Flood Risk Assessment in Henley-on-Thames

Our team at Unda use years of experience to prepare clear, technically robust flood risk assessments and drainage strategies for commercial and residential sites across Henley-on-Thames. A flood risk assessment in Henley-on-Thames shows a planning authority, a lender or an insurer how flooding could affect a site and how that risk will be managed for the whole lifetime of the development.

Henley sits on the River Thames at the foot of the Chiltern hills, where the river runs past Marsh Lock and on towards Remenham and Medmenham. We assess every source that matters here: the Thames itself, the surface water that overwhelms the town-centre streets after intense rain, and the chalk groundwater that rises through the Assendon valley behind the town. We deal directly with the Environment Agency on the main river, with Oxfordshire County Council as Lead Local Flood Authority on surface water and drainage, and with South Oxfordshire District Council as the authority that decides your application. We guide architects, planning consultants, developers, landowners, home buyers and businesses through to a decision, whether the report is needed for planning, a property purchase, a mortgage or insurance, BREEAM, or a surface-water drainage strategy.

If you need a quote or want to talk a site through, contact one of our experienced flood risk consultants on 01293 214444.

Flood risk assessment in Henley-on-Thames at a glance

• A town built along the Thames. Henley lines the west bank of the River Thames, so the streets nearest the water — around Mill Lane, Ferry Lane, Thameside and the Wargrave Road — sit in Flood Zones 2 and 3.
• Storm Henk reached within a centimetre of the record. In January 2024 the Marsh Lock gauge peaked at 4.27 m, just 1 cm below the February 2014 record, and a formal flood investigation followed.
• Three sources, not one. The river, surface water in the town centre and chalk groundwater in the Assendon valley can each reach a Henley site, so a single desktop map rarely satisfies a planning officer here.
• No town flood scheme. Henley has no major alleviation scheme of its own, so riverside sites are assessed as undefended — the recorded flood level does the work a defence elsewhere might.
• Flood Zone 1 is not a free pass. Since the 2025 PPG update, surface water risk can trigger an assessment even on a Flood Zone 1 site.

Check flood risk anywhere in Henley-on-Thames

Henley-on-Thames is outlined below. Screen any postcode in or around the town against Environment Agency data for rivers and sea, surface water and the planning flood zones, with a climate outlook to 2125. It is an indicative first pass, not a substitute for a site-specific assessment.

Flood risk in Henley-on-Thames: river, surface water and groundwater

River flooding from the Thames dominates Henley, followed by surface water in the built-up streets and, behind the town, the chalk groundwater that rises through the Assendon valley. Most sites here face more than one source at once, which is exactly why a national map rarely settles the question on its own.

Henley occupies the west bank of the Thames where the river leaves the Chiltern gap, with Marsh Lock immediately downstream and the broad regatta reach upstream towards Temple Island. When the Thames runs high it backs up against the low ground beside the river and into the streets that drain to it, while the chalk hills behind the town hold water that feeds the Assendon Stream and can emerge as groundwater long after the rain has stopped. Understanding which of these drives the risk on a given plot, and how it behaves, is the heart of a credible fluvial flood risk assessment.

• Rivers (fluvial). The Thames is the main river through Henley and the dominant source of risk; the Environment Agency runs a flood warning area for the river from Henley down through Remenham to Medmenham. Land in Flood Zones 2 and 3 along the frontage almost always needs an assessment.
• Surface water (pluvial). Now the most widespread flood risk in England. Intense rain overwhelms the drains in the town centre and runs off the valley sides toward the river. Since the 2025 PPG update, surface water on the Flood Map for Planning can trigger an assessment even on a Flood Zone 1 site.
• Groundwater. Henley sits at the foot of the Chilterns, and the Environment Agency operates a separate groundwater flood alert for the Henley and Assendon area. Chalk groundwater rises from below and shows on no standard national map, so it has to be assessed in its own right — see checking groundwater flood risk.
• Ordinary watercourses. The Assendon Stream — a Chiltern winterbourne that flows only when groundwater is high — and the Hambleden Brook downstream are ordinary watercourses, which fall to the Lead Local Flood Authority rather than the Environment Agency. Our note on critical drainage areas explains when these bring stricter requirements.

The streets beside the river are the textbook case for why a single map is never enough. The frontage around Mill Lane, Ferry Lane and Thameside sits in Flood Zone 3, squeezed between the Thames and surface water that struggles to drain by gravity when the river is high, while the chalk behind the town can bring groundwater up from below in a wet winter. A scheme there has to reconcile all three. Our notes on the sources of flood risk and on surface water and planning set out how these sources interact.

Recognised high-risk areas in and around Henley-on-Thames

Several parts of the town are repeatedly identified as higher-risk, and a flood risk assessment in Henley-on-Thames has to reflect that recorded picture rather than rely on generic mapping.

• The riverside town centre. Mill Lane, Ferry Lane and Thameside, together with the open ground at Mill Meadows, take on water from the Thames when the river is high, and the streets behind them flood as the drains back up.
• The Wargrave Road and Marsh Lock. The low ground along the Wargrave Road south of the town, near Marsh Lock, lies in the Thames floodplain and floods in the larger events.
• Remenham and Mill End. The Berkshire bank opposite Henley and the meadows downstream toward Medmenham sit within the same Thames flood warning area.
• Lower and Middle Assendon. The Assendon valley behind the town is the focus of the Henley and Assendon groundwater flood alert, where chalk groundwater and the winterbourne stream affect homes and cellars in a wet winter.

This list is not exhaustive. As climate change and new development add pressure to the system, the number of locations needing careful assessment keeps growing, which is why the right scope on a specific site matters far more than a town-wide generalisation.

A history of flooding in Henley-on-Thames

These dated events are why the authorities here scrutinise development so closely, and why a flood risk assessment in Henley-on-Thames has to be evidence-led rather than generic. Each one is recorded in the local flood record a developer now designs around.

• January 2003. Thames flooding affected riverside Henley and the wider reach, part of the run of events that prompted further modelling of how the river behaves downstream.
• July 2007. The summer floods that struck the Thames catchment above Henley were the defining modern event for the valley, with around 3,000 homes flooded across Oxfordshire and the river running high through the town.
• February 2014. Prolonged Thames flooding set the modern benchmark for Henley: the Marsh Lock gauge reached its highest recorded level, and the streets nearest the river were under water for days.
• January 2024 — Storm Henk. Some of the worst flooding for a decade. The Thames broke its banks, residents were evacuated and businesses closed; the Marsh Lock gauge peaked at 4.27 m, just 1 cm below the 2014 record. Because commercial premises flooded internally, Oxfordshire County Council carried out a formal Section 19 flood investigation covering Henley-on-Thames and Goring.

A pattern runs through all of it: ground that has flooded once tends to flood again. A robust assessment treats this recorded history as evidence, not background colour.

When you need a flood risk assessment in Henley-on-Thames

People across Henley commission a flood risk assessment for a range of reasons, each as important as the next. A flood risk assessment in Henley-on-Thames might be needed to support a planning application, a property purchase, an insurance or lending decision, a BREEAM certification or a commercial transaction.

A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct reports. They often travel together in a planning submission, but they answer different questions: is the site safe, should the development be here, and where does the surface water go. Where development is proposed in a higher-risk area, a Sequential and Exception Test justifies the site's location; where runoff has to be managed, a surface-water drainage strategy sets out how. We prepare each alongside your flood risk assessment so the case to the council holds together.

Planning and drainage policy in Henley-on-Thames

Two kinds of authority set the rules. The Environment Agency leads on the main river; Oxfordshire County Council, as Lead Local Flood Authority, leads on surface water, groundwater and ordinary watercourses; and South Oxfordshire District Council determines the planning application against its own Local Plan.

South Oxfordshire District Council adopted the South Oxfordshire Local Plan 2035 in December 2020, and its Policy EP4 (flood risk) directs development towards the areas at lowest risk of flooding and requires sustainable drainage on major development. Oxfordshire County Council is a statutory consultee on surface water drainage for major development, applies its Local Standards and Guidance for surface water drainage, and runs a pre-application drainage advice service that is well worth using before a layout is fixed. Our overview of the role the LLFA plays explains how this consultation works in practice.

The flood-zone evidence base sits in the Strategic Flood Risk Assessment that South Oxfordshire shares with the Vale of White Horse. Your report must satisfy national policy — the NPPF and its Planning Practice Guidance — together with Policy EP4 of the Local Plan. Where a higher-risk site is involved, the Sequential and Exception Tests apply, and on drainage the four pillars of SuDS shape what the LLFA will accept.

What your flood risk assessment in Henley-on-Thames must show

• Risk assessed from every credible source, using the Flood Map for Planning and local data.
• An appropriate location, tested against the Sequential Test where policy requires it.
• Safe for its lifetime, once the climate change allowances are applied.
• No increase in flood risk elsewhere, with attenuation and, where needed, compensatory storage.
• Residual risk managed, with safe access, egress and finished floor levels. Get one of these wrong and the application stalls, as our note on LLFA drainage objections shows.

Flood risk assessments for every situation in Henley-on-Thames

Whatever has brought you here, Unda prepares the right report for it across Henley-on-Thames, each to the same standard. A flood risk assessment, a Sequential or Exception Test and a drainage strategy are three different reports, and we prepare all three alongside the right form of assessment for buyers, lenders and BREEAM.

Flood risk assessments

• Flood risk assessment for planning. Site-specific reports that satisfy the NPPF, the Environment Agency and the Lead Local Flood Authority. Groundwater and, where needed, flood modelling are assessed within it rather than as separate reports.
• Flood risk assessment for property purchase. Independent, property-specific advice for buyers and their conveyancers where a search has flagged Thames or surface-water risk.
• Flood risk assessment for insurance. Property-level evidence for insurers, brokers and underwriters where premiums have risen or cover has been restricted.
• Flood risk assessment for BREEAM. Assessments structured around BREEAM Pol 03, which is credit-driven rather than safety-driven, so a planning report alone rarely satisfies it.
• Groundwater flood risk. A defensible design water table for the chalk-valley sites behind the town and any basement — assessed as a source within the assessment, or as a standalone hydrogeological study on complex sites.

Sequential and Exception Tests

The Sequential and Exception Tests are a separate, locational report about whether a development should be in a higher-risk location at all. The Sequential Test defines an area of search, identifies reasonably available lower-risk sites and justifies the proposed one; the Exception Test is the second gate where development still needs to proceed. A clean site-specific assessment showing the development is not actually within an area at risk can remove the need at application stage.

Drainage strategy and SuDS

• Sustainable drainage strategy (SuDS). Surface-water engineering that calculates existing versus proposed runoff and proposes SuDS to hold discharge within the council's standards.
• Preliminary drainage design. The early, outline read that flags infiltration issues before they affect layout and land value.
• Discharge of drainage conditions. The detailed stage to discharge a pre-commencement or pre-occupation drainage condition.
• Flood modelling. 1D and 2D hydraulic modelling — a technique that feeds the assessment, Sequential Test or drainage design where national mapping is not enough.

Early in a drainage strategy it helps to know whether the ground will take water: our free geology and ground conditions map and BRE 365 infiltration rate calculator give an indicative read before a trial pit, and the drainage calculators help size the scheme — though on the Chiltern chalk a high water table can rule a soakaway out, so infiltration testing matters. You can screen a postcode on our flood risk map, and every report is overseen by a chartered flood risk consultant who stays involved past the report to answer objections.

Development and flood risk assessment in Henley-on-Thames

Henley's development pressures shape the assessment you need. Much of the town is a conservation area within the Chilterns National Landscape, so the kind of site you bring forward is the difference between a clean approval and an objection that holds an application up for months.

• Riverside and town-centre infill. Schemes near the Thames frequently meet Flood Zone 3 and surface-water risk together, needing finished floor levels, safe access and egress, and often careful mitigation design within a conservation-area setting.
• Conversions and heritage sites. Listed buildings and former industrial plots along the river bring flood risk and heritage constraints together, and usually need modelling to prove floor and access levels.
• Valley-side and edge-of-town release. Larger sites on the slopes behind Henley turn on surface water and the Assendon Stream rather than the main river, and on the right drainage information for planning.
• Drainage on the chalk. Parts of the catchment sit on permeable chalk where infiltration may work, but a high seasonal water table can rule it out, so a soakaway needs infiltration testing and a defensible design water table first.

Climate change is not optional here. A Henley assessment must apply the Environment Agency's allowances for higher river flows and rainfall over the development's lifetime — generally 100 years for housing — which sets floor levels, freeboard and how much attenuation a drainage scheme needs.

Home buyers and businesses come to us too. A flood risk question can affect a purchase and house prices, and a conveyancing search is not the same thing as a planning-grade assessment. Knowing which you actually need saves time and money, and our guide on doing your own assessment explains where the line falls.

How Unda helps with flood risk assessment in Henley-on-Thames

We act as the accurate translation layer between your scheme and the requirements set by the Environment Agency, Oxfordshire County Council, South Oxfordshire District Council and national planning policy. We are experts on the town's flooding, not a generic report mill.

A flood risk assessment in Henley-on-Thames from Unda assesses every credible source — the Thames, surface water, the chalk-valley groundwater behind the town, sewers and any ordinary watercourse on the boundary — compares modelled flood levels against your proposed floor levels, sets safe access and egress, and recommends mitigation the regulators will accept. We are a CIWEM Business Partner with chartered members, and a senior consultant signs off every report. Where the council needs it, a surface-water drainage strategy sits alongside the assessment as a separate report so the two cases stay consistent and an objection has nowhere to land.

Unda has completed more than 5,000 flood risk assessments since 2014 and is rated 5.0 on Google. To see how the firm works and who signs off your report, read more about Unda and our chartered flood risk consultants. Henley sits within our wider flood risk assessment work across Oxfordshire.

Henley-on-Thames flood resources and contacts

The most reliable Henley flood information comes from the lead authorities and the official flood-warning services. In a flood emergency that threatens life, always call 999; for a power cut, call 105.

• Check for flooding in Henley-on-Thames. Environment Agency live warnings, river levels and maps at check-for-flooding.service.gov.uk; sign up for flood warnings or call Floodline on 0345 988 1188.
• Thames flood warning area. The Environment Agency's River Thames for Henley, Remenham and Medmenham warning area covers the river frontage.
• Groundwater in the Assendon valley. The separate groundwater flood alert for the Henley and Assendon area covers chalk groundwater behind the town.
• Lead Local Flood Authority. Oxfordshire County Council's flood team leads on surface water, groundwater and ordinary watercourses, with guidance and reporting on the Oxfordshire Flood Toolkit, including the 2024 Section 19 flood investigation for Henley-on-Thames.
• Local planning authority. South Oxfordshire District Council's adopted Local Plan 2035 and its flood advice pages.
• Live river levels. Whether the Thames is running high right now via our live river levels map.

For broader policy context, our notes on the Environment Agency's planning role and the difference between the planning and long-term flood maps are a good starting point.

Flood risk assessment in Henley-on-Thames: frequently asked questions