Flood Risk Assessment in Erith

Erith sits on the south bank of the tidal Thames in the London Borough of Bexley, a short way downstream of Thamesmead and a short way upstream of Slade Green and the Dartford boundary. Land this low and this close to a tidal river does not stay dry by accident, and a flood risk assessment in Erith has to start from that fact rather than from a flood zone map alone.

We prepare flood risk assessments in Erith for planning applications, property purchase, mortgage and insurance, and BREEAM, alongside sequential and exception tests and surface-water drainage strategies. Contact one of our experienced flood risk consultants on 01293 214444 or enquiries@unda.co.uk and we will get back to you within 60 minutes.

Erith's own sea wall gave way in the storm surge of February 1953, sending water across the marshes and leaving more than 400 people in neighbouring Belvedere homeless in a single night — which is why a flood risk assessment in Erith still has to treat the defences as something that can fail, not as a reason to stop looking.

Flood risk assessment in Erith at a glance

Four sources of flooding matter to a site in Erith, and each is reviewed by a different authority. The table below summarises how they apply before we go into the detail.

Check flood risk anywhere in Erith

Before commissioning a site-specific report, our free flood risk map gives an indicative first read on rivers and sea, surface water and flood zone for any Erith postcode, with climate change projections to 2125. Our live river levels map shows the nearest Environment Agency gauges in near-real time, our geology and ground conditions checker gives an early read on whether the river gravels beneath a site are likely to support infiltration drainage, and our drainage calculators and BRE 365 infiltration rate calculator help size a scheme once ground conditions are known. None of these replace a site-specific flood risk assessment in Erith, but they are a useful starting point.

Do you need a flood risk assessment in Erith?

Most sites fronting the Thames in Erith will need one. The common triggers are:

• Your site is on the riverside marshes or within Flood Zone 2 or 3. Almost every site fronting the tidal Thames in Erith falls into one of these categories.
• Your site is a hectare or more, even where the Flood Map for Planning shows Flood Zone 1. This covers most of the larger regeneration parcels around Erith Quarry and the wider Bexley Riverside area.
• Your site is smaller than a hectare, but the Flood Map for Planning flags surface water risk. This is increasingly common across Erith's low-lying streets since the Environment Agency's May 2026 surface water update.
• You are extending, converting or changing the use of an existing building on marsh-edge land. Flood Zone 1 does not automatically rule out the need for an assessment.
• London Borough of Bexley, as your Lead Local Flood Authority, has asked for a drainage strategy or SuDS proforma alongside your application, or the Environment Agency has raised the tidal Thames as a statutory consultee.

Flood risk in Erith: the tidal Thames, the River Cray and surface water

A flood risk assessment in Erith has to read several sources together rather than relying on a single flood zone. The tidal Thames dominates, but it is not the only source that matters here.

The tidal Thames

Erith lies within reach of North Sea tidal surges pushed up the Thames Estuary, the same mechanism that overwhelmed the town's sea wall in 1953. The Thames Barrier, upstream at Woolwich, and the wider tidal defence network reduce that risk substantially, but Erith's riverside land still sits in Flood Zone 3 on the Flood Map for Planning because the zone is drawn on the natural floodplain, defences or not. A site-specific assessment has to model what happens if a defence downstream of the Barrier is overtopped or breached, not just accept that the tide is currently held back.

The River Cray and Crayford Ness

The River Cray, which gathers the smaller River Shuttle on its way north through the borough, reaches the tidal Thames at Crayford Ness, close to Erith's eastern boundary with Slade Green. In prolonged or heavy rain the Cray can back up onto the low ground near its confluence, and a flood risk assessment for a site in eastern Erith or Slade Green has to consider that fluvial contribution alongside the tidal Thames itself.

Surface water and groundwater on the marsh

Erith's marshland and the streets built on reclaimed ground drain slowly, and the Environment Agency's Thames Estuary 2100 evidence has specifically flagged a need to upgrade the Erith Marshes drainage system as sea levels and rainfall intensity both increase. Beneath that same ground, river gravels and alluvium hold water close to the surface, which matters for basements and for any drainage strategy that hopes to rely on infiltration rather than a piped connection.

A history of flooding in Erith

Erith's flood history is not abstract. The North Sea storm surge of February 1953, the worst tidal flood disaster in England's living memory, breached the sea wall near the Callender Cable Company's jetty in Erith, tearing a hole in the defence large enough to take a London double-decker bus and sending water across the Erith and Belvedere marshes. More than 400 people in Belvedere alone were left homeless by the time of high tide the following day. Records also point to an earlier Thames flood in 1928, when the river burst its banks upstream and fourteen people drowned in central London basements, and to flooding along this stretch of river as far back as 1527.

The scale of the 1953 damage is also why Erith sits at a genuine administrative boundary in the Environment Agency's long-term Thames planning. The Thames Estuary 2100 Thamesmead Policy Unit runs from Woolwich Ferry Terminal to Erith Pier, and the Dartford and Erith Policy Unit picks up from Erith Pier onward to Greenhithe. A flood risk assessment for a site near the pier can genuinely sit at the meeting point of two separate long-term defence strategies, which is worth checking rather than assuming.

Flood defences protecting Erith

A new sea wall, built higher than the water that overwhelmed the old one, has protected Erith's riverside since shortly after 1953, and it now forms part of a wider network of tidal walls, embankments and gates along the Thames that the Environment Agency rates as giving a very high standard of protection. That standard is not the same as zero risk. The residual flood risk that remains once a defence is in place is exactly what a site-specific assessment in Erith has to quantify: what happens if a surge exceeds the design standard, if a defence is breached the way it was in 1953, or if water arrives from the marshes' own surface water and groundwater rather than over the wall.

The Environment Agency's Thames Estuary 2100 evidence for both policy units either side of Erith Pier has specifically identified the need to improve drainage across the Erith Marshes as sea levels rise and fluvial flows increase, which is a useful reminder that "defended" is a description of today, not a permanent guarantee. Anyone assessing a site here should also understand how the wider estuary is managed: our guide to the Thames Barrier explains the barrier itself, and our piece on the Thames Tideway Tunnel covers the sewer-side improvements now reducing pressure on the river during storms.

Which flood zone is your Erith site in?

The Environment Agency's Flood Map for Planning splits land into zones by the annual probability of river and sea flooding, and Erith spans more than one of them.

Our explainer on flood zones sets out how these bands are defined nationally; in Erith the zones only tell part of the story, because they ignore the surface water and groundwater sources covered above.

Planning policy and your Lead Local Flood Authority in Erith

The London Borough of Bexley adopted its current Local Plan on 26 April 2023, with Policy DP32 covering flood risk management and Policy DP33 covering sustainable drainage systems. Both draw on Bexley's Strategic Flood Risk Assessment Level 1 and its more detailed Level 2 assessment for Erith and Slade Green, which was prepared specifically to inform the borough's Erith Area Action Plan and site allocations.

Bexley is Erith's Lead Local Flood Authority and the statutory consultee on surface water drainage for major development, working alongside the Environment Agency wherever the tidal Thames is involved. Since April 2019 every London borough, Bexley included, has required the London Sustainable Drainage Proforma alongside a drainage strategy for major applications, and our guide to the SuDS proforma explains what that form needs to show. Applications should also be prepared in line with national NPPF flood risk policy, which sits above the local plan and shapes how the sequential and exception tests apply to any Flood Zone 3 site in Erith.

Development and flood risk assessment in Erith

Erith is one of the more active regeneration fronts in south-east London, and most of the activity sits close to the water. Erith Quarry, a former chalk quarry off Fraser Road, is being redeveloped by London and Quadrant and the Anderson Group for several hundred new homes across its phases, alongside a new primary school and public open space; because part of the site is a Grade 1 Site of Importance for Nature Conservation and carries contamination and landfill gas legacy issues, its flood risk assessment and drainage strategy have to sit alongside that wider technical picture rather than stand alone. A short distance north, the Erith Riverside scheme, designed by Pollard Thomas Edwards, has been granted planning permission for riverside regeneration directly on the tidal Thames frontage.

Both schemes sit within the Bexley Riverside Opportunity Area, which the Greater London Authority identifies as having indicative potential for around 6,000 new homes and 19,000 new jobs along this stretch of the Thames. That scale of ambition, on land this exposed, is exactly why a flood risk assessment in Erith needs to be built on genuine local evidence rather than a generic template: the Environment Agency, Bexley as Lead Local Flood Authority, and lenders and insurers further down the line will all expect it.

What your flood risk assessment in Erith must show

• Climate change allowances. Design flood levels for the tidal Thames and the River Cray need to be tested against the Environment Agency's climate change allowances, not just today's conditions.
• Finished floor levels. National policy expects new floor levels set at a safe freeboard above the design flood level, referenced to Ordnance Datum.
• Safe access and egress. A route away from a flood event, not just a dry building, for the lifetime of the development.
• No increase in flood risk elsewhere. Where a scheme raises ground levels on the marsh, compensatory flood storage may be needed so displaced water does not simply move onto a neighbour.
• Every source of flooding, not just the river. Surface water and groundwater both need their own evidence in Erith, alongside the tidal Thames.
• The residual, defence-failure case. Given Erith's own history, a credible assessment has to show what happens if a defence is overtopped or breached, not only what happens while it holds.

How Unda helps with flood risk assessment in Erith

Our flood risk consultants bring together tidal, fluvial, surface water and groundwater evidence into a single report that satisfies the Environment Agency and the London Borough of Bexley, whether the site is a single infill plot or part of a larger Bexley Riverside allocation. Where flood modelling is needed to establish accurate levels near the tidal defences, or a surface water drainage strategy is needed to satisfy Bexley's SuDS requirements, we handle both alongside the core assessment.

Read more about who we are and how our flood risk consultants work with developers, planning consultants, homebuyers and insurers across England and Wales.

Flood risk assessments for every situation in Erith

We prepare flood risk assessments in Erith for planning, for property purchase, for insurance, and for BREEAM, alongside sequential and exception tests, surface-water drainage strategies and the discharge of drainage planning conditions. Erith's own flood history means groundwater and flood modelling often feature too, and both are covered within the wider assessment rather than treated as separate reports.

Erith sits between two of our other local guides: the tidal Thames continues upstream through Woolwich and Greenwich, and downstream through Slade Green towards Dartford, each with its own local flood risk detail.

Frequently asked questions

Local flood risk management and emergency contacts in Erith

• London Borough of Bexley. Lead Local Flood Authority for surface water, groundwater and ordinary watercourses, and the first point of contact for drainage strategy queries.
• Environment Agency. Statutory consultee for the tidal Thames and main rivers; sign up for flood warnings covering the tidal Thames in the boroughs of Bexley and Greenwich free of charge.
• Floodline. Call 0345 988 1188 for live flood warning information at any time.
• Thames Regional Flood and Coastal Committee. Oversees flood and coastal erosion risk management investment across the wider Thames catchment, including the Erith frontage.
• Thames Water. Report sewer flooding or a suspected blocked or collapsed public sewer serving Erith.
• In an emergency. Call 999 if you or anyone else is in immediate danger from flooding, or 105 to report a power cut.