blue-green infrastructure is a connected network of water features and vegetation, blue and green working together, that manages runoff while adding amenity, biodiversity and cooling to an urban area.

Retrofitting a dense town centre is where the idea gets tested. Street trees in structural soil, a rain garden in a parking bay and a de-culverted ditch handle summer heat and surface water together, and that combined return is the case for landscapes designed to soak up rainfall.

See also: Amenity, Rain garden, Nature-based solutions

A borehole record is the log of ground and groundwater conditions written down when a borehole or well is drilled. Strata, depths and water strikes are all captured, and records like these sense-check screening maps against real levels.

Age is the first thing to check. A 1968 log drilled in August tells you very little about a February water table, and a single record two fields away tells you less again, so treat archive holes as a hint and read them next to the national groundwater flood risk mapping.

See also: Water table, Aquifer, BGS Susceptibility to Groundwater Flooding

BRE Digest 365 is the standard method for soakaway infiltration testing. A trial pit is dug, filled with water and timed as it drains, and those fill-and-drain timings are converted into a design infiltration rate that the sizing of a surface water soakaway is then based on.

Three fill-and-drain cycles per pit, and the slowest one governs. Our BRE 365 infiltration rate calculator does the arithmetic straight from your recorded timings, which heads off the usual argument about whether the design rate came from the best test on site or the worst one.

See also: Infiltration rate, Trial pit, Infiltration

A breach is the sudden failure of a section of flood defence or embankment, and water arrives faster and deeper than it would from gradual overtopping. Breach is modelled as a residual-risk scenario rather than a headline flood outline.

Speed is what makes a breach different, and it is the reason evacuation plans fail. Behind a raised defence the ground is often lower than the river, so water spreads across a site in minutes with no warning worth acting on. Residual flood risk has no sharper example.

See also: Embankment, Residual flood risk, Overtopping

A BREEAM flood risk assessment is a site-specific flood study prepared to support the Pol 03 credit. The site examined is the same one a planning FRA covers, but the structure and scoring follow BREEAM's criteria.

Reusing the planning FRA wholesale is the usual shortcut, and assessors reject it. The credit asks specific questions about run-off and pollution that a planning report has no reason to answer in that form, so a BREEAM flood risk assessment is quicker to write properly than to retrofit.

See also: Flood Risk Assessment (FRA), BREEAM Pol 03, Flood Consequences Assessment (FCA)

BREEAM Pol 03 is the 'Flood and surface water management' credit, which covers flood risk, surface water run-off and minimising watercourse pollution. The site's pre-development flood risk classification sets a ceiling on the credits achievable.

Check the classification before anyone promises a rating. A site in Flood Zone 3 caps what Pol 03 can deliver whatever the drainage design does, and that ceiling belongs in the tracker at concept stage rather than the week a BREEAM flood study is due.

See also: BREEAM flood risk assessment, Allocated site, Area of search

The British Geological Survey is the United Kingdom's national geological survey. BGS produces the geological, hydrogeological and borehole data that flood and drainage assessments rely on: bedrock and superficial geology maps, drilling logs and groundwater records. Assessments of groundwater flood risk and ground conditions start there.

BGS is not a consultee. Nothing it publishes carries a planning determination, and yet its susceptibility mapping is quoted back at us by case officers as though it were a decision. Much of our job is explaining what groundwater flood risk maps can and cannot support.

See also: Borehole record, Aquifer, BGS Susceptibility to Groundwater Flooding

BS 6297 percolation testing is the British Standard method for establishing whether ground can absorb and treat foul effluent, and the test produces a percolation value used to size a drainage field. BRE 365 surface water testing is different.

One trial pit will not do, and neither will a soakaway test borrowed from the drainage strategy. The two tests use different holes and different pass criteria, so a package treatment plant sized on the wrong value discharges effluent the ground cannot take. BS 6297 testing exists to stop exactly that.

See also: Drainage field, Percolation, Percolation value (Vp)

Consent from the sewerage undertaker, the water company that holds the public sewer, to construct over or close to that sewer. The agreement attaches conditions designed to protect the asset from the proposed structure.

Planning permission does not grant it. The undertaker decides separately, and we have watched foundations get redesigned late because nobody checked the sewer record at layout stage, a sequencing problem covered in our guide to how building control and planning permission handle drainage.

See also: Adoptable drainage, Sewer adoption, Section 104 agreement

Part H of the Building Regulations governs drainage and waste disposal. Its standard is adequacy, and adequacy is a narrower question than the one planning asks. Part H approval and a discharged planning drainage condition are separate hurdles, and clearing one does not clear the other.

Developers conflate the two constantly. A scheme can satisfy building control and still sit on an undischarged condition, which is why our note on building control, planning permission and drainage sets out which approval bites when.

See also: Right to connect, SuDS adoption, Adoptable drainage

Adjustment of a hydraulic model until its results match observed flood levels or flows. Calibration is what allows a modeller to say the predictions are reliable, and it is the difference between a tested model and a merely plausible one.

Gauged data is the constraint. Many small watercourses have none, so verification falls back on flood marks, photographs and anecdote, and the Environment Agency's 2026 river modelling technical standards expect that limitation to be recorded rather than quietly skipped.

See also: Model User Report (MUR), EA river modelling technical standards, Sensitivity testing

A call for sites is the stage where a local planning authority invites landowners, promoters and developers to put forward land for possible allocation in the emerging Local Plan. Submissions are then assessed against the plan's evidence base before any site reaches a draft allocation.

Flood risk sifts the list early. A site in Zone 3 submitted with no supporting evidence tends to be screened out at the strategic stage, which is why we advise promoters to run the Sequential and Exception Tests logic before the submission deadline rather than after the draft plan lands.

See also: Exception Test, Reasonably available sites, Local Plan

A catchment is the total area of land from which rainfall drains to a given point on a watercourse. Size, steepness and geology all shape how quickly water arrives at that point and how severely it floods, which is why two catchments of equal area can behave nothing alike.

Draw the boundary wrong and every number after it is wrong. Site drainage decisions depend on which catchment you sit in and where you sit within it, and upstream interventions such as leaky dams only help downstream when the catchment above them is large enough to matter.

See also: Leaky dam, Catchment descriptors, Growth curve

Standardised physical and hydrological parameters for a catchment: area, slope, soil and rainfall. Catchment descriptors are the numbers FEH flood estimation runs on, so a wrong boundary corrupts every peak flow that follows.

Descriptors come from a national dataset, and national datasets are coarse. Where the mapped boundary misses an urban drainage transfer or a small reservoir, the estimate drifts, and correcting that by hand is part of the flood modelling work we do rather than a formality.

See also: Flood modelling, Catchment, Hydrological modelling

A catchpit is a drainage chamber with a sump below the outgoing pipe, so silt and debris carried by the flow settle out before they reach downstream pipework. The trapped material is lifted out during maintenance instead of building up through the network.

Silt kills soakaways and blinds filter media. Put catchpits upstream of anything with a small orifice or an infiltration surface, name who empties them and how often, and carry that into the surface water drainage strategy, because an unmaintained catchpit is worse than none.

See also: Surface water drainage strategy, Cover level, Flap valve

Percentile bands for selecting a climate change allowance: central at the 50th percentile for most development, higher central at the 70th for essential infrastructure, and upper end at the 95th as a precautionary figure.

Picking the band is not a free choice. Vulnerability classification and the lifetime of the development drive it, and the sensitivity test that sits alongside your chosen band is where most objections start, as our breakdown of climate change allowances in planning sets out.

See also: Climate change allowance, Essential infrastructure, Climate change adaptation

A cesspool is a sealed tank that stores foul waste with no discharge and no treatment. Everything entering it must be tankered away, so emptying is regular and expensive, which is why a cesspool is accepted only as a last-resort off-mains option.

Councils rarely welcome one. Expect to evidence that mains connection, a septic tank and a treatment plant have all been ruled out first, and that ordering of options belongs in the foul drainage strategy submitted with the application.

See also: Foul drainage strategy, Septic tank, Foul drainage hierarchy

The chalk aquifer is a fine-grained limestone formation cropping out across south and east England, and the UK's principal aquifer. Groundwater flooding is a particular hazard here because the water table can rise several metres.

Basements are where this bites. On chalk, a winter water table can sit metres above the level assumed from a single summer borehole, so we look for a full year of monitoring before signing off a below-ground design, for the reasons set out in our note on groundwater flooding and its planning implications.

See also: Principal aquifer, Water table, Groundwater flooding

Change of use means altering the use of land or buildings rather than erecting something new. Changes of use are exempt from the Sequential Test, which is where the concession ends, because the requirement for a flood risk assessment still applies in full.

Agents read the exemption as a pass. It is not, and the applications that stall are usually the ones where a more vulnerable use has been slipped into a ground floor at risk, a pattern we set out in our piece on flood risk assessments for HMOs.

See also: Flood Risk Assessment (FRA), Sequential Test, House in Multiple Occupation (HMO)

Channel conveyance is the capacity of a river channel to carry water. Reduce it, through an obstruction or temporary works in the channel, and upstream water levels rise. Flood risk rises with them.

Consent is the practical question. Whether the Environment Agency or the lead local flood authority controls works that squeeze the channel depends on the classification explained in our guide to main rivers and ordinary watercourses, and scaffolding left standing in a channel over winter has caused real flooding.

See also: Main river, Ordinary watercourse, Watercourse

The Environment Agency's public website for looking up the long-term flood risk to a property from rivers, the sea and surface water. Its surface water layers are not classed as suitable for development planning.

Homeowners find this service first and assume it settles the question. Planning officers want the Flood Map for Planning instead. The two products have drifted further apart since the changes we cover in our note on the May 2026 Flood Map for Planning update.

See also: Banded depth data, Long-term storage, Conveyancing flood report

CIRIA is the Construction Industry Research and Information Association. Its publications are the leading UK guidance on sustainable drainage, the SuDS Manual (C753) chief among them, and they are the reference point designers and reviewers work from when a drainage scheme is judged.

Guidance is not law. Nothing CIRIA writes binds a planning authority by itself, but conditions routinely name its documents, so a design that ignores the CIRIA SuDS Manual ends up defending itself twice: once on the engineering and once on why the benchmark was skipped.

See also: CIRIA SuDS Manual (C753), CIRIA C823F, CIRIA Code of Practice (C790)

CIRIA C823F, the 2025 guide 'Enabling development: getting SuDS right from the start', argues that sustainable drainage should shape a scheme from land acquisition onwards rather than being fitted around a finished layout.

Land price is where the argument bites hardest. Buy a site at a density the drainage cannot support and no amount of clever design recovers the difference. That is the case made in CIRIA's guide to early SuDS planning.

See also: CIRIA, CIRIA SuDS Manual (C753), Balancing pond

CIRIA C790 is the code of practice for property flood resilience. It sets out a six-stage process running from hazard assessment through to the operation and maintenance of the measures installed.

Products get bought before the hazard is understood, and that is the wrong order. A door barrier rated for still water does nothing against a fast lateral flow, so match the measure to the mechanism first, as our guide to property flood resilience explains.

See also: Property flood resilience (PFR), Flood Re, CIRIA

The UK's principal best-practice design guide for sustainable drainage, published by CIRIA in 2015 and still the technical benchmark that lead local flood authorities refer to when they review a surface water scheme.

A decade on, C753 still decides arguments. Treatment stages, half-drain times, maintenance schedules: the numbers officers quote back at you nearly all trace to the C753 SuDS Manual, so cite the clause you designed to and the conversation gets shorter.

See also: CIRIA, Four pillars of SuDS, Littoral zone (SuDS)

CIWEM is the Chartered Institution of Water and Environmental Management, the professional body for water and environmental practitioners. Its members prepare and sign off flood risk work, which is why chartered status appears on the front of competent flood risk assessments.

Membership is a floor. Chartered status tells a council the author answers to a professional code, and CIWEM's commentary reaches well past compliance into catchment-scale thinking such as spongy landscapes, which most flood risk assessments never consider.

See also: Antecedent conditions, Baseflow, Blue-green infrastructure

Class MA is the permitted development right for changing commercial (Class E) premises to residential (C3). The switch raises flood vulnerability, so a flood risk assessment is required in Flood Zone 2 or 3.

Prior approval is a narrower gate than full planning, and applicants assume flooding falls outside it. Flooding is one of the matters a council can refuse on, which is why we treat an office to residential flood risk assessment as the first thing we programme rather than a late add-on.

See also: Permitted development, Prior approval, Flood Zone 2

Class Q is the permitted development right allowing agricultural buildings to be converted to dwellings. Prior approval must assess flooding, and where the building sits in a flood risk area a flood risk assessment has to accompany the application before the conversion can proceed.

Barns sit in valley bottoms for a reason: that is where the farm wanted water and access. Converting one turns a building nobody sleeps in into a dwelling, and the vulnerability jump is the whole point of the barn conversion flood risk assessment.

See also: Permitted development, Prior approval, Flood Risk Assessment (FRA)

Climate change adaptation is the work of adjusting development, drainage and land management to cope with the future effects of climate change, principally heavier rainfall and higher sea levels. Adaptation deals with effects rather than causes, which is the half of the problem a planning application has to answer for.

Adaptation is where drainage design earns its keep. Oversizing a pipe today costs little against retrofitting a street later, and the numerical expression of all this in an application is the set of climate change allowances the NPPF applies to flood risk.

See also: Climate change allowance, Central / higher central / upper end allowances, Design life

An uplift added to peak rainfall and peak river flow, published by the Environment Agency and based on the UKCP18 projections, so drainage and flood risk are designed for future conditions rather than past ones.

There is no single number to reach for. The allowance depends on the development’s lifetime, its vulnerability classification and the river basin district, and choosing the wrong one is a common reason for a strategy coming back. We set out how to pick in our guide to climate change allowances for planning.

See also: Climate change adaptation, UKCP18, Peak rainfall intensity allowance

Coastal Change Management Area: a planning designation, informed by Shoreline Management Plans, covering a stretch of coast likely to change through erosion or realignment, where development is managed to suit that change.

Designation changes what is buildable and what is temporary. Councils use it to steer new development away from a retreating frontage and to grant time-limited permissions where they must, and the policy behind the line comes from the Shoreline Management Plan for that stretch.

See also: Shoreline Management Plan (SMP), Epoch, Hold the line

Coastal erosion is the wearing away of land at the coast. Government treats it as typically known, permanent and progressive. That is a different character from a flood that arrives and recedes, and on that basis coastal erosion is excluded from Flood Re insurance.

No insurer covers a cliff. Owners of eroding property have no automatic right to compensation and no Flood Re fallback, and the patchwork of rollback schemes and discretionary payments that exists instead is set out in our piece on compensation for coastal erosion.

See also: Flood and Coastal Erosion Risk Management (FCERM), Coastal Change Management Area, Tidal (coastal) flooding

The Colebrook-White equation relates flow in a pipe to its roughness, gradient and the viscosity of the fluid. The equation sits behind the pipe capacity figures used across UK sewer and pipe design.

Nobody solves it by hand. The equation is implicit, so it is iterated inside software or read off a chart, which is what our drainage calculators do, and the sensitivity worth knowing is roughness: assume a clean new pipe and you will oversell its capacity for the whole of its life.

See also: Modified Rational Method, Runoff coefficient, Wallingford Procedure

One sewer carrying foul wastewater and surface water together, which is why new surface water connections to a combined sewer are refused as a matter of course: they add to storm overflow spills and to flooding.

Separating the flows at source is now the default expectation. Which sewer you may connect to shapes the whole strategy, so start with the difference between foul, surface water and combined sewers.

See also: Combined sewer overflow (CSO), Foul sewer, Separate sewer

A combined sewer overflow is a relief point on a combined sewer. During heavy rain it discharges diluted, untreated sewage to a watercourse so the system does not back up into homes and streets. CSO spills are a growing environmental concern.

Every extra litre of surface water you connect to a combined sewer buys a share of the next spill. That is the argument for separating flows on site and keeping roofs and yards out of the foul system altogether, which our explainer on foul, surface and combined sewers works through.

See also: Combined sewer, Surface water sewer, Foul sewer

Replacement floodplain volume, provided level for level, to offset storage lost to development. Compensatory storage stops displaced water raising flood levels elsewhere, and the volume has to be matched band by band as the water rises.

Land is the constraint nobody budgets for. Compensation must sit within the floodplain, hydraulically connected and outside anyone else's ownership, and schemes fall over at this point far more often than on the arithmetic set out in our note on level for level compensatory storage.

See also: Level for level, Cut and fill, Attenuation storage

A confined aquifer sits beneath a layer of low-permeability material that seals it from above. The confining layer holds the groundwater under pressure, so water in the aquifer is not free to rise and fall like an open water table.

Pressure is the part that catches people out. Puncture the confining layer with a piled foundation or a deep excavation and water can rise well above the level where you met it, one of the mechanisms behind groundwater flooding on development sites.

See also: Aquifer, Unconfined aquifer, Water table

The point where two watercourses join to form a larger channel. Flows from both arrive together at a confluence, which makes it a common pinch point for flood risk.

Timing decides how bad it gets. Two tributaries peaking an hour apart is survivable; the same two peaking together is not; and at a confluence the channels can also fall under different regulators, a split explained in our guide to how watercourses are classified.

See also: Watercourse, Dredging, Gabion

Rivers, coastal waters, lakes and groundwaters protected under the Water Resources Act 1991. Any discharge to controlled waters is regulated, so a permit or a recognised exemption is needed before anything is released to them.

Groundwater counts, and that surprises people. A drainage field discharging to soil is still reaching controlled waters. Small treatment plants therefore sit inside the general binding rules for small sewage discharges instead of going unregulated.

See also: General Binding Rules, Environmental permit, Source Protection Zone

Intense, localised rainfall driven by strong upward air movement in thunderstorms. Convective storms can drop very high totals over small areas in short periods, and they are a common trigger for flash flooding.

Rain gauges miss these storms routinely. A single cell can flood one street and leave the next dry, so an absence of records is no evidence of low risk, and the large tracked systems described in our piece on weather bombs sit at the opposite end of the scale.

See also: Flash flooding, Annual Exceedance Probability (AEP), Fluvial (river) flooding

Traditional below-ground drainage: piped networks and buried tanks that collect surface water and move it away underground. Conventional drainage is the approach set against sustainable drainage, which works at the surface and nearer the source.

Pipes and tanks still have a place, at the bottom of the queue. Councils expect infiltration and surface conveyance to be tested and ruled out before a buried tank appears. That reasoning is what our approach to surface water drainage records for the case officer.

See also: Critical duration, Designing for exceedance, Discharge point

Conveyance covers both the movement of water from one place to another and the measure of how much flow a channel or pipe can carry. Restrict it anywhere along the route and water levels upstream of the restriction rise.

Whole routes fail, rarely single points. A culvert, a headwall, a weedy ditch and a private outfall each take their share of the capacity, and responsibility for keeping them clear follows the ownership rules in our guide to riparian duties on watercourses.

See also: Channel conveyance, Watercourse, Blockage scenario

An automated, national-scale environmental search that screens an existing property's flood risk for a buyer or lender. A conveyancing flood report is not accepted as evidence in support of a planning application.

Clients arrive holding one and ask why they need anything else. The report never looked at their site levels, their finished floor level or their proposed layout, and it was written for a lender's risk appetite, points our article on why conveyancing flood reports fail in planning goes through in detail.

See also: Flood map challenge, Flood modelling software, GIS (Geographic Information System)

Cover level is the ground or finished surface level at a manhole or chamber. Pipe depths and gradients across a drainage layout are set out by reference to that level.

Get the cover level wrong and every invert below it moves with it. Levels come off a topographic survey tied to Ordnance Datum rather than a scaled drawing, and they carry straight into the falls and cover depths we set out in the surface water drainage strategy.

See also: Manhole, Surface water drainage strategy, Catchpit

An area with known surface water drainage problems, designated by Environment Agency notification or through a council’s SFRA, where even a Flood Zone 1 site can be required to submit a flood risk assessment.

A site can sit inside one without anything on the constraints plan saying so. We check for a Critical Drainage Area before the layout is fixed, because finding out at validation costs weeks.

See also: Flood Zone 1, Flood Risk Assessment (FRA), Betterment

Critical duration is the rainfall duration producing the highest peak flow or the greatest storage requirement at a site for a chosen return period. Designs are tested across a range of durations until it is found.

Short storms hit hardest for pipes. Long ones fill tanks, so the duration that sizes your attenuation is rarely the one that sizes your network, and a strategy tested at 30 minutes alone will understate storage. We run the full duration set before any volume goes into the surface water drainage strategy.

See also: Attenuation, Peak flow, Surface water drainage strategy

A culvert is an enclosed pipe or covered channel that carries a watercourse or drainage path beneath a road, railway or development. The inlet is the weak point. Debris gathering there makes a culvert a common flood pinch point, because the flow has nowhere else to go once the opening starts to close.

Culverts block. That is why we test them as a blockage scenario rather than assume they run clear, and why on small watercourses they remain among the most under-rated sources of flood risk.

See also: Watercourse, Infrastructure flooding, Land Drainage Act 1991

Cut and fill demonstrates compensatory flood storage by slicing the floodplain into level bands, often 100mm or 200mm, and matching every cubic metre of fill in a band with an equal volume of excavation in that same band.

Fail one band and the whole exercise fails. Planners and the EA look at the band table rather than the total, because a scheme that balances overall can still steal storage at the depths that flood first. Our note on level for level compensatory storage walks through where schemes come unstuck.

See also: Compensatory flood storage, Level for level, Land raising

Deemed discharge treats certain planning conditions as approved when the authority fails to decide within the set period. Flood risk conditions are excluded from it, as are most surface water drainage conditions.

Developers hear about the deemed discharge route and assume drainage will clear itself if the council goes quiet. It will not. The exclusions bite precisely where the delays are worst, which is why our guide to discharging drainage planning conditions starts with getting the submission right first time.

See also: Planning condition, Discharge of conditions, Pre-commencement condition

Defended Zone is the 2025 TAN15 category for land behind flood defences built to a set standard, 1-in-100 for river and 1-in-200 for tidal. Welsh planning policy names it separately because defences change flood risk without removing it.

Behind a defence is not outside the risk. Overtopping and breach still have to be assessed, and a Welsh scheme sitting in a Defended Zone will be judged on what happens when the wall is exceeded, which our TAN15 guidance sets out for each development category.

See also: TAN15, Justification test, Notification Direction

Defra, the Department for Environment, Food and Rural Affairs, holds national flood risk policy, legislation and funding for England, and coordinates strategy across the responsible bodies that deliver it. Every downstream body in England works to the national framework Defra sets.

You will rarely deal with Defra directly on a planning application. Its decisions still land on your desk through the allowances, the standards and the consultee positions that follow, and our piece on Defra's role in flood risk management traces that chain from Whitehall to the LLFA.

See also: Flood and Coastal Erosion Risk Management (FCERM), Bespoke permit, CIWEM

Demountable flood barriers are temporary defences. They go up before a flood arrives and come down once it has passed, so a property keeps its everyday access and views while still gaining protection during an event.

Someone has to be there to put them up. That is the weakness planners probe: barriers depend on warning time, a person on site and somewhere dry to store the panels, so we treat them as one component within a wider property flood resilience package rather than the whole answer.

See also: Property flood resilience (PFR), Flood Re, Flood recoverability

Depression storage is the rainfall held in small hollows, ruts and kerb-side dips across a surface. Overland flow cannot begin until those hollows have filled, so depression storage is counted as an initial loss before runoff is generated.

Set depression storage too high in a model and your surface water flood extent shrinks on paper while the real one does not. On a hard urban catchment the value is close to nothing, which is why we sense-test it rather than accept a package default when building the flood model behind an assessment.

See also: Catchment, Flood Studies Report (FSR), Hydraulic gradient

Depth-duration-frequency describes how rainfall depth changes with storm duration and return period. Every design rainfall estimate rests on that relationship: pick a duration and a return period, and the DDF curve gives the depth.

Two sites can share a return period and still need very different design depths, because their DDF curves differ. That is the trap in copying rainfall figures from a neighbouring scheme, and it is why the rainfall inputs behind our flood modelling work are drawn for the site itself.

See also: Flood modelling, Normal depth, Joint probability

Design flood level is the modelled water level of the event a development is designed against, typically the 1% annual river event or the 0.5% tidal event, plus a climate change allowance. Floor levels and mitigation are set from it.

A number on a drawing is only as good as the model behind it. Freeboard sits above the design flood level, never inside it, and everything that happens in a bigger event than the one you designed for belongs in the residual risk section of the FRA.

See also: Freeboard, Design storm, Finished floor level (FFL)

Design groundwater level is the highest wet-season groundwater level adopted for design, derived from geology, records and monitoring on the site itself. Basements, floor levels and infiltration drainage are all checked against that level.

Monitoring in August tells you very little. One dry-season dip reading has sunk more soakaway designs than any other single shortcut, because the level that matters arrives in February and February is when the base of your infiltration trench sits under water. Our groundwater assessment page explains the monitoring period we ask for.

See also: Groundwater flood risk assessment, Infiltration, Soakaway

Design life is the period a development is assumed to stay in use for flood risk purposes, a minimum of 100 years for residential schemes. Climate change and residual risk are assessed over that whole period.

Pick the wrong lifetime and every allowance downstream is wrong with it. A residential scheme carries the 100 year assumption whatever the client's own horizon, and the lifetime you state drives which climate change allowance applies, as our guide to climate change allowances for planning works through band by band.

See also: Climate change allowance, H++ scenario, Peak river flow allowance

A design storm is a synthetic rainfall event of specified depth, duration and profile, built rather than observed, and fed into a drainage or flood model as the rainfall input a design is tested against.

Duration matters as much as depth. A short violent storm and a long moderate one size completely different parts of the same system, so a range of them is run behind every surface water drainage strategy.

See also: Rainfall profile, Surface water drainage strategy, HOST classification

A designated asset is a privately owned structure or feature that a risk management authority has formally designated because it affects flood risk. Once designated, it cannot be altered or removed without the authority's consent.

Buy a site with an old embankment across the back of it and the designation comes with the deed. Owners often find out only when they apply to remove it, and which body did the designating decides who you deal with, since duties are split across the authorities responsible for flood risk in England.

See also: Risk Management Authority, Highways authority, Local Planning Authority (LPA)

Designing for exceedance plans deliberately for storms larger than the drainage system's capacity. Levels and routes are arranged so the excess runs safely over land instead of into buildings.

Every system fills up eventually. The question is where the water goes next, and kerb lines, road crossfalls and the fall of a car park decide that long before any pipe does, so exceedance routes belong in the surface water drainage strategy at layout stage.

See also: Exceedance, Surface water drainage strategy, Exceedance flow routing

Detailed drainage design is the stage that sets out how a scheme is actually built: hydraulic sizing, component layout and construction detail. It suits a planning condition well, once the strategy behind it is settled.

Councils sometimes ask for construction-level detail at application stage. That is the wrong order. Pushing back with a clear strategy plus a condition covering the detail is usually accepted, as our note on the drainage information needed for planning conditions explains.

See also: Preliminary drainage design, Colebrook-White equation, Conventional drainage

A normally dry, vegetated hollow that stores runoff during a storm and releases it at a controlled rate, with some water quality treatment along the way.

Dry between storms means easier mowing, easier inspection and a simpler adoption conversation than a wet pond. It can double as playing space for most of the year, which counts for a lot when space is being fought over in the surface water drainage strategy.

See also: Extended detention basin, Balancing pond, Runoff

Dewatering is the temporary or permanent removal of groundwater so construction can proceed below the water table. Near a groundwater body it needs flagging early in the design, since the work can require consent and that consent takes time to obtain.

Basements are where this bites. A deep dig in a gravel aquifer can pull water from beneath neighbouring buildings and change the very levels your design assumed, which is why we settle the dewatering question inside the groundwater assessment instead of leaving it to the contractor.

See also: Water table, Aquifer, Design groundwater level

Diffuse pollution arises across a wide area from many dispersed sources, such as runoff from roads and farmland, rather than from a single identifiable discharge point. SuDS are the usual means of intercepting and treating it.

No single outfall to point at makes it hard to regulate and easy to ignore. Tyre wear, brake dust and road salt arrive with every storm, so water quality earns its place alongside quantity, amenity and biodiversity in the four pillars of SuDS.

See also: Amenity, Four pillars of SuDS, Pollution pathway

The discharge hierarchy ranks the ways of getting surface water off a site. Rainwater reuse comes first, then infiltration to ground, then discharge to a watercourse, then a surface water sewer, with a combined sewer only as a last resort.

Skipping a rung is the most common reason a drainage strategy comes back. An LLFA will want infiltration testing before it accepts a watercourse connection, and the ranking works alongside the SuDS management hierarchy that governs how water is treated on the way out.

See also: SuDS hierarchy, Rainwater harvesting, Downpipe disconnection

Discharge of conditions is a formal application asking the authority to approve the details a planning condition reserved. Where the condition is pre-commencement, approval has to be in hand before lawful building can begin, so the drainage details behind it sit on the critical path.

Determination takes weeks that most build programmes have not allowed for. Start the drainage submission while the main application is still running, because a refusal to discharge means resubmission and another wait, as our guide to discharging drainage conditions makes clear.

See also: Planning condition, Deemed discharge, Section 73 variation

A discharge point is the location where surface water or foul flows leave a site and enter a receiving watercourse, a sewer or the ground. Its position, level and permitted rate shape everything upstream of it.

Third-party land is the usual sticking point. You need evidence of a legal right to discharge as well as a physical outfall. An unproven connection holds up approval however well the calculations behind the surface water drainage strategy stack up.

See also: Surface water drainage strategy, Discharge rate, Outfall

The controlled rate at which surface water is allowed to leave a developed site for a watercourse or sewer, normally restricted towards the greenfield rate so that everyone downstream is left no worse off.

Agree it early. The rate sets the storage volume for the entire scheme, so it is the first number fixed in a surface water drainage strategy and the one defended hardest through the flow control detail.

See also: Flow control, Surface water drainage strategy, Discharge point

Downpipe disconnection redirects a roof downpipe away from the sewer into a soakaway, a water butt or a permeable area, so less flow reaches the drainage network.

Cheap, quick and unfashionable. A single house achieves little on its own, though a street of them takes a measurable load off a combined sewer, which is why source control near the top of the SuDS hierarchy starts with the roof.

See also: Source control, Water butt, SuDS hierarchy

A drainage field is a network of perforated pipes laid in gravel trenches that disperses treated effluent into the ground. Its area comes from percolation testing carried out to BS 6297, so the trench layout follows the measured infiltration rate rather than a rule of thumb.

Fail the percolation test and no amount of package treatment plant will save the scheme. Wet ground, a shallow water table or a nearby well and the drainage field is out, which sends you to a different disposal route entirely. The method sits in our guide to BS 6297 infiltration testing.

See also: Percolation, BS 6297 percolation testing, Package treatment plant

A drainage strategy is a site-specific report showing how a development will manage surface water, and often foul, to a standard the local planning authority and its drainage consultee accept. Usually it is submitted with or inside the FRA.

Two consultees, two sets of expectations. The LLFA reads it for rates and volumes while the water company reads it for the connection, and a strategy that satisfies one and ignores the other stalls, which is why our drainage strategy service starts with who will be reading it.

See also: Surface water drainage strategy, Gravity drainage, Greenfield runoff rate

Dredging is the removal of accumulated silt and sediment from the bed of a watercourse, carried out to restore the channel's conveyance, meaning the volume of flow it can carry.

Ask why the silt arrived in the first place. Dredging buys conveyance back for a season or two and then the channel refills, and on a main river the work needs consent anyway, so the split described in our piece on main rivers and ordinary watercourses decides who you ask.

See also: Main river, Ordinary watercourse, Watercourse

A dry swale is a shallow vegetated channel with an engineered filter layer in its base. Runoff is conveyed and filtered as it passes, and the channel drains down between storms rather than holding standing water.

Standing water is what residents complain about, and a dry swale answers that objection before it is raised. The filter layer needs a positive outlet or an infiltration base beneath it, so we fix the drain-down route while the levels are still soft in the surface water drainage strategy.

See also: Swale, Runoff, Blue roof

Dry weather flow is everything moving through a drainage system that rainfall has not directly caused. Foul flows from properties and groundwater infiltration into the pipes make it up, the baseline a system carries after days without rain.

Infiltration is the part people underestimate. An old sewer in a high water table can run half full before a drop of rain lands, which eats the headroom you assumed for a new connection, and the differences between foul, surface water and combined sewers decide how much that matters.

See also: Combined sewer, Foul flow, Misconnection

The Environment Agency's river modelling technical standards set out how a hydraulic model submitted as flood risk evidence must be built, calibrated, sensitivity-tested and documented before the Agency will accept it.

Modellers used to work to habit. The standards turn that habit into a checklist the EA can mark against, and a model without a documented sensitivity run now comes back regardless of how good the results look, which our summary of the EA river modelling technical standards goes through.

See also: Model User Report (MUR), Calibration, Sensitivity testing

An embankment is an engineered earth bank raised above the surrounding ground, built either to contain high river levels within the channel or to resist coastal flooding of the land behind it.

An embankment protects the land behind it up to its design standard and no further. Above that, or if it breaches, everything then turns on the depth, the velocity and the warning time assumed in the design, and all of that is assessed separately as residual flood risk.

See also: Residual flood risk, Breach, Flood risk asset

The Environment Agency manages flood risk from main rivers, the sea and reservoirs in England. On planning applications in the higher flood zones it is a statutory consultee, so its position on a scheme carries weight with the authority making the decision.

Consultee is a precise word. The EA advises and objects, though the decision belongs to the local planning authority, and an objection is not a refusal even if it usually behaves like one, as our page on the EA's role in flood risk planning explains.

See also: Local Planning Authority (LPA), Flood risk standing advice, Statutory consultee

Environment Agency flood data products are numbered supplies of flood data, from the free Flood Map extract through the detailed FRA dataset (Product 4) to the underlying hydraulic model reports, outputs and inputs (Products 5, 6 and 7).

Order Product 4 on day one. Lead times run into weeks and a site-specific FRA cannot be written from the online Flood Map alone, so the request goes in before the fee proposal is signed, and our explainer on Products 4, 5, 6 and 7 covers when model outputs are worth buying too.

See also: Environment Agency (EA), Banded depth data, Gauging station

Environmental Impact Assessment is the statutory process for identifying the significant environmental effects of certain developments before planning permission is granted. Only certain categories of development are caught, and where a scheme is caught the assessment must be produced and taken into account before the decision is made.

Flood risk is usually one chapter of an Environmental Statement, and it is the chapter that most often has to be rewritten. The tests it must satisfy come from national policy, so the same NPPF flood risk requirements apply whether or not your scheme is an EIA development.

See also: NPPF (National Planning Policy Framework), Planning Practice Guidance (PPG), Previously developed (brownfield) land

An environmental permit is the consent required to carry out activities that may affect the environment. Flood risk activities and discharges to controlled waters both fall inside the regime, so the permit sits alongside planning permission rather than replacing it.

Planning permission does not give you a permit. Work in, over, under or near a watercourse is assessed separately and on different grounds, and the timescales rarely match the planning programme, which is why our guide to flood risk activity permits pushes for an early pre-application conversation.

See also: Flood Risk Activity Permit (FRAP), Bespoke permit, Standard rules permit

In shoreline management planning an epoch is a defined future time period, short, medium or long term, over which a coastal policy is set. Policies can change from one epoch to the next.

A hold the line policy in the first epoch and managed realignment in the third is a common pattern, and it changes what a site is worth. Check the epoch your development's lifetime runs into, because our explainer on shoreline management plans shows how the later ones drive coastal defence funding.

See also: Hold the line, Managed realignment, Shoreline Management Plan (SMP)

Essential infrastructure is the flood risk vulnerability category for uses that must keep working during a flood, such as electricity substations and transport links. The Exception Test applies to it even in the functional floodplain.

Classification is done early or done twice. Developers describing a data centre or a pumping station loosely can find the category shifts under them at consultation, and the difference between essential infrastructure and a less vulnerable use decides whether the Exception Test bites, as our guide to the NPPF Annex 3 vulnerability classes sets out.

See also: NPPF Annex 3, Functional floodplain, Exception Test

Evapotranspiration combines evaporation from surfaces with transpiration from plants, and both return water to the atmosphere before it becomes runoff. Green roofs and vegetated SuDS rely on it to cut runoff volume rather than simply delay it.

Losses to air are small but real. On a wet British winter day they are close to nothing, so we never claim them in a design storm, though over a year they matter enough that vegetated systems earn their place under the four pillars of SuDS.

See also: Green roof, Four pillars of SuDS, Bioretention

Exceedance is any flow or event that surpasses a set design capacity or threshold. The common case in drainage is runoff that overtops what the piped system can carry, so the surplus travels overland instead and takes whatever fall the finished site offers.

Every scheme exceeds its design at some point. Pretending otherwise is the mistake we see most often in outline drainage layouts, and an LLFA will ask where the water goes on the day it happens, which is why exceedance belongs in the surface water drainage strategy from the first sketch.

See also: Designing for exceedance, Exceedance flow routing, Runoff

Exceedance flow routing sets the above-ground paths surface water takes once a drainage system's design capacity is used up. Roads, kerb lines and shaped levels carry the excess safely past buildings rather than into them.

Sketch it early. Too many layouts stop at the pipe network and leave the overland route to chance, and the fix is nearly always cheap at design stage, provided the route is drawn into the surface water drainage strategy rather than discovered on site.

See also: Exceedance, Surface water drainage strategy, Overland flow route

A two-limbed planning test for development that cannot avoid higher flood-risk land: the wider sustainability benefits must outweigh the flood risk, and the development must be safe for its lifetime without making risk worse elsewhere.

It only comes into play once the Sequential Test has been failed or exhausted, and both limbs have to pass. We run the Sequential and Exception Tests as one piece of work for that reason.

See also: Sequential Test, Call for sites, Reasonably available sites

An extended detention basin holds runoff for longer than attenuation alone would require. The extra time lets silt and the pollutants bound to it settle out before water leaves the basin.

Slow water drops its dirt. The trade is footprint and a longer drawdown, which conflicts with how quickly a basin needs to empty before the next storm, so the outlet detail gets settled inside the surface water drainage strategy rather than left to the planting plan.

See also: Detention basin, Attenuation, Runoff

The FDA1 Foul Drainage Assessment is a standard form that records and justifies foul drainage arrangements for a planning application where no mains connection is proposed. Only some authorities require it, so the form is not universal.

Off-mains does not mean unregulated. A septic tank or package plant still needs a discharge point that works in the ground you actually have, and the FDA1 is where that case gets made, alongside the wider foul drainage strategy.

See also: Foul drainage strategy, Septic tank, Foul drainage hierarchy

Fetch is the distance of open water over which wind blows to build waves. The longer the fetch, the larger the waves that arrive, which feeds directly into wave height and the coastal flood risk at a shoreline.

Geometry does the work here. A site facing a long open reach of estuary takes a different wave climate from one tucked behind a headland a mile away, and that difference only grows as still-water levels climb, a trend we set out in our look at sea level rise and coastal flooding in the UK.

See also: Sea level rise, Still water level, Tidal flap

A filter drain, often called a French drain, is a trench filled with permeable granular material, often with a perforated pipe laid in the base. Runoff enters through the stone, which filters silt out of it, and the trench then conveys the water onward.

Stone blinds with silt. That is the failure mode nobody budgets for, and a filter drain that has not been rodded or had its surface layer lifted in a decade will pond long before the design storm arrives, which is the argument for maintaining drains before surface water flooding starts.

See also: Filter strip, Block paving, Dry swale

A filter strip is a gently sloping vegetated band that takes runoff from an adjacent hard surface as sheet flow. Spreading the water thin slows it and strips out silt and pollutants before they reach anything downstream.

Sheet flow is the whole trick. Let runoff concentrate into a channel at one low point, usually because the kerb edge was never built dead level, and the strip treats nothing. Useful but fragile, and best placed first in the sequence set out in the four pillars of SuDS.

See also: Four pillars of SuDS, Suspended solids, Evapotranspiration

Finished floor level is the set level of a building's internal ground floor. Where flood risk applies, it is raised above the design flood level with freeboard added, so habitable space stays dry in a flood.

Raise it too far and you have steps at the front door, a level-access problem and a Building Regulations conversation; raise it too little and the freeboard is gone in one climate uplift. That balance is really an argument about residual flood risk, and it is worth having early.

See also: Freeboard, Residual flood risk, Overtopping

First flush is the opening portion of runoff in a storm, the dirtiest part, which lifts contaminants that have built up on surfaces since the last rainfall. SuDS are commonly designed to capture and treat that volume.

Long dry spells make it worse, because oils, brake dust and bird mess sit on a car park for weeks and then all leave at once. Catching that first volume is why treatment sits beside quantity in the four pillars of SuDS.

See also: Four pillars of SuDS, Reed bed, Simple Index Approach

A flap valve is a hinged one-way valve fitted to an outfall. Water discharges freely when levels allow, and the flap shuts against reverse flow when the receiving river or tide rises above the pipe.

Flaps jam. Silt, a twig, a bent hinge, and the thing that was meant to keep the river out is propped open on the one night it mattered, so we treat tide-locked or backed-up outfalls as a storage question inside the surface water drainage strategy and never rely on the valve alone.

See also: Outfall, Surface water drainage strategy, Gully

Flash flooding develops within minutes to a few hours. Heavy localised rainfall over small, steep catchments drives it, and water reaches the low ground almost as fast as it falls. The speed is the danger, because there is little time to respond.

Warning systems struggle with it. A river gauge upstream gives hours of notice on a big lowland catchment and almost none on a steep upland one. Our rundown of the different types of flooding keeps flash flooding separate from ordinary fluvial risk for that reason.

See also: Fluvial (river) flooding, Pluvial (surface water) flooding, Reservoir flooding

A flood alleviation scheme is an engineered project built to reduce flood risk to an existing community. Most combine defences, upstream storage and works to the channel itself.

Being behind a scheme is not the same as being out of Flood Zone 3. The mapping usually still shows the undefended extent, planners still ask the Sequential Test, and the standard of protection carries a design limit above which water arrives anyway, which is the whole subject of residual flood risk.

See also: Standard of protection, Residual flood risk, Washland

Flood and Coastal Erosion Risk Management, or FCERM, is the framework of policy, funding and activity for managing flood and coastal erosion risk in England. Defra leads it, and the Environment Agency and other authorities deliver it.

Money is the part that bites. Which schemes get built, and which communities wait another decade, follows the funding rules rather than the depth of the water, and the 2026 changes rewrite how that allocation works, as we set out in our explainer on the new FCERM funding guidance.

See also: Coastal erosion, Defra, Environment Agency (EA)

The Flood and Water Management Act 2010 is the principal legislation governing flood risk management in England and Wales. The Act created lead local flood authorities and provided for the Schedule 3 SuDS approval regime, which remains uncommenced in England to this day.

Schedule 3 is the ghost at the feast. Successive governments have promised commencement, developers have planned for it, and English SuDS approval still runs through planning conditions and section 106 instead, a history we unpick in our guide to the Flood and Water Management Act 2010.

See also: Schedule 3, Planning condition, Flood Risk Management Plan (FRMP)

A flood bund is a retaining bank, wall or dyke raised to protect an area against flooding up to a set standard. Beyond that standard the bund is simply a line on a plan.

Two questions decide whether a bund is worth building: what happens when it is overtopped, and where the water that gets behind it drains to. Pumping, a gravity outfall or a slow puddle for three days are very different answers, and all three sit under residual flood risk.

See also: Residual flood risk, Flood alleviation scheme, Flood gate

A Flood Consequences Assessment is the Welsh counterpart to an English FRA, required under TAN15 and Planning Policy Wales. It sets out levels, depths, access and residual risk so Natural Resources Wales can review the consequences of flooding.

Welsh applicants often arrive with an English FRA and expect it to pass. It will not. The emphasis in Wales falls on what happens to people and property when water arrives, which is why a Flood Consequences Assessment asks for depths, access and a route out in more detail than a screening report does.

See also: Planning Policy Wales, TAN15, Preliminary Flood Risk Assessment (PFRA)

A flood defence is a structure or scheme built to reduce the risk of flooding from rivers or the sea: a wall, an embankment, a barrier, a storage area, or a combination of those. Defences reduce risk. They do not remove it.

Ask two things about any defence near your site: its standard of protection, and its condition. Both change over time, one because the climate allowance moves and one because maintenance budgets do, and what is left over after both is residual flood risk.

See also: Embankment, Standard of protection, Residual flood risk

The Flood Estimation Handbook is the standard UK methodology and dataset for estimating flood frequency and design rainfall at a catchment. FEH outputs give the design flows and rainfall depths that flood risk work is built on, from a small greenfield runoff calculation to a full river model.

Catchment descriptors do a lot of quiet work here, and a badly chosen donor catchment skews the peak flow long before anyone draws a cross-section. We check them. That scrutiny is part of any hydraulic flood modelling we produce.

See also: Catchment descriptors, Flood modelling, QMED

Flood forecasting predicts flooding from rainfall and river conditions ahead of time. The output drives flood warnings to the public and the operational decisions that emergency responders make, from closing barriers to moving people.

Planning officers are starting to lean on forecasting in a way they did not five years ago, particularly where a safe refuge or a managed evacuation is the only workable answer on a site. Improved surface water forecasting shifts that conversation again, as our note on what SWFFIP means for planning explains.

See also: SWFFIP, Flood Forecasting Centre, Rapid response catchment

The Flood Forecasting Centre is a joint Environment Agency and Met Office operation that provides real-time flood forecasting and public guidance for England and Wales. One roof, two organisations, meteorology and hydrology read together.

Most people meet the Centre through the daily Flood Guidance Statement, which is public and worth reading during a live event. For planning work the relevance is narrower: forecasting shapes what a warning and evacuation plan can credibly promise, a point we pick up in what SWFFIP means for planning.

See also: Flood forecasting, SWFFIP, Environment Agency (EA)

A flood gate is a movable barrier that is closed to hold back floodwater. The same idea scales from a gate across a road or a gap in a flood wall down to a demountable panel fitted over a doorway on a single property.

Someone has to close it. That sentence is the entire weakness of the flood gate as a strategy, because a gate needs a warning, a person, a key and enough time, and the failure of any one of those puts you straight back into residual flood risk territory.

See also: Flood wall, Residual flood risk, Flood bund

Flood hazard rating combines flood depth, velocity and a debris factor into a single measure of the danger a flood poses to people. Results fall into bands that run from low hazard through to danger for all.

Depth alone flatters a site. Shallow still water in a garden is an inconvenience; the same depth moving at pace down a driveway will take an adult off their feet, and hazard rating is how that distinction gets written into an assessment of residual flood risk.

See also: Residual flood risk, Hard engineering, Levee

A flood map challenge is the formal route for contesting an Environment Agency flood zone designation where the mapping does not reflect conditions on the ground. Better site-specific evidence carries it, typically topographic survey and hydraulic modelling.

Odds improve when the national model has simply missed a level, an old embankment or a culvert that has been there since the war. Weak challenges are built on opinion and cost months, so the bar set out in our note on the flood map challenge process is worth checking before a client commits.

See also: Homebuyer flood risk survey, Insurance flood risk assessment, LiDAR

The Flood Map for Planning is the Environment Agency's national online map of flood zones from rivers and the sea. Planners and applicants use it to screen whether a site needs an FRA and to apply the flood-risk tests. The map screens at a strategic scale. It never forecasts what happens at a single property.

Zone boundaries are drawn from a national model, so they land where the model says rather than where the ground falls, and a level survey will often disagree with the shading. Treat it as a starting point. Our guide to the Flood Map for Planning covers what it does and does not show.

See also: Flood Map for Planning (Wales), Flood zones, Planning validation

The Flood Map for Planning (Wales) is Natural Resources Wales's official flood risk map for planning decisions in Wales. It replaced the Development Advice Map and divides land into flood zones set by annual probability of flooding from rivers, the sea and surface water.

Surface water sits inside the Welsh zones, which catches English consultants out more than anything else about working in Wales, because the English Flood Map for Planning leaves it to a separate dataset. The zone letters differ too. Both points are set out in our explainer on Welsh flood zones.

See also: Flood Map for Planning, Planning Policy Wales, Flood zones

Flood modelling is the building and running of hydrological and hydraulic models to simulate how floodwater behaves at a site: levels, extents, depths, velocities and hazard. The work earns its place where national flood maps do not resolve the risk closely enough to answer a planning question.

Cost frightens people off, and sometimes rightly. A model is worth commissioning when the site sits on a zone boundary, when the mapped extent looks wrong against the levels, or when a refusal turns on depth rather than principle, and we scope flood modelling against that test rather than as a default.

See also: 1D modelling, 2D modelling, Hydraulic modelling

Flood modelling software covers the computer tools used to predict where floodwater goes, how deep it gets and how fast it moves. HEC-RAS, TUFLOW, Flood Modeller and InfoWorks ICM are the packages used for flood risk assessments and defence design work.

The software is never the argument. A one-dimensional model built with care beats a two-dimensional one run on bad topography, and reviewers at the Environment Agency will ask about your boundary conditions long before they ask which package produced the plots, a point we return to in our guide to flood modelling software and techniques.

See also: Flood modelling, Hydrograph, Hydrology

A Flood Performance Certificate is a proposed rating of how well a home resists flooding and recovers afterwards, along the lines of an energy performance certificate. The audience is insurance, and it would not replace a planning FRA.

Proposed is doing a lot of work in that sentence, since no certificate exists to buy today. If the scheme lands, the resilience measures a homeowner installs start to have a resale value attached, which is the interesting part of the Flood Performance Certificate idea.

See also: Areas Benefiting from Defences, Demountable flood barriers, Flood hazard rating

Flood Re is a government-backed reinsurance scheme that keeps home flood insurance affordable in areas at risk, by letting insurers pass the flood element of a policy into a central pool. Homes built since 2009 are excluded, and the scheme is scheduled to close in 2039.

The 2009 cut-off catches people out constantly. Buy a new-build on a floodplain and Flood Re will not help you, which is deliberate policy to stop the scheme subsidising development in the wrong place, and our guide to the Flood Re scheme walks through who qualifies.

See also: Flood resistance, Demountable flood barriers, Flood recoverability

Flood recoverability covers property flood resilience measures that accept water entering a building but limit the damage and shorten the repair, such as raised electrics and resilient plaster. The approach suits places where flooding could be deep.

Letting water in sounds like surrender until you price the alternative. Keeping deep water out of a solid wall house means hydrostatic pressure on masonry that was never designed for it, so a recoverable interior is often the honest answer, which is where property flood resilience earns its keep.

See also: Property flood resilience (PFR), Flood Re, Flood resistance

A flood relief channel is an artificial channel that carries part of a river's flow around a settlement during high flows. The Jubilee River is one such scheme. Diverted water lowers flood risk through the town. It does not remove it.

Relief channels move the problem as much as they solve it, and people downstream notice. Anyone developing between the old river and the new cut needs to know which channel governs their levels in a big event. We treat that as a residual flood risk question and answer it with levels rather than assurances.

See also: Residual flood risk, Channel conveyance, Design flood level

Flood resistance covers property flood resilience measures aimed at keeping water out of a building altogether: flood barriers, non-return valves and airbrick covers. The approach works best where flooding is shallow.

Resistance and recoverability are usually specified together, since barriers buy you time and a resilient interior covers the night nobody was home to fit them. Deep water changes the calculation entirely, because pressure on the fabric becomes the bigger risk, a distinction our page on property flood resilience sets out.

See also: Property flood resilience (PFR), Non-return valve, Flood Re

A flood return period is the long-run average interval between floods of a given size, such as a 1-in-100-year flood. The number describes probability rather than a timetable, and the odds stay the same the year after a flood.

Clients hear 1-in-100 and assume a century of quiet. Two big floods in five years is perfectly consistent with the statistics, and the arithmetic of annual probability over a design life is worth doing before a committee asks. Nobody enjoys that question cold.

See also: Annual Exceedance Probability (AEP), Convective rainfall, Receptor

A Flood Risk Activity Permit is the Environment Agency permission needed for work on, under, over or near a main river, a flood defence or the floodplain in England. It was formerly called flood defence consent, and the Environment Agency is the body that decides the application.

Proximity is the trigger, not contact, so you can need one without touching the channel. A FRAP is never granted after the event either, which makes it a bad thing to discover late.

See also: Bespoke permit, Environmental permit, Standard rules permit

A Flood Risk Assessment is a site-specific technical report required for many planning applications. It assesses flood risk to and from the development from all sources, and shows how the design keeps people and property safe across the development's lifetime without pushing risk onto anyone else.

Scope is where these go wrong. A report that covers the river and ignores surface water, groundwater and the sewer network will come back from the LLFA, so we set the sources out at the start of every FRA we prepare for planning and agree them before modelling begins.

See also: Strategic Flood Risk Assessment (SFRA), BREEAM flood risk assessment, Preliminary Flood Risk Assessment (PFRA)

A flood risk asset is a structure or landscape feature with a flood or coastal-erosion management function. Walls, embankments, culverts, gates and pumping stations all qualify, and every one of them has to be maintained to keep working.

Ownership is the awkward part. Half the walls and culverts we come across on site have no named maintaining party, which turns a defended site into a question about residual risk behind defences that the assessment has to answer honestly.

See also: Embankment, Safe access and egress, Finished floor level (FFL)

A Flood Risk Management Plan is the statutory plan prepared for each river basin district. It sets out the risk of flooding from all sources across that district and the measures proposed to manage it.

Planning officers rarely quote an FRMP at you. Its value is background: the district-scale measures come from the same split of duties between the Environment Agency, lead local flood authorities and water companies that governs who manages flood risk in England. Read it before you argue about whose problem a watercourse is.

See also: Flood and Water Management Act 2010, Surface Water Management Plan (SWMP), Designated asset

Flood risk standing advice is national Environment Agency guidance that fixes the EA's position in advance for lower-risk, lower-complexity development. Local planning authorities apply it to decide many applications without going back to the EA for a bespoke consultation response.

Silence from the EA is not approval. Where standing advice applies, the case officer is the one weighing your report against it, and the tests are the same ones set out in the EA's role in flood risk planning. Meet them, or expect a holding objection anyway.

See also: Environment Agency (EA), Minor development, Ordnance Datum (AOD)

Flood risk vulnerability classification is the NPPF Annex 3 system grading every development type by the consequences of flooding, across five categories. Paired with the flood zone, it decides whether a use is acceptable or needs the Exception Test.

Classification drives more refusals than most applicants expect. A ground-floor flat is more vulnerable than the shop below it, so the same building can sit on two different sides of the policy line, which is why we settle the category first and check it against the Annex 3 categories.

See also: NPPF Annex 3, Exception Test, Highly vulnerable development

The Flood Studies Report of 1975 was the predecessor to the Flood Estimation Handbook. FSR rainfall and runoff estimation methods are still referenced occasionally in flood hydrology, and the abbreviation appears in reports written long after the FEH replaced it.

Check which method a legacy study used before you reuse its numbers. FSR-derived flows and FEH-derived flows can differ enough to change a design, and any figure carried into a new model has to be defended, so we rebase them as part of the hydraulic modelling work.

See also: Flood Estimation Handbook (FEH), Hydraulic modelling, Hydrology

A flood wall is a built wall that defends an area against flooding up to a specified standard of protection. The standard is the design point: the wall holds water back to the level it was designed for, and the protection it offers stops at that level.

Walls fail. Overtopping, a breach or a closure nobody shut in time all belong in the assessment, and a scheme sitting behind one is really a scheme about the risk that remains once a defence is in place. Design the ground levels accordingly.

See also: Overtopping, Standard of protection, Flood gate

A flood warning system is the Environment Agency's three-tier public alerting service. It escalates from Flood Alert, meaning flooding is possible, through Flood Warning, meaning flooding is expected, to Severe Flood Warning, meaning danger to life.

Warnings only work if someone acts on them. A flood plan that assumes residents will wake, register for alerts and move cars at three in the morning is optimistic, and the gap between issuing a warning and people responding is one of the failures traced in the Pitt Review.

See also: Pitt Review, Floodline, Flood Performance Certificate (FPC)

Land with a low probability of river or sea flooding, below a 0.1% annual chance or less than 1 in 1,000, where most development is steered, although large sites and surface water risk can still trigger an FRA.

Zone 1 describes rivers and the sea. Nothing else. Surface water, groundwater and sewer flooding are not mapped by it, and that is the misreading of the flood zones we correct most often.

See also: Flood zones, Flood Zone 3a, Flood Zone 3b (functional floodplain)

Flood Zone 2 is land with a medium probability of flooding: between a 0.1% and 1% annual chance from rivers, or 0.1% to 0.5% from the sea. A site in Zone 2 usually needs a site-specific Flood Risk Assessment and has to pass the Sequential Test.

Zone 2 looks benign on a map and rarely is in practice. Access and escape routes cross it, finished floor levels get argued over, and the Sequential Test still has to compare your site against alternatives, which is where the difference between Zones 1, 2 and 3 starts to bite.

See also: Flood Zone 1, Flood Zone 3a, Flood Zone 3b (functional floodplain)

Land with a high probability of flooding, a 1% or greater annual chance from rivers or 0.5% or greater from the sea, where development needs an FRA, the Sequential Test, and the Exception Test for the more vulnerable uses.

Sitting in 3a does not rule development out. It does mean proving the scheme stays safe for its lifetime, usually through finished floor levels, safe access and compensatory storage, and our guide to the flood zones sets out what each one demands.

See also: Flood zones, Flood Zone 2, Flood Zone 1

Flood Zone 3b, the functional floodplain, is land where water has to flow or be stored during floods. Its extent is defined locally through the Strategic Flood Risk Assessment, and development is heavily restricted to water-compatible and essential infrastructure uses.

Councils draw 3b themselves. Two authorities on opposite banks of the same river can map it differently, because the SFRA sets the extent, so check the local evidence base rather than the national map, and read how Zone 3b differs from Zone 3a before you plan a layout.

See also: Functional floodplain, Flood Zone 2, Flood Zone 3a

Flood zones are the Environment Agency's classification of land by the probability of flooding from rivers and the sea, drawn as if flood defences did not exist. There are four: Zone 1 for low probability, Zone 2 for medium, Zone 3a for high, and Zone 3b, the functional floodplain.

The undefended assumption catches people out. A site behind a large embankment still sits in Zone 3a on the published map, so the zone tells you about the river rather than about your actual exposure, and the way each zone is defined matters more than the colour on screen. Surface water is mapped separately.

See also: Functional floodplain, Flood Zone 1, Flood Zone 3a

A floodable void is the open, unobstructed space beneath a raised building, formed by stilts or an undercroft. Floodwater passes through it freely while the occupied floor sits dry above the design flood level.

Voids get filled in. Owners board them for bike storage or wall them for security within a few years, and the flow route the consent depended on disappears, which is why the design and the planning conditions have to be written together with stilts and undercrofts as mitigation in mind.

See also: Flood warning system, Sea wall, Soft engineering

Floodline is the Environment Agency's telephone and online service, providing flood warnings and advice to the public. It is a public-facing information channel rather than a source of design data for a flood risk assessment.

Put the number in the flood plan. A resident holding an evacuation leaflet at midnight needs one phone line and one instruction, and the case for exactly that kind of plain public communication runs right through the Pitt Review into the 2007 floods.

See also: Pitt Review, Flood warning system, Floodable void

Floodplain compensation is replacement flood storage volume provided to offset floodplain lost to development. It is designed level for level, band by band, so water displaced at each level has somewhere to go and flood risk elsewhere is not increased.

Volume alone will not do it. Compensation has to be hydraulically connected to the floodplain it replaces and available at the right levels, which the flood model has to demonstrate rather than assert. Digging a hole uphill fools nobody.

See also: Manning's n (roughness coefficient), Probable Maximum Flood (PMF), Rainfall-runoff model

A flow control is a device or restriction that limits the rate at which water leaves a storage feature to an agreed discharge rate. An orifice plate and a vortex control are the common forms, and the whole storage volume is sized around whatever rate the control passes.

Small controls block. There is a floor below which an orifice becomes a maintenance liability, so on tight sites we either accept a slightly higher discharge or move to a vortex unit, and the choice belongs in the surface water drainage strategy where the LLFA will look for it.

See also: Vortex flow control (Hydrobrake), Discharge rate, Orifice plate

Fluvial flooding, also called river flooding, happens when flow in a river or watercourse exceeds the capacity of the channel. Water overtops the banks, or breaks through them, and spreads across the land on either side until the channel can carry it again.

Blockage does most of the damage on small watercourses. A trash screen packed with branches can put water over a bank well below the flow the channel was built for, which is why we look upstream at structures as well as at the numbers when working through the different sources of flood risk.

See also: Flash flooding, Sewer flooding, Infrastructure flooding

A forebay is a small pre-treatment cell set at the inlet of a pond or basin. Coarse sediment drops out there, before the water reaches the main storage, which keeps the silt in one place that can be dug out.

Leave the forebay out and the whole basin becomes the silt trap. Desilting a planted basin costs far more than clearing a small concrete cell, so the maintenance argument wins when we set out the treatment train in a drainage strategy.

See also: Treatment train, Geomembrane, Geotextile

The foul drainage hierarchy is the ranked order for disposing of foul wastewater under the Building Regulations: public sewer first, then a package treatment plant, then a septic tank with a drainage field, and a cesspool as a last resort.

Skipping a step down the list needs evidence. Councils expect a written refusal from the water company or a distance and cost case before they accept anything below public sewer, and that trail is what the foul drainage strategy for a site is there to record.

See also: Foul drainage strategy, Cesspool, Package treatment plant

A foul drainage strategy is the report confirming where a development's foul water will discharge, to the public sewer or to an off-mains system, and showing that the chosen route is available, acceptable and buildable.

Buildable is the word that trips schemes. A sewer forty metres away behind three gardens you do not own is not a connection, and easements, levels and pumping all have to be settled inside the foul drainage strategy before a condition is discharged.

See also: FDA1 Foul Drainage Assessment, Foul drainage hierarchy, Foul flow

Foul flow is the wastewater discharged from sanitary appliances and processes, carried away by the foul drainage system. Basins, WCs, showers, kitchen and trade appliances all contribute, and the total sets the size of the pipes and any pumping.

Rainwater must stay out of it. Surface water plumbed into a foul run is one of the commonest causes of overload downstream, and water companies check for it before agreeing a connection, which is why we keep the two systems separate from the first sketch in the foul drainage plan for the site.

See also: FDA1 Foul Drainage Assessment, Foul drainage hierarchy, Foul drainage strategy

A foul sewer carries wastewater, meaning sewage, from properties to a treatment works. It takes no rainwater by design. New development must generally connect its foul drainage to the public sewer where that connection is possible, which makes the sewer records an early check rather than a late one.

Old records lie. A pipe marked foul on an asset plan turns out to be combined once you lift the covers, and getting that wrong changes both the connection agreement and the volumes, so it pays to know how foul, surface water and combined sewers differ before you design around them.

See also: Combined sewer, Combined sewer overflow (CSO), Separate sewer

The four pillars of SuDS are the objectives every sustainable drainage scheme should deliver together: water quantity, water quality, amenity and biodiversity. They come from the CIRIA SuDS Manual and are carried into the 2025 National Standards.

Three out of four is a common submission. Quantity gets modelled to two decimal places while amenity and biodiversity get a sentence each, and reviewers now push back on that, so we design the planting and the public space alongside the storage when applying the four pillars.

See also: Amenity, Wetland (SuDS), CIRIA SuDS Manual (C753)

Freeboard is a safety margin of height added above the estimated flood level when the finished floor levels of a building are set, and the figure used is typically 300mm to 600mm. The margin exists to allow for uncertainty in the flood level estimate it sits above.

Freeboard is an allowance, not armour. It exists to cover what the model cannot see, so treating it as protection means you have quietly moved into residual flood risk without planning for any of it.

See also: Finished floor level (FFL), Residual flood risk, Flood alleviation scheme

The functional floodplain is Flood Zone 3b: land that floods in extreme events, or that is deliberately used to store and convey floodwater when a river is out of bank. Most forms of development are not appropriate there, which is why the extent of 3b is argued over so early.

Nobody gets a house built in 3b. That is the blunt version, and it is close enough to true that we would rather test the extent at the pre-application stage than after a layout is fixed, and the local SFRA matters here more than the published flood zones.

See also: Flood Zone 3b (functional floodplain), Flood zones, Flood Zone 2

A gabion is a wire mesh cage packed with stone, stacked to form erosion-resistant protection to a river bank or a slope. The stone gives weight and the mesh holds it in place, so the face resists scour.

Consent comes before construction. Bank works usually need permission from the Environment Agency or the lead local flood authority depending on whether the channel is a main river or an ordinary watercourse, and mesh corrodes, so somebody has to own the replacement in thirty years.

See also: Scour, Main river, Ordinary watercourse

A gauging station is a fixed point on a river where flow or level is measured continuously. The record feeds flood forecasting and hydrological analysis, and a long record carries more weight in a flood estimate than a short one.

Level is not flow. A stage record has to be converted through a rating curve before it means anything hydrologically, and ratings drift as channels change, which is worth remembering when you watch live river levels climb during a storm.

See also: Telemetry, NaFRA2, Recorded flood extents

The General Binding Rules are the Environment Agency's statutory conditions that allow a small sewage discharge from a septic tank or a package treatment plant without an environmental permit. Compliance depends on limits for discharge volume, the level of treatment and where the outfall and drainage field sit.

Fall outside any one of the limits and you need a permit. Discharging to a watercourse rather than the ground, or exceeding the volume the rules allow, takes a site off the standard route and an application that assumed otherwise loses months, so we test it against the general binding rules.

See also: Controlled waters, Environmental permit, Environment Agency (EA)

Geocellular storage is built from modular plastic cells assembled below ground into a lightweight void that stores surface water for attenuation or infiltration. The arrangement holds a large volume in a compact footprint, and that is the whole appeal.

Access for maintenance decides whether crates are acceptable. Adoptable schemes and many LLFAs want inspectable units with a silt trap upstream, and the cells sit under highways or gardens where nobody will want to dig later, so we fix the position and the maintenance route inside the surface water drainage strategy.

See also: Attenuation, Surface water drainage strategy, Offline storage

A geomembrane is an impermeable plastic sheet used to line a SuDS feature where infiltration must be prevented. Water is then held and released at a controlled rate instead of soaking into the ground below the basin, pond or trench.

Lining changes the sums. An unlined basin loses volume to the ground during a storm and a lined one does not, so the storage has to be resized the moment a liner goes in, which is a revision we would rather make while drafting the surface water drainage strategy than on site.

See also: Surface water drainage strategy, Forebay, Hydrodynamic separator

A geotextile is a permeable synthetic fabric laid within drainage and earthworks to do one of four jobs: separate one material from another, filter fine particles out of moving water, drain water along its own plane, or reinforce weak soil. The job chosen decides the grade specified.

Wrap a stone trench in the wrong fabric and it silts up. Filtration and separation grades are not interchangeable, and a blinded geotextile is effectively a liner nobody designed, so specify it properly when the infiltration feature is set out in the site drainage strategy.

See also: Micropool, Rill, Blue roof

GIS, a geographic information system, is digital mapping software that brings spatial data from many sources into one place for analysis. In flood risk assessment, modelling and SuDS design it informs site selection and drainage decisions.

Layers argue with each other. Historic mapping, LiDAR, published flood extents and the utility records rarely agree on the same ground, and resolving those conflicts is most of the work, as anyone who has stacked them knows from using GIS in flood risk work.

See also: LiDAR, Topographical survey, Check Your Long Term Flood Risk

Glacial till is a superficial deposit often mapped as clay, sand and gravel, which typically behaves as low-permeability ground. Judging soakaway feasibility from that map description alone is a common trap.

The word gravel does the damage. A drainage strategy built on a desk study over till tends to collapse at the first trial pit, so book the infiltration testing early and let ground conditions decide whether a soakaway works before the layout depends on one.

See also: Trial pit, Permeability, Superficial deposits

A Grampian condition is a planning condition that prevents development from starting until a specified action has been carried out, often on land the applicant does not control. For drainage and flood works, it is one lawful route to securing off-site works that a permission depends on.

A condition cannot make a neighbour say yes. If the off-site outfall depends on a landowner who has already refused, there is no reasonable prospect of the works happening and the condition will not save the application, which is the same test applied to conditions promising betterment.

See also: Betterment, Planning condition, Pre-commencement condition

Gravity drainage moves foul or surface water flows along falling gradients from inlet to outfall on the difference in level alone, with no pump, rising main or power supply anywhere in the run.

Levels decide. Get the outfall invert surveyed before you commit to a layout, because a scheme that falls half a metre short ends up with a pumping station. That means an adoption headache and a maintenance liability, both of which the surface water drainage strategy has to justify to the LLFA.

See also: Surface water drainage strategy, Outfall, Drainage strategy

A green roof is a roof covered with vegetation and a growing medium. Rainfall is intercepted at the surface before it reaches a pipe, what does run off is attenuated and treated on the way, evapotranspiration returns part of the water to the air, and the build-up adds biodiversity and insulation into the bargain.

Depth decides. A thin extensive system and a deep intensive one behave nothing alike in a summer storm, whatever the manufacturer’s retention figure says, so the two are never interchangeable in a calculation even though both sit under interception in the four pillars of SuDS.

See also: Evapotranspiration, Interception, First flush

Greenfield land has never been built on, typically farmland or open countryside. Runoff from it is lower than from developed ground, and the planning considerations that apply differ from those on brownfield land, which is why the distinction carries weight in both site selection and drainage design.

Undeveloped does not mean unconstrained. A greenfield site in Flood Zone 3 still has to clear the Sequential Test, and the fact that nothing has ever been built there tends to make alternative sites easier for a council to point at, which is where our NPPF flood risk guidance starts.

See also: Previously developed (brownfield) land, Sequential Test, Allocated site

The greenfield runoff rate is the rate at which rainfall runs off a site in its undeveloped state, and it is the benchmark that post-development discharge is normally restricted to, which is why the difference between it and the developed rate sets how much attenuation storage a scheme has to find.

You calculate it from catchment descriptors rather than measure it, which is how two consultants produce different answers for the same field. Ours are stated and justified in the surface water drainage strategy.

See also: Runoff, Greenfield runoff volume, Attenuation storage

Greenfield runoff volume is the total volume of water that would leave a site in its undeveloped state for a given event. That figure sets the long-term storage requirement for the drainage design.

Rate and volume are separate tests, and passing one does not pass the other. A tank sized only to hold back the peak flow still pushes far more water out over six hours than the field ever did. We size both before the surface water drainage strategy is issued.

See also: Runoff, Greenfield runoff rate, Long-term storage

Groundwater flood risk assessment is specialist, hydrogeologist-led work covering a source that national flood maps do not show. It establishes a defensible design groundwater level and tests that level against the proposal.

Basements are where it bites. A scheme with a lower ground floor slab sitting below the winter water table will not survive validation on a desk study alone, and our groundwater flood risk assessment pages set out the monitoring evidence councils ask for.

See also: Design groundwater level, Water table, Flood Risk Assessment (FRA)

Groundwater flooding occurs when the water table rises to or above ground level and water emerges from below rather than arriving overland. It is common in permeable geologies such as chalk, slow to arrive and slow to leave, and often persists for weeks rather than hours.

It lingers long after the rain stops. On a chalk site that can mean weeks of standing water in an excavation, which is why groundwater flooding is usually the risk that decides whether a basement is possible at all.

See also: Water table, Groundwater recharge, Artesian

Groundwater monitoring records how the water table moves through the seasons, usually with trial pits or boreholes read across the winter peak. The purpose is evidence: a drainage scheme relying on infiltration has to be shown to work in every season, including the wet weeks when the water table sits at its highest.

Timing is the trap. Winter readings cannot be manufactured in June, so a condition requiring a full seasonal record can hold a start on site for the best part of a year, which is the delay our note on groundwater monitoring and planning delays is about.

See also: Winter monitoring window, Piezometer, Trial pit

Groundwater recharge is the replenishment of underground water stores as rainfall infiltrates through the soil. Sustained recharge in a wet period lifts the water table, and a raised table is what triggers groundwater flooding.

Recharge runs on a lag of weeks or months, so the worst groundwater levels often arrive long after the rain that caused them, a pattern set out in our explainer on groundwater flooding causes and planning implications. Chalk catchments are the classic case.

See also: Groundwater flooding, Water table, Interflow

A growth curve describes how flood magnitude rises with return period. Scaled up from an index flood, the curve puts a number on events rarer than anything in the gauged record.

Steepness matters more than people expect. On a flashy catchment the curve climbs hard between the 1 in 100 and the 1 in 1000 event, so a design that looks comfortable at the lower return period is swamped at the higher one, and our flood modelling work reports both.

See also: Flood modelling, Rating curve, QMED

A gully is the road or yard inlet that takes surface water off the paved surface and passes it to a sewer, normally a grating over a chamber with a silt trap below the outlet.

Gullies fail quietly. A grating half blocked with leaf litter still looks like drainage from a car window, and the first anyone knows is a flooded kerb line. Cleansing and spacing belong in the maintenance schedule behind the surface water drainage strategy.

See also: Surface water drainage strategy, Headwall, Inspection chamber

The H++ scenario is a credible-maximum climate change case, around 1.9m of sea level rise to 2100, used to stress-test nationally significant infrastructure and major schemes against a plausible worst case.

Do not design to it. H++ is a sensitivity test rather than the allowance you build to, and confusing the two produces finished floor levels a metre higher than anyone needs, which is a point worth checking against the climate change allowances for planning.

See also: Climate change allowance, Sea level rise allowance, Central / higher central / upper end allowances

The time an infiltration or storage feature takes to empty to half its volume. It is checked to confirm the feature recovers before the next storm arrives.

A soakaway can pass on headline infiltration rate and still fail here. A feature that has not emptied has no capacity left, which is one of the questions behind whether you need infiltration testing at all.

See also: Infiltration testing, Infiltration, Soakaway

Hard engineering manages flood and coastal risk with built structures. Walls, embankments and barriers are the standard examples, each designed to hold water back from land and property.

Every wall has a design standard, and above that standard water arrives anyway. That gap between the defence and the true worst case is residual risk, which planning still expects a scheme to address through levels, layout and a route out, as our piece on residual flood risk explains.

See also: Embankment, Residual flood risk, Flood bund

A headwall is the retaining structure built at the inlet or outlet of a culvert or pipe. It holds the surrounding ground in place, protects the bank and bed against erosion at the point where flow enters or leaves, and fixes the pipe end so the invert stays where the design put it.

Outfalls get drawn as an arrow and left there. In practice you need the headwall detail, the erosion protection below it and, on a watercourse, the consent from the drainage authority before anything is built, all of which we pin down in the surface water drainage strategy.

See also: Outfall, Surface water drainage strategy, Invert level

Highly vulnerable development is the NPPF vulnerability category covering basement dwellings, emergency command centres and caravans used as permanent homes. Flood Zone 3 is barred to it, and in Flood Zone 2 it passes only through the Exception Test.

The category catches schemes people assume are ordinary housing. Convert a lower ground floor into a self-contained flat and the whole unit reads as highly vulnerable, which in Zone 3a ends the application, a trap set out in our guide to NPPF Annex 3 vulnerability classification.

See also: Less vulnerable development, More vulnerable development, NPPF Annex 3

The highways authority maintains and drains the public highway, including roadside ditches, and sits among the risk management authorities because highway runoff is a flood risk in its own right.

Connecting into a highway drain is not a right. The authority can refuse, or demand a payment and a maintenance agreement, and finding that out after the layout is fixed costs weeks. We check ownership early against the split of duties described in who manages flood risk in England.

See also: Local Planning Authority (LPA), Designated asset, Lead Local Flood Authority (LLFA)

Hold the line is the shoreline management policy of keeping the coast where it is. Existing defences are maintained or upgraded, so the mapped coastline stays in its present position rather than moving landward or seaward. Shoreline management plans assign the policy to a defined stretch of coast.

A policy is not a promise of funding. Hold the line for the current epoch can turn into managed realignment in the next one, and a development banking on the defence lasting a century should read the epoch tables in the relevant shoreline management plan first.

See also: Epoch, Shoreline Management Plan (SMP), Managed realignment

A homebuyer flood risk survey is an independent, property-specific assessment for a buyer. It judges how flooding would affect that particular building rather than the postcode around it, and explains what the conveyancing search result actually means.

Search reports paint whole streets red. Ground levels change by a metre across one garden, and the difference between a house that floods and the one next door comes down to a doorstep. That is what an independent flood risk survey before purchase settles.

See also: Topographical survey, Conveyancing flood report, Environment Agency flood data products

HOST, the Hydrology of Soil Types classification, sorts UK soils by their hydrological behaviour. The class assigned to a site feeds estimates of infiltration potential and percentage runoff.

Soil mapping is a starting point and nothing more. HOST can indicate free-draining ground on a site whose trial pits fill and stay full, which is why infiltration is only ever assumed in a surface water drainage strategy once real tests back it.

See also: Percentage runoff, Runoff, Hydrology

A house in multiple occupation (HMO) is a property shared by several unrelated occupants. Creating one can trigger a flood risk assessment or a drainage strategy, because the change is read as an intensification of use and it raises foul discharge from the building.

Size misleads people here. A six-bedroom conversion is a small job on paper, yet the council can still ask for a full flood risk assessment and evidence that the existing foul connection copes. Our note on flood risk assessments for HMOs works through that position.

See also: Flood Risk Assessment (FRA), Change of use, Class MA

Hydraulic capacity is the maximum flow rate a pipe, channel or structure can convey before it surcharges or overtops. Beyond that point the water has to go somewhere else.

Capacity is a number that ages. Sewers that coped forty years ago now serve extra roofs, extra paving and a climate uplift on top, so the question we answer in a surface water drainage strategy is where the excess goes once the pipe is full.

See also: Surface water drainage strategy, Surcharge, Drainage strategy

Hydraulic gradient is the rate at which hydraulic head changes with distance along a flow path. That slope is what drives water through a channel, a pipe or an aquifer: no gradient, no movement.

Downstream conditions reach back upstream. A high tide or a full river flattens the gradient at an outfall and the pipe stops delivering its design flow long before anyone sees water on the ground, which is the sort of interaction our flood modelling is built to pick up.

See also: Flood modelling, Hydraulic modelling, ReFH2

Hydraulic modelling simulates how water moves through channels and across floodplains. The output is a set of predicted flood levels, extents, depths and velocities for one site, and it is commissioned when broad-scale national mapping does not resolve the risk on the ground.

Modelling earns its cost when it changes a map. We have taken sites out of Flood Zone 3 by showing that the mapped extent came from a coarse national run rather than the local channel survey, and that is the case our flood modelling service is built to make.

See also: Flood modelling, Hydraulic gradient, Hydrological modelling

A hydrodynamic separator is a proprietary chamber that uses the flow pattern inside it to strip sediment, oils and floating debris out of surface water runoff before discharge.

Treatment by catalogue is the easy option and often the wrong one. A separator handles the pollutants, then leaves you with a chamber that needs tankering, so we would rather see a swale or a filter strip doing the same job inside the surface water drainage strategy.

See also: Swale, Runoff, Filter strip

A graph of how flow or water level at a point changes through and after a rainfall event, showing the size of the peak, the moment it arrives, and the total volume of runoff behind it.

Most people read the peak and stop there. Volume under the curve is what sizes storage, and reading it properly is a large part of what flood modelling software exists to do.

See also: Peak discharge, Time to peak, Flood modelling software

Hydrological modelling estimates how much water arrives at a site. Design flows and design rainfall come out of it, climate change uplifts included, and those numbers are the input the hydraulic model then routes.

Get the hydrology wrong and the hydraulics are wasted. FEH catchment descriptors are generated automatically and can put an urban extent or a soil class well off the mark, so we check them against the real catchment before any of it feeds the flood modelling.

See also: Flood modelling, Catchment descriptors, Linked 1D-2D modelling

Hydrology is the study of where water occurs, how it moves and how it is distributed, from rainfall to runoff to river flow. Flood estimation and drainage design both rest on hydrological analysis.

Textbook hydrology and working hydrology part company quickly. In UK practice the subject arrives as a set of tools, catchment descriptors, rainfall models and routing packages, which is the ground covered by our guide to flood modelling software and techniques.

See also: Flood modelling software, Catchment descriptors, Catchment

An impermeable surface, tarmac or a roof for instance, lets no water pass through it. Rain that would have soaked away instead runs off, faster and in greater volume than from the ground it replaced.

One driveway changes nothing. Ten thousand driveways rebuild the catchment, and that slow accumulation of paved front gardens, extensions and yards is a large part of why surface water flooding in English towns keeps getting worse.

See also: Surface water flooding, Surface water drainage strategy, Detailed drainage design

Infiltration is the passage of surface water down into the soil and the ground beneath it. Infiltration-based SuDS, soakaways above all, only work where that ground is permeable enough to take the water at the rate the design assumes, which is a question of testing rather than assumption.

Clay says no, and it usually says so early. A single trial pit that empties overnight is not evidence either, because BRE Digest 365 asks for three separate fills in the same hole, which is the standard our note on whether you need infiltration testing works to.

See also: Infiltration testing, Soakaway, BRE Digest 365

An infiltration basin is a shallow, normally dry hollow that holds runoff and lets it soak into the ground. Storage and disposal happen in the same feature, so the basin attenuates the flow and removes the water from the system.

Basins eat land. Halve the infiltration rate and the footprint roughly doubles, which is why the layout should wait for the test results discussed in our note on infiltration testing rather than the other way round.

See also: Infiltration testing, Infiltration, Infiltration trench

Infiltration rate is the speed at which water soaks into the ground, quoted in metres per second or millimetres per hour. The figure decides whether infiltration drainage is viable, and it sets how large the soakaway has to be.

Units trip people up constantly. The same ground reads as impressive in millimetres per hour and hopeless in metres per second, so check which unit the ground investigation report is quoting before drawing any conclusion from it, then see what the rate does to storage in our BRE 365 infiltration rate calculator.

See also: Infiltration, BRE Digest 365, Soakaway

Infiltration testing measures the rate at which the ground on a site absorbs water. The measured rate is what proves whether infiltration drainage such as a soakaway will work there, and it is the figure any soakaway design is sized from, so the test governs whether surface water can go into the ground.

Settle this early. A failed result means redesigning the drainage strategy rather than tweaking it, so it pays to know whether you need infiltration testing before the layout is fixed.

See also: Infiltration, Soakaway, Half-drain time

An infiltration trench is a long, narrow excavation filled with permeable material that stores surface water in the voids and lets it soak away into the surrounding soil.

Trenches silt up. Fine material washes into the voids from the first winter onwards and the storage quietly disappears. A proper inlet, a silt trap and honest maintenance access matter as much as the numbers that come out of infiltration testing in the first place.

See also: Infiltration testing, Infiltration, Infiltration basin

Infrastructure flooding happens when water escapes from a man-made system built to store or convey it: a canal, a culvert, a water main, a pumping station. The cause is normally structural failure or a fault in the asset.

National flood maps do not show it. A burst main or a failed pump produces a flood with no return period and no mapped extent, which is why we treat infrastructure separately when running through the sources of flood risk for a site.

See also: Culvert, Pluvial (surface water) flooding, Reservoir flooding

An inspection chamber is a smaller access point on a drainage system, set at ground level so the pipe below can be looked into and rodded clear from the surface. Chambers are not usually sized for a person to enter, so the work is done from above.

Entry is the practical difference. A manhole is designed to be climbed into and a chamber is not, and that one distinction drives cover size, benching, ladder provision and confined-space procedure, all of which get scheduled in the surface water drainage strategy.

See also: Manhole, Surface water drainage strategy, Penstock

An insurance flood risk assessment is a property-level report written for insurers, brokers or underwriters. It gives site-specific evidence of flood exposure in place of the automated postcode-level screening that usually sets a premium.

Postcodes are blunt instruments. A house on a rise at the top of a flagged postcode carries a fraction of the risk the model assigns it, and an insurance flood risk assessment gives an underwriter something concrete to price against instead of a red dot.

See also: Flood Risk Assessment (FRA), Flood map challenge, GIS (Geographic Information System)

Interception is the SuDS objective of dealing with the first few millimetres of rainfall on site, at least 5mm, so that ordinary showers never reach the drainage network. Water quality improves as a result.

Attenuation does not deliver it. A tank at the bottom of the site still passes every small storm through the pipe network, so interception has to be designed in at the surface, which is where the four pillars of SuDS put it.

See also: Four pillars of SuDS, Green roof, Tree pit

Interflow is shallow water that has soaked into the soil and then travels sideways through the subsurface towards a watercourse. It sits between surface runoff and deep groundwater, faster than one and slower than the other.

Spring lines at a break of slope are usually this. Householders report it as groundwater and consultants sometimes write it up as surface water, when the water is moving through a permeable layer above a clay bed. Our article on groundwater flooding and its planning implications sets the mechanism in context.

See also: Spring line, Groundwater flooding, Perched water table

An Internal Drainage Board is a local public body managing water levels and land drainage across a low-lying, flood-prone district. IDBs operate pumps, channels and sluices, and they issue land drainage consent inside their district.

Boards are easy to miss. An IDB district does not appear on the standard planning search, and a scheme discharging into a board channel without consent can be stopped after approval, which is the risk our note on why IDBs matter for planning sets out.

See also: Land drainage consent, Risk Management Authority, Pumping station

Invert level is the level of the lowest internal point of a pipe, channel or culvert, taken at the inside of the barrel rather than the outside. Gradients between manholes and the hydraulic calculations behind a drainage design are all set from one invert to the next.

Cover, soffit and invert are three different levels on the same structure. Confusing them is one of the more expensive errors a drainage drawing can carry, so each is stated explicitly in the surface water drainage strategy.

See also: Manhole, Surface water drainage strategy, Proper outfall

Joint probability analysis estimates the combined likelihood of two or more flood sources arriving together: high river flow coinciding with a high tide is the classic pairing, and the combined event carries a probability of its own.

Treat the two sources separately and you will understate the risk in an estuary. We test the pairing inside the flood modelling for tidal reaches, because the design case is rarely the worst river event or the worst tide on its own.

See also: Flood modelling, SAAR, Stage

The Justification Test was the former TAN15 route to showing that development in a flood-risk area in Wales was acceptable. The 2025 TAN15 replaced it with a plan-led framework of acceptability criteria, including depth and velocity thresholds.

Older Welsh applications still cite it, and that alone can cost you a round of comments. Check which version of the policy applies before you draft anything: our TAN15 guidance sets out the criteria that now decide acceptability in Wales.

See also: TAN15, Defended Zone, Notification Direction

The Land Drainage Act 1991 is the primary legislation governing ordinary watercourses in England and Wales. Land drainage consent, riparian duties and the powers of internal drainage boards all sit within it.

Most developers meet the Act the moment a culvert, headwall or outfall touches a ditch. Consent under it is separate from planning permission and can be refused after your approval lands, so read the ordinary watercourse consent route before levels are fixed.

See also: Ordinary Watercourse Consent (OWC), Culvert, Ordinary watercourse

Land drainage consent is permission from an internal drainage board or LLFA to alter a watercourse, discharge into a board's drain, or build near a maintained channel. Planning permission is a separate matter entirely.

Boards charge for it. They also refuse it, and a refusal arriving after planning consent leaves a scheme with an approved layout it cannot drain, which is why internal drainage boards belong in the pre-application conversation instead of the scramble to discharge conditions eighteen months later.

See also: Internal Drainage Board (IDB), Risk Management Authority, Ordinary Watercourse Consent (OWC)

Land raising lifts ground levels on a site so that development sits above the design flood level, and because the raised platform displaces water, it must usually be balanced by compensatory flood storage to stop that water going somewhere else.

Import fill without a compensation scheme and you have moved the flood rather than removed it. Volume taken out of the floodplain has to come back at matching elevations, which is the level for level principle that decides whether a raised platform is acceptable.

See also: Level for level, Compensatory flood storage, Cut and fill

A lateral drain is the length of drain running from the edge of a property's curtilage to the point where it connects to the public sewer. Private drainage inside the curtilage stops there, and the public sewer takes over at the far end, so the lateral is the pipe that bridges the two.

Homeowners usually meet the term during a blockage, when responsibility for a pipe is disputed. Which sewer it joins matters as much as where it runs, and the difference between foul, surface water and combined sewers decides what may lawfully go into it.

See also: Combined sewer, Section 104 agreement, Dry weather flow (DWF)

The county or unitary authority responsible for local flood risk from surface water, groundwater and ordinary watercourses, and a statutory consultee on surface water drainage for every major development.

On major schemes the consultation response usually settles the drainage design. Local expectations vary more than the national standards suggest, so we start from what the LLFA asks for in that county.

See also: Local Flood Risk Management Strategy, Local Planning Authority (LPA), Major development

A leaky dam is a deliberately permeable barrier placed across a small stream to slow flow, store water and push high flows onto the floodplain. Lowering the downstream flood peak is the aim. The technique belongs to natural flood management.

Placement decides everything. Put a run of them in the wrong reach and you send woody debris towards a culvert that was already the pinch point, and our guide to leaky dams therefore starts with catchment position long before construction detail.

See also: Catchment, Culvert, Beaver dams

Less vulnerable development is the flood risk vulnerability category covering uses such as shops, offices, restaurants and general industry, all of which face fewer flood-zone restrictions than residential or highly vulnerable development.

Get the class right early. A mixed scheme takes the vulnerability of its most sensitive use, so one flat above a shop drags the whole building up the NPPF Annex 3 table and can pull an otherwise simple application into the Exception Test.

See also: Highly vulnerable development, More vulnerable development, NPPF Annex 3

A levee is a raised bank alongside a river channel, natural or engineered. Its job is to hold flood flows inside the channel and keep them off the ground beside it.

UK practice tends to say embankment or flood bank. Levee is the North American word, and the one school geography uses, though whatever the label the important question is the same: what happens to the land behind when the bank is overtopped, which is residual flood risk.

See also: Embankment, Residual flood risk, Flood defence

A Level 1 SFRA is a broad, strategic-scale flood risk assessment covering a planning authority area and built from existing datasets. Initial screening and the plan-level Sequential Test are what it supports.

Screening is not assessment. Mapping at this scale is drawn around allocations and will not resolve a red line boundary, though it is still worth mining, because strategic flood risk assessments often hold modelled levels, historic flood records and defence details that a council will never send you unprompted.

See also: Strategic Flood Risk Assessment (SFRA), Level 2 SFRA, Sequential Test

A Level 2 SFRA is a more detailed, site-specific strategic flood risk assessment that includes modelling. Councils produce one where development in higher-risk flood zones cannot be avoided.

Level 2 follows a failed sequential search. It gives depth, velocity and hazard where a Level 1 gave only a zone boundary, and the two-level SFRA structure explains why a council may be sitting on far more modelling than its published plan documents ever show.

See also: Strategic Flood Risk Assessment (SFRA), Level 1 SFRA, Flood zones

Level for level is the principle in compensatory flood storage that lost storage must be replaced at the same flood levels rather than as an equal total volume. Flooding happens progressively as water rises, which is why elevation matters.

Volume-for-volume schemes fail here, and they fail late, usually in a consultation response. Build the check as a stage curve: storage lost against storage gained at each modelled level, through the design event. Our note on compensatory flood storage works through it.

See also: Compensatory flood storage, Land raising, Cut and fill

LiDAR is high-resolution airborne laser survey data giving detailed ground elevations over wide areas. Flood models take it as terrain input, and it allows modelled flood levels to be compared against levels measured on a site.

Free coverage tempts people. Skip the topographic survey on anything with a threshold level in it and you inherit vegetation and building artefacts, because the flood modelling work still needs surveyed finished floor levels before anyone can say whether a property floods.

See also: Flood modelling, Topographical survey, GIS (Geographic Information System)

Linked 1D-2D modelling couples a 1D channel model with a 2D floodplain model, the default for detailed UK flood risk assessments wherever in-channel flow and out-of-bank spreading both matter.

A 1D model says the water left the bank. Where it went next, how deep it got and how fast it moved comes from the 2D domain, which is what the modelling we run for flood risk assessments has to show when a safe access route or a dry escape stair is in question.

See also: 2D modelling, 1D modelling, Hydrological modelling

The littoral zone is the shallow, planted marginal shelf around the edge of a SuDS pond or wetland, roughly 0.1 to 0.5m deep. Wetland vegetation roots in that band. Treatment and biodiversity both depend on it, so the shelf is a working part of the design rather than an edge detail.

Steep pond edges kill the planting, and the treatment goes with it. Agree the shelf gradient and the water level range early, because the planting depth has to survive the drawdown that the surface water drainage strategy sets for the pond.

See also: Wetland (SuDS), Pond (SuDS), Dry swale

A Local Flood Risk Management Strategy is the statutory document in which a Lead Local Flood Authority sets out how it will manage local flood risk from surface water, groundwater and ordinary watercourses.

Read your county's version before designing anything. Discharge rate preferences, infiltration expectations and the local line on adoption often sit in there and not in the planning policy, and the LLFA remit explains who holds the pen.

See also: Lead Local Flood Authority (LLFA), Flood and Coastal Erosion Risk Management (FCERM), Risk Management Authority

A Local Plan is a council's statutory development plan. Policies and site allocations for the area sit in it, and a Strategic Flood Risk Assessment is the evidence that informs where allocations can go and what the plan says about flood risk.

Allocation in an adopted plan does not settle flood risk at application stage. Sites are allocated with caveats attached, and the caveat usually traces back to the SFRA behind the plan, where the council recorded what a developer would have to prove.

See also: Strategic Flood Risk Assessment (SFRA), Flood Risk Assessment (FRA), Regional Flood Risk Appraisal (RFRA)

A Local Planning Authority is the council responsible for determining planning applications and applying flood-risk and drainage policy, and it must consult bodies such as the Environment Agency, LLFAs and internal drainage boards on the applications that engage them.

Officers decide, consultees advise. Worth remembering when an LLFA objection lands: an objection is not a refusal, and a case officer can approve against it or condition around it. The guide to who manages flood risk in England sets out the split.

See also: Highways authority, Environment Agency (EA), Lead Local Flood Authority (LLFA)

A local validation list is the council's published list of documents required to validate a planning application, and because requirements vary between authorities, it determines which flood risk and drainage documents you must submit.

Validation is where a fortnight disappears. Drainage information is the item most often missing, and the usual triggers behind an invalidated application are a missing drainage strategy or an assessment that stops at Flood Zone 1.

See also: Planning validation, Flood Zone 1, Local Plan

Long-term storage is additional storage that controls the extra volume of runoff a development produces during extreme events. Release is at very low rates, close to greenfield. The storage sits alongside attenuation and answers volume rather than peak flow rate.

Miss it and your basin is sized for the peak while the volume keeps arriving. This is the element reviewers query most on tight sites, so we set the split between attenuation and long-term volume while the drainage strategy is still on paper.

See also: Peak flow, Attenuation, Online storage

Made ground is ground consisting of material deposited by human activity, such as fill or waste. Composition varies across a site and with depth. Infiltration and stability both change from one part of a site to the next as a result.

Soakaways on fill are a gamble. Material can drain freely in one trial pit and hold water in the next, so our ground conditions and soakaway testing uses several tests at the depth the structure will actually sit, instead of one convenient hole near the site entrance.

See also: Trial pit, Bedrock, Alluvium

A main river is a watercourse the Environment Agency has designated on the statutory Main River Map for the part it plays in regional drainage. Designation brings regulation with it: work on, under, over or near the channel needs a Flood Risk Activity Permit from the Agency.

Designation is administrative, not a measure of size. A small stream can be main river while a much larger one is ordinary, and the difference between the two decides who regulates your work.

See also: Main River Map, Watercourse, Revetment

The Main River Map is the Environment Agency's statutory map recording which watercourses are designated as main rivers. Designation sets the regulatory and maintenance regime that applies to a channel. Status therefore decides who you apply to before any work in the bed or bank.

Designation is not size. A ditch can be main river while a sizeable brook is not, so the split between main rivers and ordinary watercourses is worth checking on the map before you assume the LLFA is your consenting body and start drafting a consent application.

See also: Main river, Ordinary watercourse, Siltation

In dual drainage design, the minor system is the piped network that handles frequent storms, while the major system is the above-ground route that safely conveys exceedance flows once the pipes are full.

Kerb lines, road crossfalls and plot levels are drainage, whether anyone drew them that way or not. Designing the pipes alone leaves the overland route to chance, so we fix the exceedance path inside the surface water strategy and keep flows off door thresholds.

See also: Exceedance, Hydraulic capacity, Oversized pipe

Major development crosses defined thresholds: 10 or more dwellings, a residential site of 0.5 hectare or more, 1,000 square metres or more of floorspace, or a site of 1 hectare or more. Any one triggers statutory LLFA drainage consultation.

Site area catches people out. A single house on a hectare of paddock is major development, and that pulls in the LLFA as a statutory consultee, with a formal response on drainage and a full strategy expected at application stage, not once conditions are being discharged.

See also: Water-compatible development, Less vulnerable development, Minor development

Managed realignment is a shoreline management policy of deliberately repositioning the coastline landward in a controlled way. Creating saltmarsh habitat is one reason for it. Reducing the cost of holding a defence line is another, and both are judged across a whole frontage.

Landowners hear abandonment. The policy is closer to choosing where the coast will sit in fifty years, and because a shoreline management plan sets that policy stretch by stretch, it is the document to read before promoting land behind an ageing embankment.

See also: Shoreline Management Plan (SMP), Epoch, Hold the line

Managed retreat is a flood and coastal risk strategy that moves people and assets away from areas at high risk. Acquiring and demolishing repeatedly flooded homes is one form of it, chosen instead of defending those homes in place.

Britain has few worked examples. The arithmetic is blunt when one appears: holding a defence for a handful of houses eventually costs more than moving the households, which is the reasoning the Clydach Terrace case made public.

See also: Managed realignment, Fetch, No active intervention

A manhole is an access chamber on a drain or sewer, sized for a person to climb into. From it the pipe network below is inspected, maintained and rodded clear.

The pipes decide where manholes go. Not the site layout. Junctions, changes of direction, changes of gradient and long straight runs each need one, and all of them are scheduled in the surface water drainage strategy.

See also: Cover level, Inspection chamber, Invert level

Manning's n is the roughness coefficient used in open-channel flow calculations to represent the resistance that a channel bed, its banks or the floodplain offer to flow. The value chosen influences modelled water levels.

Roughness is the easiest number to fiddle and the hardest to defend. Reviewers ask where a value came from, so we record the source for every reach of the flood models we build and run a sensitivity test around it.

See also: Unit hydrograph, Afflux, Areal reduction factor (ARF)

A micropool is a small, permanently wet pool at the outlet of a pond or wetland. Holding water there improves settlement of sediment and removal of pollutants before discharge.

Small feature, disproportionate effect on maintenance. Sediment gathers where the water is deepest, so a micropool concentrates silt in one place a machine can actually reach, and we put it where the drainage strategy for the site puts the outfall, not wherever the pond edge happens to fall.

See also: Outfall, Wetland (SuDS), Extended detention basin

Minor development covers smaller works such as development within the curtilage of a dwellinghouse and non-domestic extensions up to 250 square metres, where flood risk is often handled with lighter-touch treatment under standing advice.

Standing advice still sets a bar. A householder extension in Flood Zone 3 needs flood resilient construction and a finished floor level someone has justified, and the Environment Agency's role in planning explains when a consultee response arrives instead.

See also: Environment Agency (EA), Major development, Water-compatible development

A misconnection sends foul flow into a surface water sewer, or surface water into a foul sewer, and connections made the wrong way round like this are a common cause of pollution.

Washing machines and downpipes cause most of them. On new build the risk sits with plot drainage more than with the strategy, and knowing how foul, surface water and combined sewers differ is what stops a fitter teeing a kitchen waste into the nearest pipe in the trench.

See also: Surface water sewer, Foul sewer, Combined sewer

A Model User Report accompanies a flood model and records its build, assumptions, calibration and limitations, so that a reviewer such as the Environment Agency can trust the result and adopt the model.

Skip it and the model is unreviewable, however good the hydraulics are. Agency reviewers work from the report first and the results files second, which is why the EA river modelling technical standards put such weight on documentation.

See also: Calibration, EA river modelling technical standards, Sensitivity testing

The Modified Rational Method estimates peak runoff from small urban catchments. Derived from the rational formula, it is the method behind many pipe sizing calculations on small networks.

Catchment size is the limit. Push it onto a site with storage, a flow control and a long time of concentration and the answer drifts, so use it for a first pass and check the number in one of our drainage calculators before it reaches a submitted design.

See also: Wallingford Procedure, Flow control, Colebrook-White equation

More vulnerable development is the vulnerability category that covers most housing, hospitals, care homes, schools and hotels. Uses in the class need the Exception Test in Flood Zone 3a. In the functional floodplain they are not permitted.

Care homes are the trap. Clients read them as ordinary residential, put them on a site in Flood Zone 3a, then discover late that the scheme has to pass the Exception Test as well as the Sequential Test, because the vulnerability classification decides both and decides them early.

See also: Highly vulnerable development, Less vulnerable development, Water-compatible development

NaFRA2 is the Environment Agency's second National Assessment of Flood Risk, in force since March 2025. It models defended and undefended scenarios and brings smaller watercourses into the mapping. The Flood Map for Planning that applicants and councils read is built on it.

Zones moved when it landed. Sites that sat comfortably in Flood Zone 1 under the old mapping now show a risk band, and arguments now turn on which vintage of data an assessment used, so read what NaFRA2 changed before relying on a 2024 report.

See also: Gauging station, Homebuyer flood risk survey, Insurance flood risk assessment

Natural flood management works with a catchment rather than against it. Leaky dams, new woodland, wetlands and reconnected floodplain all slow and store rainwater upstream, so the flood peak arriving downstream is smaller.

Nobody wins a planning consent on leaky dams alone. NFM shaves peaks and buys time, and it works best beside engineered storage rather than in place of it, a distinction our guide to natural flood management sets out for anyone weighing a catchment scheme against on-site attenuation.

See also: Runoff Attenuation Feature (RAF), Working with Natural Processes, Wetland (SuDS)

Natural Resources Wales is the environmental regulator for Wales and the direct counterpart of the Environment Agency in England. NRW produces the flood mapping, maintains flood defences, and acts as the statutory consultee on flood risk when Welsh planning authorities determine applications.

Cross the border and the rules change. An FRA written to English NPPF wording will get short shrift from NRW, whose flood map products and TAN15 expectations differ from the Environment Agency's, so we build Welsh submissions around the NRW approach to flood risk in planning from the first sketch.

See also: Statutory consultee, Environment Agency (EA), Flood Map for Planning (Wales)

Nature-based solutions use natural systems and processes to manage flood and drought risk, with wider environmental benefits attached. Policy promotes them alongside conventional hard defences rather than in place of them.

Councils like the phrase because it sounds cheap. It rarely is, at least at first: a wetland that also treats runoff and holds carbon needs land, and land is the expensive bit, so the case for sponge-like landscapes that soak up rainfall gets made at catchment scale rather than plot by plot.

See also: Wetland (SuDS), Blue-green infrastructure, CIWEM

No active intervention is one of the shoreline management policy options. Defences are left unmaintained and the coast is allowed to evolve naturally, so the response to erosion and tidal flooding becomes adaptation rather than protection.

Property owners rarely hear this policy until it shows up in a mortgage valuation or a search. Where a frontage is designated for no active intervention, the designation sits in a public document and shapes what a council will support, so read the shoreline management plan before committing to a coastal site.

See also: Shoreline Management Plan (SMP), Tidal (coastal) flooding, Coastal Change Management Area

A non-return valve is fitted into a drainage run to let water leave a property while blocking anything trying to come back. Floodwater and sewage surcharging the sewer are held on the outside of the valve.

Fit one and forget it, and it will fail. Non-return valves silt up, jam part-open and need checking before every flood season, which is why they belong in a maintained property flood resilience package alongside barriers and raised electrics rather than as a single bought fix that nobody revisits.

See also: Property flood resilience (PFR), Flap valve, Flood resistance

Normal depth is the depth a channel settles at under uniform flow, where the water surface slope matches the bed slope. Hydraulic calculations use it as a reference condition against which real, varied flow is compared.

Real channels almost never sit at normal depth. Bridges, weirs and downstream water levels pull the profile away from it, and that gap between the tidy hand calculation and the modelled backwater is the reason a scheme near a structure needs proper hydraulic flood modelling rather than a spreadsheet.

See also: Backwater, Flood modelling, Depth-duration-frequency (DDF)

The Notification Direction is a 2025 provision of TAN15 in Wales. It opens a call-in window for highly vulnerable development proposed on undeveloped land in Flood Zone 3, before the local decision is final.

Welsh schemes that once sailed past on a local decision now pause here. The trigger is narrow, so check early whether the use falls in the highly vulnerable category and whether the land is genuinely undeveloped, both of which our TAN15 guidance sets out.

See also: TAN15, Highly vulnerable development, Defended Zone

The National Planning Policy Framework is the government's overarching planning policy for England. Its flood risk chapter sets out the sequential, risk-based approach to where development goes, the vulnerability classification that decides what may be built where, and the drainage expectations applied to new development.

The chapter itself is short. Almost all the detail sits in the Planning Practice Guidance behind it, so applying the two together is most of what our NPPF flood risk advice covers.

See also: Planning Practice Guidance (PPG), Planning Policy Wales, Environmental Impact Assessment (EIA)

NPPF Annex 3 carries the flood risk vulnerability categories, written in as policy in December 2024. Paired with the flood zones, the categories decide which land uses are appropriate in which zone.

Getting the category wrong is the quickest way to lose a Sequential Test. A care home and the flats above a shop sit in different categories, and a basement bedroom shifts the answer again, so we settle classification against the vulnerability classification guide before the layout is fixed.

See also: Flood risk vulnerability classification, Essential infrastructure, Less vulnerable development

Nutrient neutrality is a planning requirement that a development must add nothing to the nutrient load reaching protected water habitats. Nitrogen or phosphorus from the scheme is calculated and then offset, so the catchment sees no net increase from the new homes or floorspace.

Timescales are the killer here. Offset land or credits can take longer to secure than the rest of the application put together, and a scheme in an affected catchment that leaves nutrient neutrality until validation will sit in limbo for months while the credits are chased.

See also: Water Framework Directive (WFD), Section 106 (Water Industry Act 1991), Storm overflow

Offline storage sits away from the main flow route and stays empty until a threshold or diversion point is reached. Once filled, it releases water back into the system through a control structure as flows drop.

Keeping the basin dry most of the year is the selling point. Dry means usable: amenity, sports pitches, planting that survives and a maintenance regime the management company will actually follow, so we set the diversion level to protect all of that in the surface water drainage strategy.

See also: Surface water drainage strategy, Geocellular storage, Online storage

Ofwat is the economic regulator of the water and sewerage companies in England and Wales. Prices and company performance are its business, so the limits on what a water company charges customers and what it must deliver in return are set by Ofwat rather than by the companies themselves.

Developers meet Ofwat at one remove. Price review settlements shape how much capacity a sewerage undertaker can fund, and as our note on the role of the water authorities in drainage planning sets out, a connection refusal often owes more to an investment cycle than to anything about your site.

See also: Water and sewerage company, Section 106 (Water Industry Act 1991), Storm overflow

Online storage sits in the main flow route, so every flow passing through the system also passes through it. Water fills the volume as levels rise and drains out again as they fall, with no diversion needed.

Simplicity is the trade. Online storage needs no diversion weir and no split flow, and it silts up faster and sits wet for longer, a trade-off that decides whether the pond goes on the main route in a site-wide surface water strategy or off to one side.

See also: Offline storage, Attenuation storage, Geocellular storage

Any river, stream, ditch, drain or culvert not designated a main river. Regulated by the lead local flood authority or an internal drainage board, with works needing Ordinary Watercourse Consent.

Anything that alters flow needs consent, down to a temporary dam. Unlike a main river, an ordinary watercourse is regulated locally, so standards and timescales change from one county to the next.

See also: Watercourse, Ordinary Watercourse Consent (OWC), Main river

Ordinary Watercourse Consent is the written permission required under the Land Drainage Act 1991 before the flow of an ordinary watercourse is obstructed or altered. Consent comes from the lead local flood authority, which holds that power for watercourses outside the main river network.

Two permissions, two timetables. Planning permission does not carry consent with it, and a culverting or outfall proposal that assumes otherwise can find the drainage design unbuildable after committee, so we flag ordinary watercourse consent at strategy stage and build the programme around the determination period.

See also: Ordinary watercourse, Land Drainage Act 1991, Section 24 agreement

Ordnance Datum is the national height reference, set at approximately mean sea level at Newlyn, against which flood levels and finished floor levels are quoted in metres. Quoting levels as metres AOD keeps a survey, a model output and a proposed floor level directly comparable.

Site datum causes more grief than any other survey error we see. A level taken off an arbitrary peg cannot be compared with an Environment Agency flood level, and every modelled level in the Environment Agency's flood risk data for planning is quoted in metres AOD. Tie your topographic survey in first.

See also: Environment Agency (EA), Topographical survey, Flood risk standing advice

An orifice plate is a plate with a fixed opening, set at the outlet of a storage feature so water can only leave as fast as the hole allows and the rest is held back in storage. It is the simplest way to throttle a discharge to an agreed rate.

Size is the whole argument. Too generous and the site discharges above its agreed rate, too mean and the opening becomes a blockage risk in its own right, which is the point at which vortex controls and other proprietary devices take over, and whichever a site ends up with is justified in the surface water drainage strategy.

See also: Surface water drainage strategy, Flow control, Drainage strategy

An outfall is the point where a drain, pipe or channel discharges into a watercourse, a sewer or the sea. Its capacity, and the water level sitting outside it, govern how freely the whole upstream system can drain.

Design the outfall last and you will redesign the scheme. Tidal or fluvial levels can drown it for hours, and the storage upstream then has to hold everything arriving in that window, so we fix outfall and receiving water levels in the drainage strategy for the site before any pipe is sized.

See also: Proper outfall, Headwall, Penstock

An overland flow route is the path surface water takes across the ground once the drainage system runs out of capacity. Designing and protecting those routes sends the excess away from buildings instead of through them.

Kerb lines, driveway falls and a raised threshold decide where water goes when pipes are full. Most schemes draw the routes and then lose them at detailed design, when a landscape architect flattens a level or a plot boundary wall lands across one, so check them against the surface water strategy at every revision.

See also: Vortex flow control (Hydrobrake), Exceedance flow routing, Flow control

An oversized pipe is deliberately larger than conveyance alone requires, so the spare bore holds volume and attenuates flow. The extra diameter buys storage inside the pipe run rather than in a tank or basin.

Cheap volume, expensive maintenance. A large diameter at shallow gradient holds a useful amount, and it also traps silt where nobody wants to go, so adopting authorities push back when the attenuation case in a drainage strategy rests on pipe volume rather than on a basin anyone can inspect.

See also: Attenuation, Drainage strategy, Major and minor drainage systems

Overtopping happens when a flood rises above the crest of a defence and spills over the top, beyond the standard the defence was built to hold. Everything behind the line is then exposed, a headline residual risk.

Defences do not fail politely. Water arrives fast and builds behind the wall with nowhere to drain. Finished floor levels, an escape route and a plan for who moves where still matter on a defended site, all of which our note on residual flood risk spells out in detail.

See also: Finished floor level (FFL), Residual flood risk, Flood wall

A package treatment plant is an off-mains foul system that provides secondary treatment rather than settlement alone. The effluent it produces is cleaner, and that quality is what allows a discharge to a watercourse in places where a septic tank on its own would not be accepted.

Running costs are what catch owners out. A plant needs power, servicing and periodic desludging, and it will breach its permit quietly if any of that slips, so we set out the whole-life obligations in the foul drainage strategy alongside the sizing.

See also: Septic tank, Foul drainage strategy, Foul drainage hierarchy

Peak discharge is the maximum rate of flow passing a point in a river during a flood event, usually quoted in cubic metres per second. That figure marks the top of the flood hydrograph.

One number, many methods. A statistical analysis, a rainfall-runoff model and a gauged record will rarely give the same peak discharge, and the choice between them shapes every water level downstream, so our rundown of flood modelling tools is worth reading before you commit to one.

See also: Hydrograph, Time to peak, Rainfall-runoff model

Peak flow is the highest flow rate reached in a pipe, channel or watercourse during a rainfall or flood event. Cutting the peak flow leaving a site is the whole purpose of attenuation storage.

Volume and peak are different arguments, and reviewers conflate them constantly. You can hold the discharge rate down to greenfield and still send more water downstream over the day, which long-duration storms expose, so we test both in the surface water drainage strategy we prepare before it reaches the LLFA.

See also: Attenuation, Discharge rate, Attenuation storage

The peak rainfall intensity allowance is a climate change uplift applied to rainfall figures, published by management catchment for defined storm probabilities and epochs. Surface water assessment and drainage sizing both run on the uplifted rainfall.

Pick the wrong epoch and the storage volume is wrong by a wide margin. Development lifetime drives the choice, and a residential scheme is assessed over a much longer period than a commercial shed, so we fix the epoch against the climate change allowances used in planning before sizing anything.

See also: Climate change allowance, Peak river flow allowance, Sea level rise allowance

The peak river flow allowance is a percentage uplift added to modelled river flows to cover climate change across a development's lifetime. Values are set by management catchment, and the allowance is applied whenever fluvial flood risk is assessed.

Central allowance or upper end: that choice is where objections start. An upper-end allowance can lift a design flood level enough to change the ground floor use, and the Environment Agency will ask which one you applied, so read the NPPF climate change allowances and justify the choice in writing.

See also: Climate change allowance, Peak rainfall intensity allowance, Sea level rise allowance

A penstock is a vertically sliding plate or gate that closes or partly closes an aperture, and so controls the flow passing through a channel, a culvert or a drainage chamber. Raising or lowering the plate changes the opening, which changes what gets through.

Manual control is the catch. A penstock only works if somebody winds it, so it rarely earns the primary flow control role on an unmanned site, though isolation duty on an outfall or a tank inlet suits it well, and that is how we specify it in the surface water drainage design.

See also: Outfall, Flow control, Soffit level

Percentage runoff is the share of rainfall falling on an area that reaches the drainage system, rather than soaking into the ground or being lost. A high value means most of what lands has to be conveyed or stored.

Assume 100 per cent off roofs and hard paving and you will be close enough. The argument starts on the pervious areas, where soil type, antecedent wetness and slope move the figure a long way, and an optimistic number there quietly undersizes every element of the site's surface water strategy that follows.

See also: Runoff, HOST classification, Design storm

A perched water table is a pocket of saturated ground sitting above the main water table, held up by a layer of low-permeability material beneath it. Water collects on that layer instead of draining down to the regional groundwater.

Trial pits dug in August find nothing. Come February the same pit fills within a day, and a soakaway designed on the summer visit backs up into the garden it was meant to drain, one of the mechanisms behind groundwater flooding and its planning implications.

See also: Water table, Groundwater flooding, Permeability

Percolation is the downward movement of water through soil or other porous material under gravity. A percolation test measures how quickly the ground will accept water, and that rate decides whether infiltration drainage is feasible on a site or whether the water has to go somewhere else.

Timing wrecks more percolation tests than technique. Dig in a dry spell and the ground takes water it will never take in February, so for a foul drainage field we follow BS 6297 infiltration testing, run the test when groundwater sits high, and repeat it if the result looks flattering.

See also: Percolation value (Vp), Drainage field, BS 6297 percolation testing

The percolation value, Vp, is the time in seconds for water to fall a set distance in a percolation test, and it sizes a foul drainage field. An acceptable range runs from roughly 15 to 100 seconds per millimetre.

Below 15 the ground drains too fast for the soil to treat anything, and above 100 the field waterlogs. Either end kills an off-mains scheme outright. Run the BS 6297 test procedure before the layout is drawn, because the result can move the house.

See also: Percolation, BS 6297 percolation testing, Drainage field

Permeability measures how easily water moves through a soil or rock. High permeability favours infiltration drainage, so soakaways can take the runoff, while low permeability usually forces a piped or stored solution instead. Ground conditions decide which of the two applies.

Soil maps flatter the ground. Perched water, a shallow water table or a thin permeable layer sitting over rock will all beat a textbook description, so we test on site and report what the ground conditions and soakaway testing actually show. Clay can still work.

See also: Glacial till, Soakaway, Porosity

Permeable paving is a hard surface built to let rainfall pass through it into a stone sub-base and the ground below, instead of shedding that water across the surface and away as runoff. Used as a SuDS source-control feature, it deals with rain where it falls.

The sub-base holds water. That makes permeable paving storage as much as surface, and treating it as a finish is how the maintenance obligations that come with it get quietly missed, which is what our guide to permeable paving in planning applications covers what councils expect to see.

See also: Sub-base, Block paving, Source control

Permitted development is development allowed without a full planning application, often through a prior approval route rather than a free pass. Flood risk survives the shortcut: in the higher flood zones a flood risk assessment can still be required before prior approval is granted.

Office to residential is where this bites hardest. A change of use lifts the vulnerability classification, and a building that passed as offices in Flood Zone 3 looks very different once people sleep in it. That is the question an FRA for an office to residential conversion has to answer.

See also: Prior approval, Class MA, Change of use

A piezometer is an instrument installed in the ground to measure groundwater pressure, and from that the level of the water table, at one specific depth rather than across the whole soil profile. What comes back is a direct reading of groundwater at that depth, taken from below ground.

One reading tells you almost nothing. You need a record long enough to catch a seasonal high, which is why groundwater monitoring requirements hold up more planning applications than most people expect.

See also: Groundwater monitoring, Water table, Standpipe

The Pitt Review is Sir Michael Pitt's 2008 inquiry into the 2007 floods. Its 92 recommendations led to the Flood and Water Management Act 2010 and to the creation of lead local flood authorities.

Every drainage condition on a decision notice traces back to 2007. The Act that followed made county and unitary councils lead local flood authorities, so your surface water details land with them rather than the Environment Agency. Our look at the Pitt Review's legacy and gaps weighs what still has not been done.

See also: Flood and Water Management Act 2010, Floodline, Flood warning system

A planning condition is a requirement attached to a planning permission that must be complied with. Drainage details are commonly reserved by condition, to be submitted and approved before construction starts or as the build proceeds.

Approval is not the finish line. A pre-commencement drainage condition can take weeks to discharge, and breaking ground before it is signed off puts the permission itself at risk. Our note on the discharge of drainage conditions sets out what councils actually ask for.

See also: Pre-commencement condition, Deemed discharge, Discharge of conditions

Planning Policy Wales is the Welsh Government's national planning policy document, and TAN15 sits underneath it as the technical advice on development and flood risk. Flood risk decisions in Wales are made against that framework.

Quoting the NPPF at a Welsh planning officer wastes everyone's morning. The tests, the zone names and the vulnerability categories all differ, and the technical detail sits in TAN15, which our advice on applying TAN15 works through. Learn the Welsh vocabulary before you write a word.

See also: TAN15, NPPF (National Planning Policy Framework), Flood Map for Planning (Wales)

Planning Practice Guidance (PPG) is the government's detailed online guidance sitting behind the NPPF. Its Flood Risk and Coastal Change section explains how national flood policy is applied in practice, and it is the material a case officer reaches for whenever the NPPF wording alone leaves a judgement open.

Officers quote it constantly. The NPPF gives you the principle and the PPG gives you the method, which is why an assessment that cites policy without the matching NPPF and PPG advice on flood risk usually comes back with questions attached.

See also: NPPF (National Planning Policy Framework), Greenfield land, Allocated site

Planning validation is the administrative check that an application contains every required document before the council registers it. A missing flood risk assessment, drainage strategy or proforma is enough to cause invalidation.

Nobody assesses your scheme until it passes. Weeks disappear at this stage, and we have watched perfectly good schemes sit unregistered over a blank drainage proforma, which is how an application gets invalidated on flood risk and drainage.

See also: Local validation list, Flood Risk Assessment (FRA), Flood Map for Planning

Pluvial flooding, more often called surface water flooding, occurs when intense rainfall overwhelms both the ground's ability to absorb it and the capacity of the drainage system, before any of that water reaches a watercourse. It is now the most widespread flood source in England.

Flood Zone 1 tells you nothing about it. A site can sit miles from the nearest river and still take water off the road outside, so the separate sources of flood risk each need their own answer in an assessment. Lumping them together is the commonest failure we correct.

See also: Surface water flooding, Sewer flooding, Flash flooding

A pollution pathway is the route a pollutant takes from its source to a receptor such as a river or an aquifer. The source-pathway-receptor model is used to identify those links and, where possible, break them.

Break the pathway and the receptor is safe, whatever the source keeps doing. That logic sits under the treatment train, the water quality strand of the four pillars of SuDS, and it explains why a single interceptor bolted on at the outfall rarely satisfies an LLFA.

See also: Treatment train, Four pillars of SuDS, Diffuse pollution

A SuDS pond holds a permanent body of water, stores runoff above it and attenuates flows, while treating that water as sediment settles out and plants and micro-organisms take up pollutants.

Depth is the argument. A permanent water body in public open space brings safety edges, marginal shelves and a named maintenance funder into the conversation early, so we fix the pond's role inside the surface water drainage strategy before anyone draws it as a landscape feature.

See also: Littoral zone (SuDS), Retention pond, Balancing pond

Porosity is the proportion of a soil or rock's volume made up of voids capable of holding water. It governs how much water a granular sub-base or an aquifer can store.

Storage is not the same as drainage. A clay can be highly porous and still refuse to let water through, so the figure that decides a soakaway is the infiltration rate measured on site. We test ground conditions for soakaway design instead of reading numbers off a table.

See also: Infiltration rate, Aquifer, Alluvium

Positive drainage moves water away actively through a pipe or channel, under gravity or by pumping, rather than relying on infiltration into the ground beneath. The route and its outfall are chosen by design, not left to the site.

Where infiltration testing fails, positive drainage is what remains, and the discussion turns immediately to who owns the receiving pipe and what rate they will accept. All of that belongs in the site's surface water drainage strategy. Consent first, layout second.

See also: Surface water drainage strategy, Outfall, Drainage strategy

A pre-commencement condition must be discharged before any development starts, and drainage details are among the commonest subjects for one. It cannot be imposed without the applicant's written agreement.

Agree to one casually and you have handed your programme to the LLFA. Discharge takes weeks, longer where the details were never properly designed, so the discharge of drainage conditions deserves a line in the build programme from the day consent lands.

See also: Planning condition, Section 73 variation, Deemed discharge

Preliminary drainage design is the high-level but site-specific strategy expected at outline application stage. It sets out the disposal method, whether infiltration is feasible, and indicative discharge rates and destinations.

Outline does not mean vague. A layout that cannot physically drain gets found out at reserved matters, when the storage has nowhere left to sit, so the drainage information needed for planning permission should be settled while the masterplan can still move.

See also: Detailed drainage design, Discharge rate, Floodplain compensation

A Preliminary Flood Risk Assessment (PFRA) is a high-level review of past floods and the potential consequences of future ones, prepared to identify areas of significant flood risk across an authority's area.

Do not confuse it with the site-specific assessment your application needs. A PFRA is strategic screening, produced at authority level, from the same family as what a Strategic Flood Risk Assessment does, and neither answers the questions an officer will put to your red line.

See also: Strategic Flood Risk Assessment (SFRA), Flood Risk Assessment (FRA), Level 1 SFRA

Previously developed land, usually called brownfield, is land that is or was occupied by a permanent structure together with its associated infrastructure. Certain agricultural and natural land uses fall outside the definition.

Brownfield status wins no argument on flood risk. A previously developed site in Flood Zone 3 still faces the Sequential Test, and the NPPF and PPG position on flood risk is where that gets settled. Old hardstanding raises a second question: what did the site drain like before.

See also: Greenfield land, Sequential Test, Environmental Impact Assessment (EIA)

A principal aquifer is a body of rock, typically chalk, limestone or sandstone, that stores and transmits significant volumes of groundwater. Sites sitting above one are more prone to groundwater flooding.

Winter is when it shows. Levels in a chalk aquifer keep rising for months after the rain that fed them, so a trial pit dug in August tells you very little, and groundwater flooding reaches the basement long after the storm has gone.

See also: Chalk aquifer, Secondary aquifer, Confined aquifer

Prior approval is a limited form of consent attached to certain permitted development rights. Instead of judging the principle of development, the council assesses only specified matters, flooding among them, and must reach its decision inside a set determination period.

The clock is the whole point. With that determination period running, an office to residential conversion submitted without flood risk evidence can be refused on that ground alone, so we front-load the flood risk assessment for an office to residential scheme rather than answering a holding query later.

See also: Permitted development, Class MA, Class Q

The Probable Maximum Flood (PMF) is the largest flood considered physically possible for a catchment, derived from probable maximum precipitation. Reservoir safety assessment is where it belongs, not everyday development control.

Nobody sizes a housing estate against it. The PMF is the territory of dam engineers and inundation mapping, though the same hydraulic machinery sits behind it as behind the flood modelling supporting a site-specific assessment. Scale differs. Method does not.

See also: Flood modelling, Catchment, Depression storage

A proper outfall is a legally acceptable point of discharge: a watercourse, a public sewer or, in some cases, an adopted highway drain. Without one, a drainage design has no lawful destination for its water.

Piping it onto a neighbour's field is not one. Highway drains are the usual trap, because the highway authority can refuse and frequently does, and we pin down the destination and its consent before any storage volume is set out in the surface water drainage strategy.

See also: Outfall, Surface water drainage strategy, Sump

Property flood resilience (PFR) covers measures fitted to an individual property, split into resistance, which keeps water out, and recoverability, which limits damage and speeds recovery once water gets in.

Barriers on their own are half an answer. Water finds air bricks, service ducts and the slab, and once a house has flooded it is the tiled floor and the raised sockets that decide how many months the family spends elsewhere, so property flood resilience gets specified as a pair.

See also: CIRIA Code of Practice (C790), Demountable flood barriers, Flood recoverability

Pumped drainage lifts flows with pumps where gravity discharge is not possible, typically from low-lying ground or basement areas. Power supply, storage against pump failure and long-term maintenance all become part of the design.

Adoption is where pumped schemes stall. Water companies apply their own criteria and a private pump means a management company for the life of the development, so gravity is worth chasing hard before anything gets committed to in the drainage strategy for surface water.

See also: Drainage strategy, Positive drainage, QBar

A pumping station is a structure housing pumps that lift water or drainage flows to a higher level where gravity discharge is not possible. Low-lying and defended land depends on them entirely: behind a flood bank, drained land has nowhere for its water to go without one.

Whole districts run on this kit. In fenland and other board areas the pumps are operated and charged for by the drainage board, and a development adding flow to that system will certainly hear about it, one of several reasons internal drainage boards matter for planning.

See also: Internal Drainage Board (IDB), Land drainage consent, Risk Management Authority

QBar is the mean annual flood flow, roughly a 1-in-2.3-year return period, used as a reference greenfield runoff rate. It is not interchangeable with the 1-in-1-year rate.

Swap the two and your storage volume moves. Some LLFAs ask for QBar, others for the 1-in-1-year, and that difference can decide whether a tank fits under the car park, so we state plainly which rate the surface water drainage strategy has been built on.

See also: Greenfield runoff rate, Runoff, Pumped drainage

QMED is the median annual maximum flood. In Flood Estimation Handbook terms it is the index flood, and it corresponds broadly to a flow with a 2-year return period.

Everything rarer is built on it. The growth curve is applied to QMED to reach the 1 in 100 and beyond, so an error in the index flood propagates through every result that follows, which is why flood modelling starts at the index flood rather than at the design event.

See also: Growth curve, Flood Estimation Handbook (FEH), Flood Studies Report (FSR)

A rain garden is a shallow, planted depression that collects rainwater runoff and filters it through soil and vegetation. Bioretention at this small scale is one of the few drainage components able to deliver quantity, quality, amenity and biodiversity benefits all at once.

Kerb inlets block. That single detail turns a well designed rain garden into a puddle on the highway, and it is a maintenance question rather than a design one, so the four pillars SuDS schemes are judged against put amenity and upkeep alongside the hydraulics.

See also: Bioretention, Amenity, First flush

A rainfall profile describes how intensity is distributed across a design storm, the summer and winter profiles being the usual pair. That distribution changes the peak runoff a model produces from the same total depth.

Summer profiles are peaky, winter profiles longer and wetter, and the two stress a system differently: one finds the pipe, the other finds the tank. Both should appear in the calculations behind a surface water drainage strategy.

See also: Design storm, Runoff, Percentage runoff

A rainfall-runoff model converts a rainfall input into a runoff hydrograph by accounting for losses, storage and the way a particular catchment responds. Its output is the flow shape everything downstream gets designed against.

Rubbish in, confident-looking rubbish out. Catchment descriptors and loss assumptions drive these models, and a reviewer will go straight to them, so every input behind the flood modelling gets documented rather than a hydrograph being exported and forgotten.

See also: Catchment descriptors, Flood modelling, Hydrograph

Rainwater harvesting collects rain, usually off roofs, and stores it for non-potable reuse such as irrigation or flushing. In the SuDS discharge hierarchy it takes the highest priority, ahead of infiltration and discharge to a watercourse, provided a genuine demand for the water exists on site.

Demand is the entire test. A tank sitting full because nobody draws on it gives you no attenuation when the storm arrives, so check how the SuDS hierarchy treats reuse before claiming storage credit for it. That calculation is rarely done honestly.

See also: Discharge hierarchy, SuDS hierarchy, SuDS management train

A rapid response catchment is steep and flashy, so dangerous flooding can arrive very soon after heavy rainfall. Warning time is short, sometimes far too short for anyone to be moved to safety.

Steepness beats depth here. Where the lag between rain falling and the peak arriving is measured in minutes, a safe access route that only works on paper is worthless, and better surface water flood forecasting for planning counts for more in these places than on a slow lowland river.

See also: Flood forecasting, Catchment, Flood Forecasting Centre

A rating curve links water level, or stage, to discharge at a river cross-section, so that measured levels can be converted into flow estimates. Every flow record from a gauging station depends on that relationship holding.

Extrapolation is where it turns shaky. Big floods sit above anything ever gauged, leaving the top of the curve as an estimate rather than a measurement, and that uncertainty travels straight into the flood modelling built on the gauge.

See also: Stage, Flood modelling, Growth curve

Reasonably available sites are alternatives in a suitable location that could meet the same development need within the same timeframe. The Sequential Test requires an applicant to demonstrate that none exists at lower flood risk.

Search area is the fight. Define it as your own land holding and the council rejects the exercise, define it as the whole district and you may lose the site, so the geography of a Sequential and Exception Test gets agreed with the officer first.

See also: Exception Test, Sequential Test, Call for sites

A receptor, in the source-pathway-receptor model, is the person, property, infrastructure or habitat that flooding may harm. Where no receptor is exposed there is a hazard but no meaningful risk.

Two sites with identical flood depths can carry wildly different risk, because a warehouse yard and a ground-floor flat are not the same receptor. That distinction runs through the Environment Agency river modelling standards, and it decides which model outputs actually matter.

See also: Source-Pathway-Receptor, Calibration, EA river modelling technical standards

Recorded flood extents are a mapping layer showing areas known to have flooded historically. The layer works as contextual evidence alongside modelled flood zones, never as a replacement for them.

Absence of a record is not absence of risk. Historic extents rely on someone having surveyed the water at the time, which is patchy at best, so on the Welsh flood map for planning we read the recorded layer as a sense check against the modelling.

See also: Flood Map for Planning (Wales), NaFRA2, Ordnance Datum (AOD)

A reed bed is a constructed bed of marsh plants used to treat runoff or wastewater. Treatment happens two ways: particles are trapped as flow slows through stems and substrate, and nutrients and metals are taken up by the plants growing there.

Land take is the catch. A reed bed needs area, and it needs enough water through it to keep the plants alive, so it earns its place only where treatment is genuinely required under the four pillars of SuDS design.

See also: Four pillars of SuDS, Pollution pathway, Simple Index Approach

ReFH2 is version 2 of the Revitalised Flood Hydrograph model, a rainfall-runoff method used with FEH data to generate design flood hydrographs for a catchment. It gives a peak flow and the shape of the event around it.

Version matters. Numbers from ReFH2 and from the older ReFH do not agree, and a reviewer will ask which produced yours, along with the urban and seasonal settings behind them, before accepting the flood modelling that follows.

See also: Flood modelling, Hydrograph, Joint probability

A Regional Flood Risk Appraisal (RFRA) is a strategic, regional-scale assessment of flood risk, produced historically to inform higher-level spatial planning. It sat above the more detailed Strategic Flood Risk Assessment in the evidence chain.

Regional planning went, and the appraisals largely went with it. You may still meet one buried in an older evidence base, though the document your council will actually lean on is its own Strategic Flood Risk Assessment.

See also: Strategic Flood Risk Assessment (SFRA), Flood Risk Assessment (FRA), Level 2 SFRA

Reservoir flooding arises from the failure or overtopping of a reservoir. Probability is very low, because reservoir safety is strictly regulated, yet potential extents are mapped nationally since the consequences can be severe.

Depth and speed are the problem, not likelihood. A reservoir inundation extent rarely stops a scheme by itself, though it shapes emergency planning and safe access, and it belongs with the other sources of flood risk a site-specific assessment has to screen.

See also: Fluvial (river) flooding, Infrastructure flooding, Pluvial (surface water) flooding

Residual flood risk is the risk left once defences, drainage and mitigation have done everything they can. It comes from events exceeding the design standard, from a defence breach, from assets deteriorating, and from sources nobody mitigated.

Defended does not mean safe. The bank in front of your site has a design standard and a condition, and both will be questioned, so we show how residual flood risk is managed through finished levels, escape routes and a flood plan instead of calling the site protected.

See also: Breach, Flood hazard rating, Hard engineering

A retention pond keeps a permanent pool of water between storms and holds runoff long enough for pollutants to settle out and be broken down biologically before it moves on.

The permanent pool is the point. Drain it dry and you lose the treatment, which is why the wet volume and the attenuation volume get counted separately in the surface water drainage strategy rather than merged into a single headline figure.

See also: Pond (SuDS), Balancing pond, Forebay

A revetment is a sloping facing of stone, concrete or other material laid against a bank or shoreline so that flowing water or wave attack works on the facing instead of the erodible material behind it.

Before you specify one, find out who controls the channel. Consent turns on whether the watercourse is a main river or an ordinary watercourse, a split covered in our guide to main rivers and ordinary watercourses. Get that wrong and the design stalls.

See also: Main river, Ordinary watercourse, Sluice gate

The right to connect is the near-absolute statutory entitlement, under section 106 of the Water Industry Act 1991, to connect drainage to a public sewer. Outright refusal is rare.

Plenty of applicants read that right as a blank cheque. The undertaker still fixes the point and manner of connection, and building control still has to be satisfied, so we deal with the right to connect alongside building control and planning permission rather than in isolation.

See also: Build-over agreement, Building Regulations Part H, SuDS adoption

A rill is a narrow open channel with hard edges, sometimes planted, that carries surface water between SuDS features and gives it a small amount of treatment on the way.

Conveyance features earn their keep twice over. A rill moves water visibly, which helps residents understand the drainage they live with, but the flow it takes is small, so we set it against pipe and swale options inside the wider surface water drainage strategy.

See also: Swale, Surface water drainage strategy, Geomembrane

A landowner whose property adjoins, contains or lies above a watercourse, presumed to own the bed and banks as far as the centre line, and carrying duties to keep the channel clear and not increase flood risk to others.

The duties come with the land whether or not anyone mentioned them at purchase. Most owners discover their riparian rights and responsibilities at the point they try to sell.

See also: Watercourse, Riparian planting, Flood relief channel

Riparian planting puts trees and buffer strips along riverbanks to slow runoff, hold the banks together and cut the sediment washed into the channel. Flood risk downstream falls with it.

Planting is slow. A decade on, a well established buffer strip does real work on peak flows and bank stability, and most of the catchment schemes in our piece on natural flood management lean on exactly that. Expect no credit for it in year one.

See also: Natural flood management (NFM), Riparian owner, Sediment pond

Risk Management Authorities are the bodies carrying statutory flood and coastal risk duties: the Environment Agency, lead local flood authorities, district councils, internal drainage boards, water companies and highway authorities.

Six categories, overlapping duties, and no single point of contact. The one that catches people out is the internal drainage board, because its area ignores council boundaries and its consent sits on top of everything else. Our note on internal drainage boards as risk management authorities explains where that leaves an applicant.

See also: Internal Drainage Board (IDB), Environment Agency (EA), Land drainage consent

Runoff is rainfall that travels across the ground surface instead of soaking into it, and it ends up in drains or watercourses. Development increases it. Impermeable surfaces are the reason: every roof, road and parking bay replaces ground that once absorbed water, so a greater share of each storm reaches the outfall.

Greenfield and brownfield sites start from different baselines, and that baseline decides how much storage you have to find. Get the impermeable area schedule wrong at the outset and every figure downstream of it is wrong too, which is why the runoff calculations in a surface water drainage strategy begin with a measured plan.

See also: Greenfield runoff volume, Greenfield runoff rate, QBar

A Runoff Attenuation Feature, or RAF, is a small engineered landscape feature that slows, stores and filters rural runoff. It is one building block of natural flood management.

Scale is the catch. One feature on its own changes very little, so RAFs are designed in numbers across a catchment, an approach set out in our overview of working with natural processes. Landowner agreement usually decides where they end up.

See also: Natural flood management (NFM), Working with Natural Processes, Sediment pond

A runoff coefficient sets out what proportion of rainfall falling on an area becomes surface runoff, from close to zero on permeable ground to close to one on roofs and paving, and it feeds directly into drainage calculations.

Assume every hard surface sheds all of its rainfall and you will oversize storage; assume too little and you undersize it. Reviewers check the value against the surface you claimed. The drainage calculators on our site work to the same convention, so a figure you check there matches what a reviewer expects.

See also: Runoff, Modified Rational Method, Colebrook-White equation

SAAR is Standard-period Average Annual Rainfall, a catchment descriptor giving the mean annual rainfall over a fixed standard reference period, and it is used as an input throughout flood estimation.

It is a rough measure of how wet a catchment usually is. That makes it an input to a surprising number of rainfall-runoff equations, and it turns up in the audit trail of most flood modelling work.

See also: Flood modelling, Catchment, Manning's n (roughness coefficient)

Safe access and egress means people can get into and out of a site safely while it floods, judged on the depth and velocity along escape routes. It is often the constraint that decides a flood risk case.

Dry pedestrian routes are the gold standard and rarely available. Where a route floods, the case turns on how deep and how fast, and on whether anyone would realistically use it at night, which is the sort of judgement we work through when we assess residual flood risk and safe access.

See also: Residual flood risk, Flood risk asset, Levee

Schedule 3 of the Flood and Water Management Act 2010 would make SuDS subject to approval by a SuDS Approving Body before construction. Wales commenced it. England never switched it on.

Two countries, two regimes, and one very common assumption that the English position will change soon. Until it does, SuDS approval in England rides on planning conditions rather than a separate consent, and the practical detail sits in the 2025 national standards for sustainable drainage.

See also: SuDS Approving Body (SAB), Flood and Water Management Act 2010, 2025 National Standards for Sustainable Drainage Systems

Scour is the erosion of a riverbed or bank by flowing water. It concentrates where the flow is squeezed or accelerated, so bridge piers and outfalls are the usual places to find it.

Scour damage shows up below the water line, long after the storm that caused it. A new outfall on a bank needs erosion protection and consent from whoever holds the channel, a distinction our explainer on main river and ordinary watercourse designations untangles.

See also: Main river, Ordinary watercourse, Gabion

Sea level rise is the long-term increase in mean sea level driven by climate change. Every centimetre eats into the safety margin that coastal defences were built with, which is why planning does not treat today's sea level as fixed: climate change allowances build the rise into the assessment from the start.

Design life is where this bites. A house consented today will very likely still be standing after 2100, beyond the horizon most assessments bother with, and that gap is what we take on in planning for sea level rise beyond 2100.

See also: Tidal locking, Wave overtopping, Fetch

A sea level rise allowance is the climate change figure, set by river basin district and epoch, added when assessing tidal and coastal flood risk over a development's lifetime.

Choose the epoch from the development lifetime rather than the current decade. Residential schemes run far longer than most commercial ones, so they reach into a later epoch and a larger figure, and the mapping between the two is set out in our guide to NPPF climate change allowances.

See also: Climate change allowance, Peak rainfall intensity allowance, Peak river flow allowance

A sea wall is a coastal defence built along the shoreline to protect the land behind it from wave action and tidal flooding. Crest height sets the standard of protection. Waves that overtop it, or a beach that drops away in front of it, both reduce what the wall can do.

Ownership questions arrive with the first crack. Maintenance of a private frontage wall falls to the frontager, and when a defence is not maintained or not replaced, the losses land on the property behind it, which is the ground our article on compensation for coastal erosion in the UK covers.

See also: Coastal erosion, Standard of protection, Flood wall

A secondary aquifer is a permeable stratum able to support local water supplies or contribute to river baseflow. The Environment Agency classifies it below principal aquifers in its groundwater designations.

Classification governs what you may discharge, and says nothing about how wet the ground is. A soakaway over a secondary aquifer can still be acceptable with treatment upstream of it, while the same ground can flood cellars in a wet winter, a mechanism explained in our piece on groundwater flooding and its planning implications.

See also: Principal aquifer, Groundwater flooding, Chalk aquifer

A section 104 agreement under the Water Industry Act 1991 is the route by which a new sewer, built to the water company's standards, is adopted as a public sewer. Maintenance passes with it.

Agree it early or build it twice. Design approval comes before construction, the water company inspects as work proceeds, and a bond sits behind the whole thing until adoption completes. We put section 104 in context alongside the difference between foul, surface water and combined sewers.

See also: Combined sewer, Lateral drain, Build-over agreement

Section 106 of the Water Industry Act 1991 gives a developer the right to connect foul drainage to a public sewer. The right itself is hard to refuse. What the water company controls is the point and manner of connection, and those two decisions can shift a drainage layout a long way.

Surface water is the trap. The section 106 right is often assumed to cover it, and water companies increasingly refuse surface water connections where a SuDS solution is available, a stance we look at in our note on how water authorities handle drainage in planning.

See also: Storm overflow, Ofwat, Water and sewerage company

A section 19 flood investigation is the duty placed on lead local flood authorities by the Flood and Water Management Act 2010 to look into significant flooding, then publish a record naming which authorities hold relevant responsibilities.

Published reports are free evidence. If your site sits near a road or estate that has flooded, the LLFA's section 19 record may already describe the mechanism and the responsible body, which saves guesswork. We set the duty in its wider frame in our guide to the Flood and Water Management Act 2010.

See also: Flood and Water Management Act 2010, Section 24 agreement, Natural Resources Wales (NRW)

A section 24 agreement, under the Land Drainage Act 1991, lets drainage responsibilities move or be varied between authorities, so a duty held by one body can pass to another by agreement rather than by legislation.

You will meet this one rarely, and when you do it usually explains why the body you expected to consult is the wrong one. Check who holds the watercourse before you apply, because consent and enforcement follow the agreement, as our explainer on ordinary watercourse consent sets out.

See also: Ordinary Watercourse Consent (OWC), Land Drainage Act 1991, Section 19 flood investigation

A section 73 variation is an application under the Town and Country Planning Act 1990 to change or remove a planning condition without applying for a fresh permission. The original consent stays.

Drainage conditions are a common target. Where an approved scheme cannot be built as drawn, a section 73 is often cleaner than a fresh application, though the LLFA gets to comment again and can ask for more, which is why we treat it as part of the same workflow as discharging drainage conditions.

See also: Planning condition, Discharge of conditions, Deemed discharge

A sediment pond is a small retention feature that slows runoff and gives suspended solids time to settle before the water reaches a watercourse. Flood control, water quality and biodiversity all benefit.

Silt has to go somewhere, and a pond that traps it needs emptying. Write the desilting interval into the maintenance plan at design stage, because an unmanaged sediment pond loses its volume quietly, and the wider catchment thinking behind these features is covered in our article on using nature to reduce flood risk.

See also: Suspended solids, Runoff Attenuation Feature (RAF), Natural flood management (NFM)

Sensitivity testing re-runs a flood model with inputs varied, roughness or inflows for example, to show how far the results move. A conclusion that holds across the range is a conclusion you can rely on.

Reviewers ask for it when the model is doing the heavy lifting in a case. Two or three runs with a defensible range beat a single best estimate, and expectations are tightening, as our summary of the Environment Agency river modelling standards shows.

See also: Model User Report (MUR), Calibration, EA river modelling technical standards

A separate sewer carries one flow only, either surface water or foul, and never the two together. A combined sewer does the opposite. Which one you are connecting to changes the drainage design.

Misconnections are the everyday failure here. A foul pipe teed into the surface water sewer sends sewage to a watercourse, and the fix falls on whoever owns the pipe, which is one reason we spell out the three sewer types and what each carries before design starts.

See also: Surface water sewer, Combined sewer, Misconnection

A septic tank is an off-mains foul drainage system that provides primary settlement only, separating solids from the liquid passing through. The effluent must then go to a drainage field for secondary treatment in the soil, and it may never be discharged straight to a watercourse.

Because the tank only settles, the drainage field does the real work. Proving the ground will take that discharge is a core part of the foul drainage strategy.

See also: Foul drainage strategy, Cesspool, FDA1 Foul Drainage Assessment

The Sequential Test is the planning test that steers development towards land at the lowest probability of flooding. An applicant has to show that no other reasonably available site at lower flood risk could take the proposed development before permission is given on a site at higher risk.

It is won or lost on the search area. Draw it too narrowly and the council rejects it, too widely and you cannot evidence it, and striking that balance is most of the work in a Sequential Test.

See also: Exception Test, Area of search, Reasonably available sites

Sewer adoption transfers responsibility for maintaining a sewer or drainage system to the sewerage undertaker, and it happens only where the system meets the adoption standards. Fall short and maintenance stays private.

Adoption decides who pays in twenty years, so it belongs in the viability conversation early. A private system needs a management company, a funded maintenance plan and a condition in the permission, and the sequencing of all that against building control is set out in our article on drainage approvals, building control and planning.

See also: Adoptable drainage, Build-over agreement, SuDS adoption

Sewer flooding happens when sewers are overwhelmed, blocked or surcharged and wastewater or stormwater is forced back out through drains, gullies and manholes. The water that reappears at street level came from the pipe.

Blame lands in three places at once: the water company for capacity, the highway authority for gullies, and the householder for the private lateral. Sorting which mechanism actually operated is the first job, and we set sewer flooding beside the other sources in our guide to the types of flooding.

See also: Reservoir flooding, Flash flooding, Fluvial (river) flooding

A Shoreline Management Plan sets long-term coastal management policy without statutory force. Every stretch of coast is placed in one of four categories: hold the line, advance the line, managed realignment or no active intervention.

Non-statutory does not mean ignorable. An SMP policy of no active intervention will shape what a council grants near that frontage, and it feeds straight into the sequential thinking on a coastal site, which is why our overview of shoreline management plans starts with the policy units.

See also: No active intervention, Hold the line, Managed realignment

Siltation is the gradual filling of a channel or storage feature with suspended silt and sediment, and the capacity it takes away is capacity the design assumed would be there.

Nobody surveys a ditch until it fails. A silted watercourse downstream of your outfall raises levels at the point you are discharging to, and whether anyone has a duty to clear it depends on the designation, which our piece on watercourse designation and riparian duties covers.

See also: Watercourse, Tributary, Confluence

The Simple Index Approach is the CIRIA SuDS Manual method for demonstrating water quality: score the pollution hazard from the site's land use, then match it against the treatment capability of the proposed SuDS features.

Water quality is the pillar most schemes treat last and get queried on first. One filter component rarely scores enough for a busy car park, so plan two treatment stages in series, as our piece on the four pillars of SuDS works through.

See also: Four pillars of SuDS, CIRIA SuDS Manual (C753), Reed bed

A sluice gate is an adjustable gate set into a structure in a channel or watercourse, raised or lowered to control either the rate of flow passing through the opening or the level of water held behind it, so one structure can do both jobs at different times.

A weir is fixed. A sluice moves. Either way you are putting a structure into a channel, so the consent you need depends on whether you are dealing with a main river or an ordinary watercourse.

See also: Main river, Ordinary watercourse, Dredging

An underground structure or filled pit that takes surface water and lets it infiltrate into the surrounding ground, sized from BRE Digest 365 test results, and liable to fail if it silts up or the ground around it saturates.

Two things have to be true: the ground drains, and the water table stays clear of the base. Infiltration testing settles both before a design is committed to.

See also: Infiltration testing, Infiltration, BRE Digest 365

Soffit level is the level of the crown of a pipe or culvert, the highest point of the internal bore and the level at which the pipe is full.

Confuse soffit with invert and your cover depth is out by the pipe diameter. Freeboard to a culvert soffit also decides when the structure starts running under pressure, so the level appears on every long section in a surface water drainage strategy.

See also: Surface water drainage strategy, Catchpit, Gully

Soft engineering manages flood and coastal risk by working with natural processes and natural materials rather than with built structures, so the approach shapes how a system behaves instead of holding it in a fixed line.

Hard and soft are rarely rival camps in practice. Most English schemes we see mix the two, with a natural approach upstream to buy time and a structure at the pinch point, and the upstream half of that is the subject of our article on natural flood management techniques.

See also: Working with Natural Processes, Natural flood management (NFM), Hard engineering

Soil Moisture Deficit measures how dry the soil is, and with it how much rainfall the ground can take in before runoff starts. A high deficit soaks up the first part of a storm.

Antecedent conditions decide whether a storm becomes a flood. Two identical rainfall events, one in August and one in February, produce very different hydrographs, so the starting soil condition is an input we state openly in our flood modelling.

See also: Flood modelling, Antecedent conditions, Hydrograph

Source control manages rainfall at or very close to the point where it lands, using green roofs, permeable paving or rain gardens before flows can gather and run off. It is the first stage of the SuDS management train.

Skip this stage and everything downstream grows. Storage volume, pipe sizes and the size of the tank at the bottom of the site all follow from how much you dealt with at the surface, which is the argument behind the SuDS hierarchy. Reviewers notice when the first stage is missing.

See also: SuDS management train, SuDS hierarchy, Downpipe disconnection

Source Protection Zones are mapped by the Environment Agency around groundwater abstractions such as drinking-water sources, and inside them drainage to ground and other polluting activities are restricted to protect the aquifer.

Check the zone before you design a soakaway, an infiltration basin or a septic tank drainage field. In the inner zone a discharge to ground is usually refused outright under the general binding rules for small sewage discharges. A permit application follows, and months go onto the programme.

See also: General Binding Rules, Aquifer, Unsaturated zone

Source-Pathway-Receptor is the framework behind flood and pollution risk assessment. It traces the source of the water or contaminant, the pathway it travels along, and the receptors it reaches: people, property and habitats.

Break the chain anywhere and the risk falls. That is why a scheme can sometimes pass on finished floor levels alone, and why the Environment Agency's 2026 river modelling standards push modellers to state the pathway explicitly instead of leaving it buried in a results table where nobody reads it.

See also: Receptor, Pollution pathway, Model User Report (MUR)

A spring line is the line of ground along which groundwater emerges at the surface, forming where a permeable geological layer meets the impermeable layer beneath it and the water can go no lower.

Spring lines run along contours, so they often cut straight through a sloping site's proposed access road. Emergence migrates uphill through a wet winter and dries back to nothing by August. One dry-season visit sees none of it, which is one reason groundwater flooding gets missed at survey stage.

See also: Groundwater flooding, Interflow, Aquiclude

Stage is the height of the water surface in a river or channel above a fixed datum, measured at a gauging point. A stage figure means nothing until the datum it was measured against is stated.

Stage and flow are not interchangeable. A rating curve converts between them, and a shifted river bed quietly wrecks the top end of that curve, which is why we sense-check gauged levels against every flood model we build for a riverside site.

See also: Rating curve, Probable Maximum Flood (PMF), ReFH2

Standard of protection is the flood severity, usually a return period, that a defence or drainage system is designed to withstand without flooding: typically the 1% annual event for river defences and 0.5% for tidal. Beyond it, residual risk remains.

Nothing is protected against everything. A scheme sitting behind a 1% defence still needs its residual risk worked through. The case officer will ask what happens when that defence is overtopped, and where the people inside are expected to go.

See also: Flood alleviation scheme, Flood defence, Flood wall

A standard rules permit is a Flood Risk Activity Permit issued against fixed published conditions. It costs less than a bespoke permit and comes back faster, but only where the proposed work fits every published rule.

Miss one published rule and the whole thing goes bespoke. We check the rule set against the drawings before advising a client which route to take, because the gap between the two flood risk activity permit routes is weeks of programme rather than days, and a determination period that runs on.

See also: Flood Risk Activity Permit (FRAP), Bespoke permit, Environmental permit

A standpipe is a perforated pipe set into a borehole so the groundwater level inside can be dipped and recorded over time, giving the seasonal record that a single trial pit visit cannot.

Install early. Winter is when the level you actually need to design against turns up. A standpipe put in during August tells you very little, and late monitoring is one of the commonest reasons groundwater work delays a planning decision.

See also: Window sample, Trial pit, Groundwater monitoring

A statutory consultee is a body named in law that a local planning authority must consult on defined applications and whose advice must be taken into account. Advice carries weight, though the authority still decides the application.

Objections from a statutory consultee are rarely ignored, though they are not decisions. An LLFA holding objection can sit on a file for months while a drainage strategy is reworked, so we answer the likely question before it is asked. The consultees on an application are predictable once you know the site.

See also: Local Planning Authority (LPA), Natural Resources Wales (NRW), Water and sewerage company

Still water level is the sea or water surface level with waves stripped out, the baseline to which wave effects such as run-up are added in a coastal flood risk assessment.

Quoted alone, still water level flatters a coastal site. Run-up and overtopping sit on top of it, and both the baseline and the allowance climb with sea level rise around the UK coast, so an exposed frontage needs the design year stated alongside the number.

See also: Wave run-up, Sea level rise, Tidal flap

A storm overflow is a permitted discharge point where a combined sewer releases excess flow, usually diluted sewage, to a watercourse during heavy rainfall. Without that release the network would back up into basements, homes and streets.

Every extra litre buys a little more spill. That is the argument water companies now run when they refuse a connection to a combined sewer, and it is why the water company's part in drainage approval deserves attention long before you submit anything rather than after the refusal arrives.

See also: Combined sewer, Section 106 (Water Industry Act 1991), Combined sewer overflow (CSO)

A storm surge is a temporary rise in sea level at the coast above the predicted tide, driven by low atmospheric pressure lifting the water surface and strong onshore winds pushing water towards the shore. Surges are a principal driver of tidal and coastal flooding.

Timing is everything. A surge arriving on a spring high tide produces the extreme still-water levels that coastal defences are designed against, and that combination of surge and astronomical tide is exactly what the Thames Barrier was built for.

See also: Tidal surge, Tidal (coastal) flooding, Thames Estuary 2100 Plan

A Strategic Flood Risk Assessment is a local planning authority's own mapping of flood risk from all sources across its area. It defines the functional floodplain locally and feeds both plan-making and the flood-risk tests applied to individual sites.

Read the SFRA before you trust the national maps. Councils define the functional floodplain differently, and a site the online map calls Zone 3 may turn out to be 3b in the council's own strategic assessment, which changes what you can build there from houses to almost nothing on the same red line.

See also: Local Plan, Level 1 SFRA, Level 2 SFRA

The sub-base is the layer of granular material beneath a pavement that carries the structural load. In a permeable system it doubles as the storage reservoir, holding infiltrating water in the voids until it drains away.

Depth is the lever. Getting the void ratio and the aggregate specification onto the same drawing is what turns a sketched build-up into storage a reviewer will accept, and it is the detail most often missing when permeable paving reaches a planning application and someone asks what the layer actually holds.

See also: Permeable paving, Block paving, Filter drain (French drain)

SuDS adoption is a water company or authority taking on responsibility for maintaining sustainable drainage features once they are built. In England it stays rare because Schedule 3 is uncommenced, so maintenance often falls to residents instead.

Somebody has to cut the grass in year twenty. Planning conditions increasingly ask who, and an unanswered maintenance question stalls the discharge of conditions just as effectively as a missing calculation. That surprises people who assume building control signs drainage off.

See also: Sewer adoption, SuDS proforma, Sustainable Drainage Systems (SuDS)

A SuDS Approving Body is the statutory body in Wales, operating under Schedule 3, that approves, adopts and maintains sustainable drainage schemes. Approval must be in place before construction starts, and it runs separately from planning permission.

Two consents, two timetables. Welsh schemes get caught out when planning permission lands and site work still cannot start because the SAB submission was never made, and the 2025 national standards for sustainable drainage set out what that submission has to prove on treatment, on volumes and on who maintains the thing afterwards.

See also: Schedule 3, 2025 National Standards for Sustainable Drainage Systems, British Geological Survey (BGS)

The SuDS hierarchy is the order of preference for managing surface water in a sustainable drainage design: prevention first, then source control where the rain lands, then site control, then regional control serving a wider area. Each step is only reached once the one above it has been exhausted.

Discharge destination follows the same logic, ground before watercourse before sewer. You are allowed to depart from the SuDS hierarchy, but the departure has to be evidenced at each step rather than asserted in a covering letter, and that is where most drainage strategies get sent back.

See also: Source control, SuDS management train, Discharge hierarchy

The SuDS management train handles runoff in stages: prevention and source control first, then site control, then regional control. Water is dealt with progressively as it moves through the site rather than all at once at the outfall.

Order matters more than capacity. Most drainage layouts we are asked to review start at the end, with a tank. Work the train forwards, the way the SuDS hierarchy intends, and that tank shrinks, sometimes to nothing, because every stage upstream takes a share.

See also: SuDS hierarchy, Source control, Rainwater harvesting

A SuDS proforma is a standardised summary form, separate from the drainage strategy itself, that lets a lead local flood authority check a scheme's compliance with SuDS policy at a glance. Many councils require it at validation.

Leave it out and the application never gets registered. The irritating part is that every number on the form must match the strategy exactly, and a discharge rate rounded on one and not the other is enough to draw a query, so we fill the proforma last.

See also: Littoral zone (SuDS), Sustainable Drainage Systems (SuDS), Pond (SuDS)

A sump is a pit that collects water together with the sediment it carries. Both sit at the low point until a pump lifts the water out or it soaks slowly away through the base of the pit.

Sumps fail quietly. Sediment fills the void, nobody dips it, and the pump then either runs continuously or burns out, which is why a pumped low point needs a named maintenance regime inside the surface water drainage strategy, with a stated frequency and someone's name against it.

See also: Surface water drainage strategy, Soffit level, Catchpit

Superficial deposits are the unconsolidated materials sitting above bedrock, including alluvium, glacial till and river terrace deposits. Thickness and composition vary sharply over short distances, so what lies under one corner of a site may not lie under the next.

Alluvium is the deposit that ruins infiltration schemes. It reads as permeable on a geological map and behaves like clay in a soakaway test. We would rather see ground conditions tested on site, in the right place and to the right depth, than inferred.

See also: Bedrock, Glacial till, Alluvium

Surcharge is the condition where a pipe or sewer runs completely full and goes under pressure, pushing the water level above the pipe soffit so that flow can escape upwards at manholes and gullies.

Surcharge is normal in design; flooding from it is not. Modelling software will happily report a surcharged pipe as passing, so check what the water level does at each cover. Check which network you are in as well, because foul, surface and combined systems behave differently when they fill.

See also: Hydraulic capacity, Dry weather flow (DWF), Lateral drain

A surface water drainage strategy is the report that proves how rainfall landing on a site will be controlled by rate, volume, destination and quality. It is the document English validation lists usually name by that title.

Four questions, and a strategy answering three of them fails. Quality is the one people skip, then wonder why the LLFA comes back asking about treatment stages, so we build the surface water drainage strategy around all four from the first sketch, because retrofitting treatment into a finished layout is miserable.

See also: Drainage strategy, Tank sewer, Urban creep

Surface water flooding happens when rainfall cannot drain away fast enough through the ground, the drains or the nearest watercourse, so it runs over the surface and pools in low spots. Much of it falls outside the mapped flood zones, which is why a site in Zone 1 can still flood.

Ask the neighbours. Surface water routes are local, sometimes a matter of one dropped kerb or a blocked gully, and council records rarely capture them. That blind spot is a large part of why urban flood risk in England stays badly understood.

See also: Impermeable surface, Pluvial (surface water) flooding, Convective rainfall

A Surface Water Management Plan is a Lead Local Flood Authority's area-wide assessment of local surface water flood risk, setting out how the partner organisations involved will manage it. Every plan is written at area scale rather than plot scale.

Where a SWMP exists it is worth an hour of reading, because the modelling behind it is usually better than anything you would commission for a single site at planning stage. Ask the LLFA directly; plans often sit unpublished, while the strategic flood risk assessment for the same area is public.

See also: Strategic Flood Risk Assessment (SFRA), Flood Risk Assessment (FRA), Regional Flood Risk Appraisal (RFRA)

A surface water sewer carries rainwater runoff away from a site to a watercourse or to attenuation, and it sits below SuDS in the drainage hierarchy. That ranking matters: a connection is acceptable only where infiltration and other higher-priority options have been shown to be impracticable.

Water companies ask for evidence at this point, and rightly. A line saying infiltration is unsuitable will not do: you need soakaway test results, or a recorded groundwater level, or a contamination reason, before a surface water sewer connection is agreed, and even then the discharge rate will be capped.

See also: Separate sewer, Combined sewer overflow (CSO), Foul sewer

Suspended solids are the fine particles carried along in water, and their concentration is a standard indicator both of water quality and of how well a SuDS component is actually treating the runoff passing through it.

Silt kills a treatment train. Trap it early, in a forebay or a filter strip, and everything downstream lasts longer, which is what the quality limb of the four pillars of SuDS means once you are actually building, and it is the pillar most often reduced to one line on a plan.

See also: Filter strip, Treatment train, Forebay

Sustainable Drainage Systems manage rainfall close to where it lands, copying natural processes so that both the quantity and the quality of runoff are controlled. Amenity and biodiversity come with them: a well-placed basin is open space and habitat as well as storage.

SuDS is a design philosophy before it is a shopping list of components. Bolt a plastic tank under a car park and you meet the runoff rate figure while missing three of the four pillars. Schemes like that come back for reworking, and the delay is nobody's fault but the designer's.

See also: Amenity, Wetland (SuDS), 2025 National Standards for Sustainable Drainage Systems

A shallow, broad, vegetated channel that conveys, stores and treats surface water runoff, with some infiltration where the ground allows. The vegetation slows flow and filters sediment.

Two jobs, one feature. A swale conveys and treats at the same time, which is why it often replaces a length of pipe outright in the surface water drainage strategy.

See also: Dry swale, Hydrodynamic separator, Rill

SWFFIP, the Surface Water Flood Forecasting Improvement Project, is a Flood Forecasting Centre programme bringing impact models and nowcasting into fuller operational use so that surface water flooding can be forecast with more confidence.

Better forecasting does not reduce risk on a site; it changes what a flood plan can promise. If warning lead times for pluvial events improve, evacuation and vehicle-moving procedures start to look credible rather than aspirational, which is the shift SWFFIP could bring to planning for sites relying on them.

See also: Flood forecasting, Flood Forecasting Centre, Rapid response catchment

SWIM, the Surface Water Investment Model, is an Environment Agency approach to business cases that values many small SuDS features across a wide area as one portfolio, so their combined benefit can attract FCERM funding.

Individually, a rain garden never passes a benefit-cost test. Add a few hundred across a catchment and the arithmetic changes, which is the idea behind a business case for small-scale SuDS. Retrofit programmes across an existing town are where the model earns its keep.

See also: Rain garden, CIRIA C823F, Filter drain (French drain)

The TA6 Property Information Form is the Law Society's standard conveyancing form on which a seller in England and Wales must disclose flooding they know about. The current edition asks about flooding from any source, so a one-off surface water event in a back garden falls within the question.

Sellers underestimate this form badly. Non-disclosure of a known flood is a misrepresentation a buyer can act on years later, and the honest answer costs far less than the claim. We tell clients to write down the flood history they can actually evidence.

See also: Critical Drainage Area, Water neutrality, BREEAM Pol 03

TAN15, Technical Advice Note 15: Development, Flooding and Coastal Erosion, is the Welsh Government's planning policy on flood risk and the document that governs development on flood-prone land anywhere in Wales. Its 2025 version made the Flood Map for Planning central to decisions and introduced Defended Zones.

Welsh flood risk work is its own system. Zones, acceptability thresholds and the justification tests all differ from the English ones, and applying NPPF logic to a site in Cardiff is a reliable way to get refused, so we read TAN15 on its own terms.

See also: Justification test, Defended Zone, Notification Direction

A tank sewer is an oversized length of pipe or sewer that holds surface water in line. Storage sits inside the pipe run itself, so peak flows leaving the site are attenuated without a separate tank.

Tank sewers buy storage where nothing else fits. Under an access road, say, or across a tight frontage they earn their place, but jetting access is the thing designers forget, along with the volume they can realistically claim, and both belong in the surface water drainage strategy rather than on a later detail drawing.

See also: Peak flow, Surface water drainage strategy, Weir

Telemetry is the automatic transmission of measured river level, flow or rainfall readings from a remote gauge to a central system, where the data feeds monitoring and flood forecasting in near real time.

Gauges fail in wet weather. A flat trace during a storm usually means a blocked stilling well or a dead battery rather than a calm river, which is worth remembering while you watch live river levels during an event and try to decide whether to believe them.

See also: Gauging station, Recorded flood extents, Banded depth data

The Thames Estuary 2100 Plan is the Environment Agency's long-term strategy for tidal flood risk along the Thames Estuary. It covers the future of the Thames Barrier and the walls, banks and gates behind it.

Riverside sites in London inherit dates from this plan. Defence crest levels, raising programmes and the timing of any barrier replacement all shape the levels you design to now, decades ahead of the work itself, and the Thames Barrier is only the visible part.

See also: Tidal (coastal) flooding, Storm surge, Tidal surge

Tidal flooding, also called coastal flooding, occurs when seawater moves inland across low-lying coastal land. High tides, storm surge and wave action drive it, and sea level rise is steadily raising the baseline it starts from.

Tidal flooding arrives fast and deep, then leaves on the ebb. Depth and duration behave nothing like a slow fluvial rise, so warning time, escape routes and the salt damage afterwards all need separate thought. That is where the different sources of flood risk stop being an academic distinction.

See also: Storm surge, Fluvial (river) flooding, Tidal locking

Tidal flaps are flap valves fitted to a tidal outfall, hanging shut when the sea rises so that seawater cannot travel back up the drainage system at high tide.

Flaps fail quietly. Silt, a trapped stone or a seized hinge leaves the outfall open at the worst moment, and since the tide-locked window each day lengthens with rising sea levels around the UK coast, we ask for a maintenance route to every flap we specify.

See also: Tidal (coastal) flooding, Tidal locking, Flap valve

Tidal locking is the condition where high tidal levels outside an outfall stop a drainage system or watercourse discharging at all, so water backs up behind the outfall and stays there until the tide drops far enough for the pipe to run freely again.

For any outfall under tidal influence this sets the design case, because storage has to cover the whole locked period and not simply the storm peak. Rising sea levels lengthen that period, which is the slow problem behind sea level rise and coastal flooding.

See also: Tidal (coastal) flooding, Tidal flap, Sea level rise

A tidal surge is a rise in sea level above the predicted tide, driven by low pressure and strong winds. Coastal and estuary defences can be overtopped or breached by the extra water. Storm surge is the same phenomenon under a different name.

Barrier closures tell the story. The Thames Barrier was built for exactly this mechanism, and our account of London's tidal surge defence shows how the frequency of closure has climbed since the 1980s.

See also: Storm surge, Tidal (coastal) flooding, Thames Estuary 2100 Plan

Time of concentration is the travel time for runoff from the most distant point of a catchment to its outlet. It governs the critical rainfall duration and therefore the peak flow.

Get it wrong and every duration test downstream is wrong too. A concentration time set too short pushes the model towards brief intense bursts, when the true critical event is a longer, duller storm, so the packages we use for flood modelling sweep a range of durations.

See also: Time to peak, Catchment, Peak discharge

Time to peak is the interval between the onset of rainfall and the moment river flow reaches its maximum. Small steep catchments have short times to peak and produce rapid flash flooding.

Warning time is the practical consequence. A catchment that peaks two hours after the rain starts leaves nobody time to move a car, let alone evacuate a ground floor flat, and that shapes what a safe access route has to do. The tools used to build flood hydrographs give you the number.

See also: Hydrograph, Time of concentration, Peak discharge

A topographical survey is a measured survey of ground levels and features across a site, recording where the ground sits and what stands on it. It supplies the baseline elevation data that a flood risk assessment, a drainage design and any modelling of overland flow paths are built on.

Levels come first. Ground level data controls model output more than any other input on a flood risk assessment, which makes a survey tied to Ordnance Datum a prerequisite for flood modelling, not a refinement of it.

See also: Flood modelling, Ordnance Datum (AOD), LiDAR

Trade effluent is liquid waste from an industrial or commercial process, discharged to the public sewer rather than to a watercourse. A consent from the sewerage undertaker, the water company that owns the network, is normally required before any of it flows. That consent regime sits in the Water Industry Act 1991.

Nobody flags it at outline stage. A brewery, a laundry or a car wash will meet it the moment they apply to connect, and the consent can take months, so we name the likely trade element inside the foul drainage strategy and let the programme absorb it.

See also: Foul drainage strategy, Cesspool, Foul flow

A trash screen is a grille set at a culvert or structure inlet to catch debris before it enters. Screens need clearing, because a blocked one holds water back and causes flooding.

A screen moves the risk rather than removing it: less chance of the culvert blocking internally, more chance of it blocking at the inlet, and a maintenance liability that has to sit with somebody. Who that is depends on whether the channel is a main river or an ordinary watercourse.

See also: Main river, Ordinary watercourse, Culvert

A treatment train is a sequence of SuDS components arranged so each one removes a different set of pollutants from runoff, so water quality improves stage by stage before discharge.

One pond at the end of a site is not a train. Two or three components in series, a permeable surface then a swale then a basin, is what regulators expect, because quality is one of the four pillars every SuDS scheme has to satisfy.

See also: Suspended solids, Pollution pathway, Swale

A tree pit is an engineered planting pit built to keep a tree alive and to work as a SuDS component, so it receives, stores and treats surface water runoff instead of shedding it to a gully.

Highway teams and drainage teams have to agree the levels, or the kerb line sits above the inlet and nothing ever enters the pit. Amenity is the easy sell. Treatment, storage and biodiversity all sit in there too, which is why tree pits score well against the four pillars of SuDS design.

See also: Amenity, Four pillars of SuDS, Bioretention

A trial pit is a shallow excavation dug to expose the soil profile, so ground conditions and any groundwater strike can be logged at first hand. The same pit usually hosts the infiltration test, which for soakaway design follows BRE Digest 365. Depth is limited to what stands safely.

Two pits on a hectare tells you very little. Ground varies over a few metres, and a pit sited away from the proposed soakaway can produce a rate that collapses later, so plan the positions against the drainage layout before the digger arrives. Our guide to ground conditions and soakaways sets out what to test.

See also: BRE Digest 365, Infiltration, Made ground

A tributary is a smaller watercourse flowing into a larger river or stream. Flows in the main river are the sum of its tributary catchments, so what happens upstream on each one shows up downstream.

Status does not follow size. A tributary joining a main river is often an ordinary watercourse in its own right, which puts consents with the lead local flood authority instead of the Environment Agency, so check which of the two classifications applies before you design anything in the channel.

See also: Main river, Watercourse, Main River Map

UKCP18 is the UK Climate Projections 2018, the national climate dataset published for planning and adaptation work. The Environment Agency's current flood risk climate change allowances, the uplifts applied to peak river flow and rainfall in a flood risk assessment, are derived from it.

Nobody cites UKCP18 in a planning statement. They cite the allowance table drawn from it, and the allowances used in planning move: pick the wrong epoch or the wrong river basin district and your hydraulic model is out of date before submission. Check the date on any figure you inherit.

See also: Climate change allowance, Flood Risk Assessment (FRA), Design life

An unconfined aquifer has the water table as its upper surface, open to atmospheric pressure, and takes recharge directly from rainfall soaking down through the ground above it.

Recharge from above is what makes an unconfined aquifer awkward under a development. Water levels respond to the same wet winter that fills your soakaway, so a basement or an infiltration system sized on a dry-season reading will disappoint, and the mechanism behind groundwater flooding in planning terms is exactly that seasonal rise.

See also: Confined aquifer, Aquifer, Secondary aquifer

A unit hydrograph is the runoff hydrograph a catchment produces from one unit depth of effective rainfall, and it is scaled and combined to derive flood responses for larger storms.

The assumption underneath it is linearity: double the rainfall, double the flow. Catchments do not really behave that way, which is why the method earns its place in screening and quick checks rather than in the detailed flood modelling we run for a site.

See also: Flood modelling, Hydrograph, SAAR

The unsaturated zone is the ground between the surface and the water table, where pore spaces hold air and water together rather than water alone. Its thickness shrinks as the water table rises.

Soakaways need a decent thickness of it. Squeeze that thin by February and infiltration slows to nothing while the ground above stays saturated, which is the quiet start of the groundwater flooding problem councils now ask about.

See also: Water table, Groundwater flooding, Aquitard

Urban creep is the slow spread of impermeable surfacing across a developed site over time, most often gardens paved over for parking, and an uplift for it is added when surface water storage is sized.

The volume is wrong from day one. Pave the gardens over the next twenty years and the attenuation you sized for a clean impermeable area no longer holds the storm, so we carry the uplift into the impermeable area schedule in the surface water drainage strategy.

See also: Attenuation, Surface water drainage strategy, Conventional drainage

A vortex flow control, often called a Hydrobrake, spins incoming water into a tight spiral so that outflow from a storage feature is held back to a set discharge. No moving parts. Blockage is resisted rather than invited.

Devices are selected by head and discharge together, so a change to the basin's top water level after the layout moves will change the unit you need, and the manufacturer's curve has to be redrawn. Check the outlet chamber has room for it. We size ours inside the surface water drainage strategy.

See also: Flow control, Surface water drainage strategy, Overland flow route

The Wallingford Procedure is the standard UK methodology for designing and analysing urban storm drainage networks, and the Modified Rational Method used for pipe sizing sits inside it.

Most engineers meet it second hand. The equations behind a routine pipe sizing run, and behind the drainage calculators on this site, came out of research done at Wallingford in the 1970s and 1980s, which is worth knowing when you want to sanity-check a consultant's output.

See also: Modified Rational Method, Runoff coefficient, Colebrook-White equation

A washland is land deliberately allowed to flood at high flows, holding water back so that peak flows downstream are reduced. Flood alleviation schemes use washlands as storage rather than as protected land.

Being inside a washland is not the same as being defended. The land is meant to flood, so anything built on or beside it inherits the operating rules of the scheme, and that is a textbook case of residual flood risk rather than protection.

See also: Flood alleviation scheme, Residual flood risk, Safe access and egress

A water and sewerage company holds the regional licence for public water supply and sewerage, and controls connections to the network and the capacity available for them. On flood risk it is not generally a statutory consultee.

Their answer still decides whether your scheme works, because a refused or capacity-limited connection ends a foul strategy on the spot. Ask early. The pre-development enquiry takes weeks, and our rundown of the water authorities and their role in drainage planning explains who else is in the queue behind them.

See also: Statutory consultee, Ofwat, Section 106 (Water Industry Act 1991)

A water butt is a container that collects rainwater from a roof and stores it for reuse. Each one takes a small amount of runoff out of the system at source.

The reduction only counts when the butt happens to be empty as the storm arrives. Often it is not. A water butt therefore sits at the prevention end of the SuDS hierarchy and rarely earns credit as attenuation.

See also: SuDS hierarchy, Downpipe disconnection, Rainwater harvesting

The Water Framework Directive, retained in UK law after EU exit, protects inland, coastal and ground waters by requiring each of them to reach good ecological and chemical status.

Deterioration is the test that bites. A discharge that pushes a water body down a class, or delays its recovery, becomes very hard to consent, which is the same regulatory instinct that drives the nutrient neutrality rules now holding up housing in several catchments.

See also: Nutrient neutrality, Trade effluent, Dry weather flow (DWF)

Water neutrality is a planning requirement that a new development must not increase the total amount of water abstracted from the environment for public supply. Two moves get you there: cutting the demand of the new units as far as design allows, then offsetting the remainder so the net figure comes back to zero.

Abstraction pressure is what triggers it. Where a water company's supply is already damaging a protected habitat, an otherwise policy-compliant scheme can stall for a year while an offset package is found. Our page on water neutrality in planning sets out the calculation and the evidence councils ask for.

See also: TA6 Property Information Form, Class Q, Critical Drainage Area

The water table is the level below which the ground is fully saturated. It rises and falls with the seasons, and on chalk the swing between summer and late winter can be several metres.

A single borehole reading in August proves nothing about February. That is the whole reason councils ask for winter monitoring before they accept an infiltration design or a basement, and our explanation of groundwater flooding and its planning implications covers what a defensible record looks like.

See also: Groundwater flooding, Perched water table, Unsaturated zone

Water-compatible development is the flood risk vulnerability category for docks, marinas, water and sewage infrastructure and amenity open space. Those uses are generally acceptable even in the functional floodplain, Flood Zone 3b.

Classification is where a lot of applications quietly go wrong: a marina building with a flat above it is no longer water-compatible, and the Sequential Test comes back into play. Check the use against the Annex 3 vulnerability classification before you set the layout.

See also: More vulnerable development, Highly vulnerable development, Less vulnerable development

A watercourse is any channel that water flows through, whether a river, a stream, a ditch, a drain, a culvert or a dyke. The word carries no legal status of its own.

A ditch at the back of a plot is a watercourse, and the riparian duties that come with it fall to whoever owns the land, including keeping the flow clear. Where it sits in the main river and ordinary watercourse split decides who consents your headwall.

See also: Ordinary watercourse, Main river, Culvert

Wave overtopping is the volume of water that passes over the crest of a sea wall or defence when wave run-up climbs higher than crest level. Volume per metre of crest, per second, is how it gets measured.

Defences do not have to fail for a promenade to be dangerous. Overtopping is a design allowance, tolerated up to a rate, and every centimetre of sea level rise raises the rate the same storm delivers, which is the trend traced in our work on sea level rise around the UK coast.

See also: Wave run-up, Sea level rise, Overtopping

Wave run-up is the vertical height above still water level that a wave reaches as it surges up a beach or the face of a structure. Slope and roughness change how far it climbs.

Still water level is not the flood level. Take a level straight off a tidal map, design a threshold to it and you have missed the run-up entirely, and the pressures described in our piece on UK coastal flooding widen that gap every decade.

See also: Wave overtopping, Still water level, Fetch

A weir is a low structure built across a channel with a crest that water passes over, used to hold an upstream water level or to measure flow. In drainage it doubles as a flow control, holding water back and letting it spill once a set level is reached.

Its commonest drainage job is as an overflow. Spilling only above a set level is how exceedance routes get designed into a surface water drainage strategy rather than left to chance.

See also: Exceedance, Flow control, Critical duration

A SuDS wetland is a shallow area of standing water and water-loving planting, permanent or seasonal, that stores runoff and delivers high levels of pollutant treatment and biodiversity.

Space and a reliable water supply are the two constraints. A wetland that dries out in a dry summer loses the planting and the treatment with it, so base flow matters as much as storage volume, and on quality and amenity it scores about as well as anything in the four pillars of SuDS.

See also: Four pillars of SuDS, Amenity, Sustainable Drainage Systems (SuDS)

Window sampling drives narrow steel tubes into the ground and recovers a continuous soil column through slotted windows, giving samples for logging and laboratory testing at shallow depths.

The hole is worth more than the samples on a drainage job. Fit a standpipe as the rig leaves and you have a monitoring point for the winter, which is the difference between one visit and the monitoring delays that push schemes past a determination date.

See also: Standpipe, Winter monitoring window, Piezometer

The winter monitoring window runs from roughly November to May, peaking between January and March, and is when groundwater and infiltration data have to be collected if the seasonal high water table is to be captured.

Miss it and you wait. A scheme that reaches ground investigation in June has no defensible groundwater record until the following spring, which is why we push the decision to the front of the programme. The delays caused by groundwater monitoring requirements are almost always programme delays.

See also: Groundwater monitoring, Window sample, Piezometer

Working with Natural Processes is the Environment Agency's evidence directory on natural flood management, a review of around 800 studies across 17 measures that sets out what NFM can and cannot achieve.

Cite it when a council asks you to justify leaky dams or floodplain planting, because the directory is honest about confidence levels and that honesty is what carries an argument. It also curbs overclaiming, and our summary of natural flood management pulls out the measures with the strongest evidence.

See also: Natural flood management (NFM), Soft engineering, Runoff Attenuation Feature (RAF)