Flood Risk Assessment in Chichester

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

Chichester is a place where three very different kinds of flooding meet. The River Lavant, a chalk stream that runs dry most summers, can rise quickly enough to put the city centre under water. The tide reaches well inland through Chichester Harbour at Bosham, Fishbourne and Apuldram. And the low-lying Manhood Peninsula around Selsey and the Witterings sits barely above the sea. We assess every source that matters on a given site, then deal directly with the Environment Agency and West Sussex County Council, the 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 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.

In Chichester the same rain can arrive three ways — down the Lavant off the Downs, up the harbour on a rising tide, and across coastal-plain ground that barely drains — so an assessment that reads only the river map will not stand up to scrutiny.

Flood risk assessment in Chichester at a glance

• The River Lavant is a winterbourne. Fed by groundwater in the chalk of the South Downs, it runs dry most summers and in a wet winter can rise sharply — the mechanism behind the city's worst floods.
• January 1994 was the benchmark flood. The Lavant overwhelmed Chichester in the worst flooding for around 130 years, with total damage put at roughly £6 million and the A27 and A259 closed.
• Chichester Harbour brings the tide inland. Bosham, Fishbourne, Apuldram and Itchenor sit in tidal Flood Zone 3, and several of the harbour's sea defences are near the end of their service life.
• West Sussex County Council is the Lead Local Flood Authority. It leads on surface water, groundwater and ordinary watercourses; the Environment Agency leads on the Lavant and the sea.
• Flood Zone 1 is not a free pass. Since the 2025 PPG update, surface water or groundwater risk can trigger an assessment even on a Flood Zone 1 site.

Check flood risk anywhere in Chichester

The Chichester district is outlined below. Screen any postcode 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 Chichester: the Lavant, the harbour and the coast

No single source dominates flood risk in Chichester. The River Lavant brings groundwater-driven river flooding to the city and the spring-line villages; Chichester Harbour brings the tide inland to Bosham, Fishbourne and Apuldram; the Manhood Peninsula sits low against the open sea; and surface water collects across a flat coastal plain that struggles to drain. Most sites here face more than one of these at once, which is why a credible assessment reads them together.

The River Lavant rises in the chalk near East Dean, flows west to Singleton and then south through Lavant to Chichester. Because the chalk feeds it, the Lavant is a winterbourne: it runs dry through summer and only flows once groundwater in the Downs has recharged. That makes its flooding behave like groundwater as much as a river, rising slowly over weeks and then persisting. Knowing which source drives the risk on a given site, and how it behaves over a season, is the heart of a defensible assessment — and the reason a screening map alone is never enough, as we explain in are flood maps accurate.

• Rivers and groundwater (the Lavant). The Lavant and the chalk behind it drive river flooding in Chichester, Lavant and Singleton. Because it is groundwater-fed, the risk is tied to how wet the preceding winter has been, not to a single storm.
• Tidal (Chichester Harbour). The harbour reaches inland to Bosham, Fishbourne, Apuldram, Itchenor and Chidham, placing land in tidal Flood Zone 3. The tidal range at the harbour mouth is close to 4.9 metres on a spring tide.
• Coastal (the Manhood Peninsula). Selsey, the Witterings, Bracklesham, Sidlesham and Birdham sit only a few metres above sea level and depend on maintained defences against the open Channel.
• Surface water (pluvial). Now the most widespread flood risk in England. On Chichester's flat coastal plain, intense rain overwhelms drains and rifes. Since the 2025 PPG update, surface water on the Flood Map for Planning can trigger an assessment even on a Flood Zone 1 site.

Chichester is the textbook case for why one map is never enough: a city-centre site can sit in Flood Zone 1 for rivers and the sea yet still face the Lavant's groundwater and surface water with nowhere to drain, while a harbour-edge plot reads low for surface water but sits squarely in the tidal floodplain. A scheme has to reconcile whichever sources apply. Our note on the Environment Agency's planning role sets out who assesses what.

A history of flooding in Chichester

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

• January 1994 — the Chichester flood. After an exceptionally wet winter, the Lavant catchment received around 610 millimetres of rain between October 1993 and January 1994, about 61% above average. Groundwater in the chalk rose until the Lavant overwhelmed its channel and flooded the city, the worst event for roughly 130 years. The A27 and A259 were closed; damage to West Sussex County Council alone was about £1.9 million, with total damage estimated near £6 million.
• The lesson of 1994. The flood was driven by groundwater, not by a single storm, and the culverts that carry the Lavant beneath the city could not cope once levels were high. It is why any assessment near the Lavant has to consider the state of the chalk aquifer over a whole winter, not just rainfall on the day.
• Recurrent tidal flooding around the harbour. At Bosham, deep flooding occurs along Shore Road and the High Street for roughly two hours either side of a high spring tide, a routine event rather than a rare one — a reminder that tidal risk is a constant design constraint here.
• A coast under growing pressure. Studies of the Manhood Peninsula warn that, without continued defence, large parts of Selsey, the Witterings, Bracklesham and Sidlesham could be permanently inundated by 2100 as sea levels rise.

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.

The River Lavant and the flood alleviation scheme

After 1994, the Environment Agency built defences to reduce the chance of the Lavant flooding Chichester again. They reduce the probability of flooding; they never remove it, and planning policy still expects the residual risk to be assessed for any site that relies on them.

Through Chichester the Lavant runs in two long culverts beneath the city, and serious flooding occurs when their capacity is exceeded in extreme conditions. The River Lavant flood alleviation scheme is designed to carry flood water away from the city to the east, lowering levels in storage pits and then routing flow southward toward Pagham Harbour, operated in phases as river levels rise. A development near the river or the scheme has to understand how it works and what happens if it is overtopped — the residual-risk question we set out in residual flood risk.

• Two culverts under the city. The Lavant is carried through Chichester in culverts; when groundwater is high and flow is heavy, that capacity is the limiting factor.
• A phased alleviation scheme. The Environment Agency scheme moves flood water east into storage and on toward Pagham Harbour, operated as levels rise rather than all at once.
• The map ignores defences. The Flood Map for Planning is drawn as if defences are not there, so a site protected by the scheme can still sit in a flood zone — see why the long-term flood map is not the planning map.

Tidal flooding, Chichester Harbour and the Manhood Peninsula

South and west of the city, the risk changes from river to sea. Chichester Harbour is a tidal inlet that floods its fringes on high spring tides, and the Manhood Peninsula is a low spit of land between the harbour, Pagham Harbour and the open Channel where defence, not drainage, governs whether the land stays dry.

Bosham, Fishbourne, Apuldram, Itchenor and Chidham all sit within the tidal floodplain, and several of the harbour's older sea defences at Thorney Island, Nutbourne, Chidham and Bosham are reaching the end of their service life. On the Manhood Peninsula, much of the land around Selsey, the Witterings, Bracklesham, Birdham and Sidlesham lies only a few metres above mean sea level, held by a maintained shoreline under a hold-the-line policy. For any site here, the assessment has to take the design tide level plus a climate change allowance, set finished floor levels and safe access above it, and treat the failure or overtopping of a defence as a real scenario rather than a remote one. Our note on how accurate flood maps are explains why a harbour-edge site needs measured levels, not just the national mapping.

Planning policy and your Lead Local Flood Authority in Chichester

Two kinds of authority set the rules. The Environment Agency leads on the River Lavant and the sea. West Sussex County Council is the Lead Local Flood Authority, leading on surface water, groundwater and ordinary watercourses, and Chichester District Council determines most applications against its adopted Local Plan.

West Sussex County Council is a statutory consultee on surface water drainage for major development, reviewing flood risk assessments and drainage strategies, and it runs a pre-application advice service for developers — the role we describe in what a Lead Local Flood Authority does. The district's planning framework was refreshed when the Chichester Local Plan 2021–2039 was adopted in August 2025, with Policy NE16 on flood risk and water management, Policy NE17 on water management and water quality, and Policy NE18 on water neutrality. The council also maintains Level 1 and Level 2 Strategic Flood Risk Assessments that map where the harbour tide, the Lavant and surface water constrain development.

Your assessment and drainage strategy must satisfy national policy — the NPPF and its Planning Practice Guidance — and the local framework: West Sussex County Council's flood risk management strategy and SuDS expectations, the district's Strategic Flood Risk Assessment and the Local Plan policies above. Where a higher-risk harbour or Lavant site is involved, the Sequential and Exception Tests apply, and the live policy around a failed sequential test — set out in the recent Yatton judgment — can be decisive. There is one further local wrinkle: until late 2025, much of the northern part of the district fell within the Sussex North water neutrality area, which paused some housing over abstraction near the protected Arun Valley wetlands. Natural England withdrew that position statement on 31 October 2025, but water remains a live planning consideration here under Policy NE18.

What your flood risk assessment in Chichester must show

• Risk assessed from every credible source — the Lavant and chalk groundwater, the harbour tide, surface water and any ordinary watercourse — 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 Environment Agency's climate change allowances are applied — usually 100 years for housing.
• 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 Chichester

The district takes in a cathedral city, a string of harbour villages, the Manhood Peninsula and a slice of the South Downs National Park, 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.

• City and edge-of-Chichester sites. Infill and urban-extension schemes around Chichester have to reconcile the Lavant, chalk groundwater and surface water, even where the river map reads low.
• Coastal-plain housing. Development around the Witterings, Birdham and Sidlesham turns on coastal risk, low ground levels and surface water that cannot drain by gravity, often needing tanked drainage rather than soakaways.
• Harbour-edge and conversion schemes. Around Bosham, Fishbourne and Apuldram, tidal Flood Zone 3 drives finished floor levels, safe access and egress, and the residual risk behind ageing defences.
• Rural and Downland schemes. Conversions and small sites in the north often turn on ordinary watercourses, infiltration and groundwater rather than a main river — and on the right infiltration testing before a layout is fixed.

Climate change is not optional here. A Chichester assessment must apply the Environment Agency's allowances for higher river flows, rainfall and sea levels 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 Chichester

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

A flood risk assessment in Chichester from Unda assesses every credible source — the Lavant and chalk groundwater, the harbour tide, surface water, 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.

Flood risk assessments and related reports for every situation in Chichester

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 Chichester District Council. Groundwater and, where needed, tidal or fluvial flood modelling are assessed within it rather than as separate reports.
• Homebuyer and property-purchase survey. For conveyancing, where a search has flagged Lavant, harbour or surface-water 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 report alone rarely satisfies it.
• Groundwater flood risk. A defensible design water table for chalk and coastal-plain 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 Yatton judgment explains.

Drainage strategy and SuDS

• Sustainable drainage strategy (SuDS). Surface-water engineering — calculates existing versus proposed runoff and proposes SuDS to hold discharge within West Sussex standards, 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, which on Chichester's clay and coastal-plain deposits is far from guaranteed. 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 who stays involved past the report to answer objections.

Chichester flood resources and contacts

The most reliable Chichester 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 Chichester. 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.
• West Sussex County Council (Lead Local Flood Authority). Flood risk management, SuDS guidance and developer pre-application advice at westsussex.gov.uk.
• Chichester District Council. The adopted Local Plan and its flood and water policies at chichester.gov.uk, plus the council's Strategic Flood Risk Assessment.
• River Lavant flood alleviation scheme. How the Environment Agency scheme works at gov.uk.
• Live river levels. Whether the Lavant is running high right now via our live river levels map.

For flooding from blocked or burst sewers, contact Southern Water, the area's sewerage company. For broader policy context, our notes on the Environment Agency's planning role and on riparian ownership of watercourses are a good starting point for sites with a ditch or stream on the boundary.

Flood risk assessment in Chichester: frequently asked questions