Flood Risk Assessment in Portsmouth

Our team at Unda prepares comprehensive flood risk assessments and drainage strategies for sites across Portsmouth, from Portsea Island and Old Portsmouth to Tipner, Milton and the harbour fronts. A flood risk assessment in Portsmouth shows your local planning authority that a development will stay safe from flooding for its whole lifetime, allowing for climate change, without pushing flood risk onto anyone else.

Portsmouth is unusual, and that shapes every assessment here. It is the only English city built mainly on an island — Portsea Island — and almost all of it is low-lying, with much of the island under three metres above sea level and ringed by tidal water in Portsmouth Harbour, Langstone Harbour and the Solent. Tidal flooding from the sea is the defining risk, with surface water, groundwater and sewer flooding layered on top. We read those sources together, then deal directly with the Environment Agency and Portsmouth City Council 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, a mortgage or insurance, BREEAM, or a surface-water drainage strategy.

Most of Portsea Island sits below the level of an extreme tide and stays dry only because of its sea defences, so flood risk here is about residual and future risk behind the wall — which is exactly what a planning authority will expect a flood risk assessment in Portsmouth to address.

To speak to someone now, call +44 (0) 1293 214444 or start a quote — one of our experienced flood risk consultants will get back to you within 60 minutes.

Flood risk assessment in Portsmouth at a glance

• An island city below the tide. Most of Portsmouth sits on Portsea Island, much of it less than three metres above sea level, so the sea — not a river — is the dominant flood risk.
• One of the most flood-exposed cities in England. Portsmouth is a nationally designated flood risk area, and tens of thousands of homes and businesses sit behind tidal defences that are being raised across two major schemes.
• Portsmouth City Council is your Lead Local Flood Authority. As a unitary authority it leads on surface water, groundwater and ordinary watercourses, alongside the Environment Agency on the sea and main rivers. We work with both.
• Flood Zone 1 is not a free pass. Since the 2025 update to national guidance, surface water and groundwater can trigger an assessment even where the planning flood zone looks low.

Check flood risk anywhere in Portsmouth

Portsmouth is outlined below. Screen any postcode in the city 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.

Why is Portsmouth at risk of flooding?

Tidal flooding from the sea is the dominant risk in Portsmouth. Portsea Island is low and almost surrounded by water, so a high spring tide combined with a storm surge and waves is what threatens the city — with surface water, groundwater and overloaded sewers adding to the picture inland. Most sites here face more than one source at once, which is why a single national map is never enough.

The coastal frontages of Portsea Island have a long record of flooding from wave overtopping, and the Partnership for South Hampshire Strategic Flood Risk Assessment treats tidal sources as the dominant source of potential flooding for sites across the island. Because the land is flat and low, water that gets past the defences has nowhere to drain by gravity, and rainfall that cannot soak into the clay collects in the streets. Reading which of these sources drives the risk on a given site, and how they interact, is what a credible assessment has to get right — our notes on the sources of flood risk and on whether flood maps are accurate explain why.

Portsmouth's geology adds a twist. The island sits on London Clay and Bagshot sands, with the chalk of Portsdown Hill to the north, so much of the city has low-permeability ground where soakaways struggle and a high, tidally-influenced water table reduces drainage storage. That makes groundwater and the choice of drainage strategy genuinely site-specific. Our free geology and ground conditions checker gives an early read on whether infiltration is likely to work at a postcode.

Which flood zone is Portsmouth in?

Large parts of Portsea Island fall within Flood Zones 2 and 3 on the Environment Agency's Flood Map for Planning, because those zones show the risk that would exist without defences. The defences are real and substantial, but in planning terms the land behind them is still high-risk — which is why assessments here turn on residual risk and climate change rather than on a single zone label.

The Flood Map for Planning shows tidal Flood Zones 2 (medium probability) and 3 (high probability) reaching across much of the low-lying island, while the higher ground around Portsdown Hill to the north sits in Flood Zone 1. Crucially, the flood zones ignore the sea defences, so a site can be protected to a high standard today and still be mapped as Flood Zone 3. A flood risk assessment in Portsmouth has to work with that reality: it sets out the standard of protection, the residual risk if a defence is overtopped or fails, safe access and egress, finished floor levels referenced to Ordnance Datum, and how the risk grows as sea levels rise. Where the mapping does not reflect local conditions, flood modelling can define levels more precisely. And because the September 2025 Planning Practice Guidance update confirmed that the risk-based approach applies to all sources of flooding, even a Flood Zone 1 site near Portsdown Hill can need an assessment if surface water or groundwater is flagged.

The sea defences protecting Portsmouth: Southsea and North Portsea Island

Portsmouth is defended by two of the largest coastal flood-defence schemes in the country, both led by Portsmouth City Council through Coastal Partners and grant-funded by the Environment Agency. Together they are raising the standard of protection around almost the whole island — but they manage flood risk, they do not remove it, and every development behind them still has to account for residual and future risk.

These schemes are the reason so much of Portsmouth stays dry, and they change how an assessment is written: a development on a defended frontage has to state the standard of protection it relies on, show it is safe if that defence is overtopped, and design for the sea level expected over its lifetime. You can read more about the works on the Southsea Coastal Scheme and North Portsea Coastal Scheme pages, and Portsmouth City Council sets out its wider responsibilities on its coastal defence management pages. The same residual-risk thinking that drives the national debate on defended low-lying places applies on Portsea Island: a defence reduces the probability of flooding, never the possibility.

A history of flooding in Portsmouth

Portsmouth has flooded from the sea throughout its history, and those events are why the city is defended and scrutinised so closely today. Dated local flooding is the clearest evidence that an assessment has to be led by the real coast and catchment, not a generic template.

• The early-1900s tidal floods. Old Portsmouth and Broad Street were inundated in serious tidal floods in 1910, 1911 and 1929 — events still remembered locally and part of the reason the Broad Street flood gates exist.
• Recorded flooding since 1946. The Environment Agency's recorded flood outlines show repeated tidal flooding around the older, low-lying parts of the city through the post-war decades.
• The 2014 winter storms. The storms of winter 2013–14 brought tidal flooding and overtopping to the seafront and sharply raised local awareness of how exposed the island is.
• Ongoing surface-water flooding. Portsmouth City Council's surface water work identifies a series of inland hotspots where intense rain floods streets and properties away from the coast — a reminder that the risk here is not only the sea.

Planning policy and your Lead Local Flood Authority in Portsmouth

Two kinds of authority set the rules in Portsmouth. The Environment Agency leads on the sea and any main rivers. Portsmouth City Council, as a unitary authority, is the Lead Local Flood Authority and leads on surface water, groundwater and ordinary watercourses — and is the statutory consultee on surface-water drainage for major development.

A developer here works with both, alongside the framework each one applies. The Environment Agency advises on tidal risk and the standard a defended site must meet, while the Lead Local Flood Authority reviews how surface water will be managed. Our overview of the Environment Agency's role in planning explains how their advice carries weight without their being the decision-maker.

Your assessment and drainage strategy must satisfy national policy — the NPPF and its Planning Practice Guidance — and the local framework: Portsmouth City Council's Strategic Flood Risk Assessment, its Local Flood Risk Management Strategy and the flood and drainage policies in the adopted Local Plan. Where a higher-risk site is involved, the Sequential and Exception Tests apply — a live area of policy, as the recent Yatton judgment shows. On drainage, the four pillars of SuDS shape what the council will accept, and on Portsmouth's clay the discharge route has to be justified rather than assumed.

What development needs a flood risk assessment in Portsmouth?

Portsmouth is a tightly constrained island city, so most schemes are infill, regeneration and brownfield redevelopment — often on land mapped as Flood Zone 2 or 3 behind the sea defences. The kind of site you bring forward shapes the assessment you need, and getting the scope right is the difference between a clean approval and a drainage or flood-risk objection that holds an application up for months.

• City-centre and harbourside regeneration. Schemes around the city centre, Gunwharf, Tipner and the harbour fronts frequently sit in tidal Flood Zones 2 and 3, needing finished floor levels, safe access and egress and a clear residual-risk case.
• High-density island infill. Because Portsea Island is built up and low-lying, even small sites often need to show that surface water is managed and that the development is safe behind the defences for its lifetime.
• Brownfield and change of use. Conversions and redevelopment of former industrial and commercial land across the city turn on surface water, ground conditions and the right preliminary drainage design before the layout is fixed.
• Larger and major development. Sites of 10 or more dwellings, 1 hectare or more, or 1,000 m² of floorspace trigger the Lead Local Flood Authority's role and usually need a surface-water drainage strategy as well as a flood risk assessment.

How Unda helps with flood risk assessment in Portsmouth

We are flood risk and drainage specialists with extensive experience of coastal and tidal sites across the south coast, and we know how Portsmouth's defended, low-lying island is assessed. We translate the Environment Agency, Lead Local Flood Authority and national policy requirements into a clear, robust report that keeps your application moving — and we stay involved after the report to answer objections.

Every project is overseen and signed off by a Senior Flood Risk Consultant, and we reply to every quote request within 60 minutes. We have completed more than 5,000 flood risk assessments since 2014 for clients ranging from private homeowners to national developers, and our consultants are chartered members of CIWEM. We assess every source of flooding that matters in Portsmouth — tidal, surface water, groundwater and sewer — read the Environment Agency and council data for your specific site, model flood levels where the mapping is not enough, and design proportionate mitigation that the regulators will accept. You can read more about us, meet the team in the media, or see how our flood risk consultants work.

Start a Quote

Flood risk assessments for every situation in Portsmouth

People need a flood risk assessment or drainage strategy in Portsmouth for many reasons, and we prepare each of them. 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.

• Flood risk assessment for planning. The site-specific report that supports a planning application, demonstrating the development is safe for its lifetime and does not increase risk elsewhere — see flood risk assessment for planning.
• Flood risk survey for property purchase. An independent assessment for home buyers and their advisers when a search or lender flags flood risk on a Portsmouth property — see flood risk survey for property purchase.
• Flood risk assessment for insurance. A property-specific report to support cover, premiums or a claim — see flood risk assessment for insurance.
• Flood risk assessment for BREEAM. Structured around the BREEAM Pol 03 flood and surface-water criteria — see flood risk assessment for BREEAM.
• Sequential and exception tests. The locational justification needed when development is proposed in a higher-risk part of the city — see sequential and exception tests.
• Surface-water drainage strategy and SuDS. The surface-water engineering that manages runoff within agreed limits, with discharge of drainage conditions at the detailed stage — see surface-water drainage strategy.
• Groundwater and flood modelling. A groundwater assessment is part of the wider flood risk assessment, and flood modelling feeds it where the national mapping is not precise enough.

Need a quick first pass yourself? Our free tools cover the basics: the flood risk map, the live river levels map, the drainage calculators and the BRE 365 infiltration rate calculator. Portsmouth sits within Hampshire — you can also see our wider flood risk assessment in Hampshire page for the county picture.

Local flood risk management and emergency contacts in Portsmouth

If you need to report flooding, prepare for it, or reach the right body in an emergency, the contacts below cover the organisations responsible for managing flood risk in Portsmouth. In a flood that threatens life, always call 999.

You can sign up for Environment Agency flood warnings and check live tidal and river conditions through the Government's check for flooding service, and report surface-water or drainage flooding to Portsmouth City Council through its surface water flooding pages. If you own land next to a watercourse, our guide to riparian ownership explains your responsibilities.

Frequently asked questions about flood risk assessment in Portsmouth