Flood Risk Assessment in Merseyside

Unda prepares flood risk assessments and surface water drainage strategies for residential and commercial sites across Merseyside. A flood risk assessment in Merseyside shows your local planning authority that a development will stay safe from flooding for its whole lifetime, and that it will not push flood risk onto neighbouring land.

Merseyside is a densely built, low-lying county where surface water is the dominant flood risk, the Mersey estuary and the Sefton and Wirral coasts add tidal risk, and managed rivers like the Alt and Sankey Brook drain catchments kept dry by pumping. We assess every source that matters on your site, then deal directly with the Environment Agency and the relevant lead local flood authority — Liverpool, Sefton, Knowsley, St Helens or Wirral — on your behalf, guiding developers, planning consultants, architects, homebuyers and businesses through to a decision.

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 Merseyside at a glance

• Surface water is the main risk. Merseyside carries a high surface water flood risk compared with the rest of the North West, while its river and sea risk is lower. Liverpool and Sefton form a nationally significant surface water Flood Risk Area, with around 90,000 people in the Liverpool area living where surface water can flood.
• Five lead local flood authorities. Knowsley, Liverpool, Sefton, St Helens and Wirral each manage local flood risk, working together as the Merseyside flood and coastal partnership alongside the Environment Agency and United Utilities.
• A coast that has to be defended. The 1.1km West Kirby flood wall on the Wirral and coastal schemes at Crosby and Southport show how real tidal and coastal risk is along the Mersey and the Sefton shore.
• Flood Zone 1 is not a free pass. Since the 2025 Planning Practice Guidance update, surface water risk can trigger an assessment even on a Flood Zone 1 site in Merseyside.

Check flood risk anywhere in Merseyside

Merseyside 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 Merseyside: surface water, the coast and rivers

Surface water is the leading flood risk across Merseyside. Heavy rainfall overwhelms old, undersized drains in built-up Liverpool, Bootle, Birkenhead and St Helens. The Mersey estuary and the open coast at Sefton and the Wirral add tidal risk, and the low-lying land drained by the Alt and Sankey Brook only stays dry because it is pumped. Most sites here face more than one source at once, which is why a desktop map rarely satisfies a planning officer on its own.

Because the county is so heavily urbanised, its flood risk is now dominated by surface water rather than rivers. That does not mean rivers and the sea can be ignored: the tidal Mersey, the Dee estuary on the west of the Wirral, the River Alt and the Sankey catchment all shape risk in different parts of Merseyside, and a credible assessment has to identify which source actually drives the risk on your site. You can screen any location against the live Environment Agency data on GOV.UK Check for Flooding or through our own flood risk map.

Surface water: the dominant risk

The Liverpool and Sefton area is one of the surface water Flood Risk Areas designated as nationally significant, and Southport is identified as a separate surface water Flood Risk Area in its own right. Around 90,000 people in the Liverpool area live where surface water flooding can occur, the legacy of dense Victorian development, large areas of hard standing and a drainage network that struggles in intense rainfall. This is why a pluvial (surface water) assessment and a sound drainage strategy matter so much on Merseyside sites, even those that map as low fluvial risk.

The Sefton and Wirral coast: tidal and coastal risk

The Sefton coast runs from the Mersey at Seaforth to the Ribble at Crossens, taking in Crosby, Formby and Southport, with dunes managed under long-standing coast protection orders. On the Wirral, the Dee and Mersey estuaries put places like West Kirby, Hoylake and Wallasey at tidal risk. Coastal and estuary sites bring sea-level rise and, in places, erosion into the assessment, so they often need flood modelling and careful attention to rising sea levels and tidal flooding.

Rivers and managed catchments: the Alt and Sankey Brook

The River Alt and the wider Alt/Crossens system drain a swathe of low-lying land between the Mersey and Ribble estuaries, around a third of it urban, kept dry by around 13 pumping stations that discharge to Liverpool Bay and the Ribble. In St Helens, Sankey Brook and its tributaries are the main river flood risk, draining most of the borough to the Mersey via Warrington. These are heavily modified watercourses, and on sites near them it is essential to understand both the fluvial flood risk and whether an ordinary watercourse on or near the boundary changes the picture.

Groundwater and the Sherwood Sandstone aquifer

Much of Merseyside sits on the Sherwood Sandstone, one of the most important aquifers in the country, forming sandstone ridges at Wallasey, Birkenhead and Crosby. As Victorian-era industrial abstraction has fallen away, groundwater levels beneath parts of Liverpool and the Wirral have recovered, bringing rising-groundwater and basement-flooding problems with them — groundwater flooding was reported in basements and gardens after Storm Christoph in 2021. Because groundwater rises from below and appears on no national flood map, a groundwater flood risk assessment and an early read of the ground conditions for a soakaway are often decisive on city-centre and basement schemes here.

A history of flooding in Merseyside

Merseyside's flood record is mostly a record of surface water and the coast. Intense summer downpours have repeatedly flooded Southport and Sefton, the Leeds and Liverpool Canal has burst, and Storm Christoph showed how quickly heavy rain turns into both surface water and groundwater flooding. These dated events are the evidence a planning officer expects an assessment to engage with.

• Early 1990s — Leeds and Liverpool Canal. The canal burst its banks and flooded more than 200 properties.
• July 2010 — Seaforth. Around 50 properties were flooded in this part of Sefton.
• September 2012 — Maghull and St Helens. About 40 properties flooded in Maghull, while overtopping of the Sankey Canal at Blackbrook flooded 10 homes and 2 businesses in St Helens.
• June 2016 — Southport and Birkdale. Roughly 90 minutes of torrential rain caused flash flooding, with properties flooded internally and widespread flooding of the highway.
• 2020 — Sefton. Prolonged flooding affected Southport, Maghull, Thornton, Crossens and Formby.
• January 2021 — Storm Christoph. Widespread surface water flooding across Southport was followed by reports of groundwater flooding affecting basements and gardens.

Do you need a flood risk assessment in Merseyside?

You will normally need a flood risk assessment for development in Merseyside that falls in Flood Zones 2 or 3, on sites of one hectare or more in Flood Zone 1, and increasingly on smaller Flood Zone 1 sites where the Flood Map for Planning shows surface water risk. With surface water the dominant local risk, that last trigger catches a growing number of Merseyside applications.

The flood risk assessment for planning is the report most Merseyside applications turn on, prepared to satisfy the NPPF and its Planning Practice Guidance. Knowing which flood zone a site sits in on the Flood Map for Planning is the starting point, but the map is only the first screen — the cost and scope of an assessment then depend on the sources in play, as our guide to the cost of a flood risk assessment explains.

• Flood Zones 2 and 3. Most development needs an assessment, whether the risk is from the tidal Mersey, the coast or a river such as the Alt or Sankey Brook.
• One hectare or more in Flood Zone 1. Larger sites need an assessment even in the lowest-probability zone.
• Surface water on the Flood Map for Planning. A common trigger across urban Merseyside, even where fluvial risk is low.
• A more vulnerable use or a basement. Changing use, or building below ground over the Sherwood Sandstone, can bring groundwater into scope.

Planning policy and your lead local flood authority in Merseyside

Which authority you deal with depends on where your site is. Each of the five Merseyside boroughs — Liverpool, Sefton, Knowsley, St Helens and Wirral — is its own lead local flood authority for surface water, groundwater and ordinary watercourses, while the Environment Agency leads on the main rivers and the sea. Together they make up the Merseyside flood and coastal partnership, with United Utilities responsible for the public sewer network.

For planning, the strategic evidence sits in the Liverpool City Region Strategic Flood Risk Assessment and in each borough's local flood risk management strategy and Local Plan policies. A lead local flood authority is a statutory consultee on surface water drainage for major development, so a drainage strategy that follows the local discharge hierarchy from the start is the surest way to avoid an objection. Since recent changes to the NPPF, proportionate sustainable drainage is expected on schemes of all sizes that affect drainage.

Where policy requires it, a sequential and exception test is prepared alongside the assessment to justify the location, and detailed drainage work to discharge a drainage condition follows once permission is granted. Designing to the 2025 national SuDS standards and the four pillars of SuDS from the outset keeps the local authority on side.

Development and flood risk assessment in Merseyside

Merseyside is a regeneration county, and the kind of site you bring forward shapes the assessment you need. Getting the scope right is the difference between a clean approval and a surface water or drainage objection that holds an application up for months.

• Waterfront and dock regeneration. Schemes along the Liverpool waterfront and at Wirral Waters and Birkenhead often sit close to the tidal Mersey, needing finished floor levels, safe access and egress, and sometimes compensatory flood storage.
• City-centre and basement schemes. Dense Liverpool sites over the Sherwood Sandstone bring surface water and groundwater into play, so a defensible design water table matters rather than a guess.
• Coastal Sefton and Wirral. Development at Southport, Crosby, Formby, West Kirby and Hoylake brings tidal modelling and sea-level rise into the assessment.
• Employment and logistics land. Large impermeable sites around the port, Bootle and Knowsley turn on surface water and the right drainage information for planning.

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

Homebuyers and businesses come to us too. A flood question can affect mortgage lending and house prices, and a conveyancing search is not the same thing as a planning-grade report. Knowing which you actually need saves time and money.

How Unda helps with flood risk assessment in Merseyside

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

A flood risk assessment in Merseyside from Unda assesses every credible source — the tidal Mersey and the coast, the Alt and Sankey Brook, surface water, groundwater from the Sherwood Sandstone 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, the drainage strategy sits alongside the assessment so the two cases stay consistent and an objection has nowhere to land.

Flood risk assessments for every situation in Merseyside

• Flood risk assessment for planning. Site-specific reports that satisfy the Environment Agency and your Merseyside LPA.
• Flood risk survey for property purchase. An independent, property-specific answer for buyers, lenders and their advisers.
• Flood risk assessment for insurance. Property-level evidence for brokers and underwriters where cover or premiums are in question.
• Flood risk assessment for BREEAM. Structured around Pol 03 so credits are not lost at assessment stage.
• Sequential and exception test reports. Locational justification prepared where policy requires it.
• Surface water drainage strategy and SuDS. Designed to each borough's standards from the first sketch.
• Groundwater flood risk assessment. A defensible design water table for basement and city-centre sites on the sandstone.
• Chartered flood risk consultants. Specialists who stay involved past the report to answer objections.

Merseyside flood resources and contacts

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

• Check for flooding in Merseyside. Environment Agency live warnings, river levels and maps at check-for-flooding.service.gov.uk.
• Floodline. Advice and flood-warning sign-up on 0345 988 1188.
• Report flooding to your council. Liverpool, Sefton, Knowsley, St Helens and Wirral each take reports of surface water, groundwater and ordinary watercourse flooding as lead local flood authorities — for example St Helens Council and Wirral Council.
• Strategic Flood Risk Assessment. The Liverpool City Region SFRA sets the strategic flood risk picture for planning.
• Regional context. Merseyside flood and coastal information from The Flood Hub, which serves the North West.
• Live river levels. Whether the Alt, Sankey Brook or other rivers are running high right now via our live river levels map.

For flooding from blocked or burst sewers, contact United Utilities. For broader policy context, our notes on the Environment Agency's planning role and on the sources of flood risk are a good starting point.

Flood risk assessment in Merseyside: frequently asked questions