Flood Risk Assessment in Hampshire

Our team at Unda use years of experience to prepare comprehensive flood risk assessments and drainage strategies for commercial and residential sites across Hampshire. A flood risk assessment in Hampshire shows your local planning authority that a development will stay safe from flooding for its whole lifetime, without pushing flood risk onto anyone else.

Hampshire is one of the most varied counties in England for flood risk. Its chalk hills feed the Test and the Itchen, and a long Solent coastline brings tidal and coastal flooding to its southern towns, so almost every site here has a flood question of some kind to answer. We assess every source that matters — the chalk rivers and their groundwater, the surface water that overwhelms drains in the towns, and the tidal risk along the coast from Southampton to Portsmouth and Havant — then deal directly with the Environment Agency and the relevant Lead Local Flood Authority on your behalf. 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, 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 Hampshire at a glance

• Groundwater is the dominant source inland. South Hampshire's strategic flood risk assessment names groundwater as the most significant source of flooding in the area, because so much of the county sits on water-bearing chalk.
• Two of the world's great chalk rivers run through it. The Test and the Itchen, with the Hamble and Meon, drain south to Southampton Water and the Solent; around one in four of England's chalk streams are in Hampshire.
• The Solent coast is tidal. The £180m-plus Southsea Coastal Scheme, the largest local-authority-led coastal defence project in the UK, is rebuilding 4.5km of Portsmouth seafront to protect more than 10,000 homes.
• Three flood authorities cover the county. Hampshire County Council is the Lead Local Flood Authority for the two-tier area; Southampton and Portsmouth are unitary authorities and run their own. We work with all three.

Check flood risk anywhere in Hampshire

Hampshire is outlined below. Screen any postcode in the county 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 Hampshire: chalk rivers, the Solent coast and surface water

Hampshire has no single dominant flood source. Central Hampshire is shaped by the chalk catchments of the Test and the Itchen, the south coast by the tides of Southampton Water and the Solent, and the towns by surface water that overwhelms the drains. Most sites here face more than one source at once.

The Test rises near Ashe above Basingstoke and runs roughly forty miles south to Southampton Water; the Itchen, the Hamble and the Meon drain the same way, while the Loddon in the north flows towards the Thames. These are slow, spring-fed chalk rivers, and the way they respond to rain is the key to flood risk across much of the county. Knowing which source actually drives the risk on a given site, and how it behaves, is the heart of a credible fluvial flood risk assessment.

• Groundwater. The chalk aquifer that underlies central and eastern Hampshire raises water tables that emerge in valley bottoms and flood low ground, basements and roads without ever appearing on a river map. See groundwater flooding.
• Rivers (fluvial). The Test, Itchen, Hamble, Meon and Loddon flood their valley floors. Land in Flood Zones 2 and 3 along these corridors almost always needs an assessment.
• Tidal and coastal. Southampton Water and the Solent flood the southern shore from Southampton through Fareham, Gosport, Portsmouth, Hayling Island and Havant, where sea-level rise sharpens the long-term picture. See tidal flooding.
• Surface water (pluvial). Now the most widespread flood risk in England. Intense rain overwhelms drains in Southampton, Portsmouth, Basingstoke and Winchester, and since the 2025 PPG update can trigger an assessment even on a Flood Map for Planning Flood Zone 1 site.

Our notes on the sources of flood risk and on surface water and planning set out how these sources interact, and how the Environment Agency maps the river and sea zones in flood zones explained.

Why groundwater flooding dominates inland Hampshire

More than any other county feature, the chalk shapes flood risk in Hampshire. After a wet winter the water table in the chalk rises until it reaches the surface in the valley bottoms, and the slow-responding Test and Itchen catchments are especially prone. South Hampshire's strategic flood risk assessment treats groundwater as the area's most significant flood source.

The villages repeatedly identified at high groundwater risk around Winchester include Hambledon, Kings Worthy, Hursley, Bishops Sutton and West Meon. Because groundwater rises from below according to geology rather than from a nearby river, it is the source most often missed at the start of a Hampshire scheme, and the one most likely to draw a groundwater objection from the Lead Local Flood Authority.

Groundwater appears on no national flood map, can sit at the surface for four to six weeks once it arrives, and undermines soakaway drainage because infiltration needs unsaturated ground beneath it.

That is why a clean Flood Map for Planning result is not the end of the story on the chalk. Our groundwater flood risk map guide explains how to check a site, groundwater in planning applications covers what developers must submit, and groundwater monitoring for planning sets out what happens when a season of monitoring is needed to establish a credible design water table.

A history of flooding in Hampshire

These dated events are why local authorities here scrutinise development so closely, and why a flood risk assessment in Hampshire has to be evidence-led rather than generic. Each one reshaped local policy and the defences a developer now has to design around.

• Winter 2013–14 — Romsey. Thirty-six residential and forty-four commercial properties flooded from a combination of groundwater, surface water and foul drainage, followed by the Fishlake Stream and the River Test overtopping. The Environment Agency's Romsey flood alleviation scheme followed.
• Winter 2013–14 — the Winchester villages. Littleton, Headbourne Worthy and Kings Worthy were badly affected by groundwater that winter, documented in Hampshire County Council's outer Winchester flood investigation.
• The groundwater winters. Significant chalk groundwater flooding, typically lasting four to six weeks, also occurred in the winters of 1994–95, 2000–01 and 2002–03, confirming that these episodes recur rather than being one-offs.
• The Solent coast. Tidal flooding and the threat of sea-level rise drive the £180m-plus Southsea Coastal Scheme, designed for a one-in-200-year standard and a metre of sea-level rise over the next century.

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

It matters for buyers and sellers too. A flood in the record can affect a sale, and our note on flood history declarations when selling a property explains what has to be disclosed.

The Solent coast and the defences protecting Hampshire

Hampshire's southern edge is one of the most heavily defended coastlines in England, and the schemes change what a development near them has to prove. A new defence reduces the chance of flooding, but it never removes it, and planning policy still expects the residual risk to be assessed.

• The Southsea Coastal Scheme. At over £180 million, the largest local-authority-led coastal defence project in the UK, rebuilding 4.5km of Portsmouth seafront from Old Portsmouth to Eastney to protect more than 10,000 homes and 700 businesses.
• An island city. Most of Portsmouth sits on low-lying Portsea Island, so tidal modelling of Portsmouth Harbour and Langstone Harbour governs what can be built and at what level.
• Shared coastal planning. Portsmouth, Havant, Gosport, Fareham, Eastleigh, Southampton and the New Forest co-ordinate on tidal and coastal risk across the Solent through the Partnership for South Hampshire.

The catch for developers is the one we set out in why a site stays in a flood zone after defences are built: the Flood Map for Planning is drawn as if the defences are not there, so a defended coastal site can still sit in Flood Zone 3. Sea-level rise makes this sharper, which is why an assessment applies the Environment Agency's climate allowances over the development's lifetime.

Planning policy and your Lead Local Flood Authority in Hampshire

Two kinds of authority set the rules. The Environment Agency leads on the main rivers and the sea. A Lead Local Flood Authority (LLFA) leads on surface water, groundwater and ordinary watercourses — and in Hampshire there are three, depending on where the site sits.

Hampshire County Council is the LLFA for the two-tier county area, while Southampton and Portsmouth are unitary authorities and act as their own LLFAs. A developer deals with whichever covers the site, alongside the Environment Agency and the local planning authority. Our overview of who manages flood risk in England explains how the pieces fit together.

Your assessment and drainage strategy must satisfy national policy — the NPPF and its Planning Practice Guidance — and the local framework. For evidence, developers work to the Strategic Flood Risk Assessments the planning authorities maintain: the Partnership for South Hampshire Level 1 SFRA covers the southern districts, with East Hampshire and boroughs such as Fareham and Gosport holding their own, all sitting under Hampshire County Council's Local Flood Risk Management Strategy. Where a higher-risk site is involved the Sequential and Exception Tests apply, and the right vulnerability classification has to be argued. On drainage, recent changes have removed the old development-size threshold so that proportionate SuDS are now expected on far more schemes, and getting the evidence right at application stage matters more than ever under the 2026 planning appeal rules.

What your flood risk assessment in Hampshire must show

• Risk assessed from every credible source, including the chalk groundwater that national maps miss.
• An appropriate location, tested against the Sequential Test where policy requires it.
• Safe for its lifetime, once climate change and, on the coast, sea-level rise 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.

Development and flood risk assessment in Hampshire

Hampshire is under sustained pressure for new homes and employment land, and the kind of site you bring forward shapes the assessment you need. The right scope is the difference between a clean approval and a drainage objection that holds an application up for months.

• Coastal regeneration and infill. Schemes in Portsmouth, Southampton, Gosport and Havant frequently sit in tidal Flood Zones 2 and 3, needing finished floor levels, safe access and egress, and an allowance for sea-level rise.
• Chalk-valley villages. Housing around Winchester, Alresford and the Meon valley turns on groundwater, where a defensible design water table decides what foundations and drainage are viable.
• Town surface water. Higher-density development in Basingstoke, Eastleigh and the cities makes the surface-water drainage strategy central to the application.
• Rural and agricultural schemes. Conversions and new builds across the county often turn on ordinary watercourses and infiltration rather than a main river, and on the right drainage information for planning.

Climate change is not optional here. A Hampshire assessment must apply the Environment Agency's allowances for higher rainfall, river flows and, on the coast, sea-level rise over the development's lifetime, which sets floor levels, freeboard and how much attenuation a drainage scheme needs.

Home buyers and businesses come to us too. A flood question can affect a purchase and house prices, and a conveyancing search is not the same thing as a planning-grade assessment. Our guides to how to do a flood risk assessment and how much one costs set out what to expect.

How Unda helps with flood risk assessment in Hampshire

We act as the accurate translation layer between your scheme and the requirements set by the Environment Agency, the relevant Hampshire LLFA and national planning policy. We are experts on the county's flooding, not a generic report mill.

A flood risk assessment in Hampshire from Unda assesses every credible source — the chalk rivers and their groundwater, surface water, the Solent tides and any ordinary watercourse on the boundary — models flood levels and flow routes where national mapping is not enough, compares them against your proposed floor levels, and recommends mitigation the regulators will accept. We are a CIWEM Business Partner with chartered members, and a senior consultant signs off every report and stays involved past submission to answer objections.

Hampshire's chalk-valley sites show why local knowledge counts. On one sensitive site next to a protected watercress bed fed by chalk springs, Unda was asked to demonstrate that homes could be built without harming the cress beds or worsening flood risk. By building a groundwater conceptual model we showed the shallow groundwater discharged to the perimeter channels around the beds rather than into them, and that ground previously mapped as high groundwater-flood risk could be re-ranked as medium to low, allowing the developer to build across the whole site. It is the kind of evidence-led groundwater assessment that chalk-stream Hampshire often needs.

Flood risk assessments and related reports for every situation in Hampshire

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. We prepare all three, alongside the right form of assessment for buyers, lenders and BREEAM.

Flood risk assessments

• Flood risk assessment for planning. The core report, site-specific and prepared to satisfy the NPPF, the Environment Agency and your Hampshire LPA. Groundwater and, where needed, flood modelling are assessed within it rather than as separate reports.
• Homebuyer and property-purchase survey. For conveyancing, where a search has flagged chalk, river or coastal risk and you need the actual building assessed, not a broad map.
• Flood risk assessment for insurance. Property-specific evidence for brokers and underwriters where premiums have risen or cover has been restricted.
• Flood risk assessment for BREEAM. Structured around BREEAM Pol 03, which is credit-driven rather than safety-driven, so a planning assessment alone rarely satisfies it.
• Groundwater flood risk. A defensible design water table for chalk-valley sites 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 site; 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 — a live policy area, as our note on the Sequential and Exception Tests explains.

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 required rate, following the four pillars of SuDS.
• 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. You can screen a postcode on our flood risk map, and every report is overseen by a chartered flood risk consultant.

Hampshire flood resources and contacts

The most reliable Hampshire flood information comes from the lead authorities. For an immediate threat to life during a flood, always call 999; for a power cut during flooding, call 105.

• Check for flooding in Hampshire. 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.
• Hampshire County Council flood team. Strategy, developer guidance and how to report flooding at Hampshire County Council.
• Partnership for South Hampshire SFRA. The area-wide strategic flood risk assessment evidence base at push.gov.uk.
• Southsea Coastal Scheme. The Solent's largest coastal defence project at southseacoastalscheme.org.uk.
• Live river levels. Whether the Test, Itchen or Meon are running high right now via our live river levels map.

For broader policy context, our notes on the Environment Agency's planning role and on the NaFRA2 flood risk data are a good starting point.

Flood risk assessment in Hampshire: frequently asked questions