Flood Risk Assessment in Thamesmead

Most of Thamesmead was marsh within living memory of the people who built it. The town straddles the Royal Borough of Greenwich and the London Borough of Bexley on reclaimed Erith and Plumstead Marshes on the south bank of the Thames, protected today by river walls that stand downriver of the Thames Barrier rather than behind it. That history is why flood risk here is rarely a single-source question: a site can face tidal risk from the river, fluvial risk from the historic marsh drainage system, surface water on flat, low-permeability ground and groundwater close to the surface, all at once — and depending which side of the town it falls on, it will answer to a different Lead Local Flood Authority.

We prepare flood risk assessments in Thamesmead for planning, property purchase, mortgage and insurance, and BREEAM, alongside sequential and exception tests and surface-water drainage strategies (SuDS). We assess every source that matters on this stretch of reclaimed marsh — the tidal Thames, the Erith and Plumstead Marshes drainage system, surface water, groundwater and the sewer network around Crossness — then deal directly with the Environment Agency, the Royal Borough of Greenwich and the London Borough of Bexley on your behalf, whichever authority your site falls under. Our flood risk consultants know Thamesmead's regeneration sites well, and guide architects, developers, planning consultants, home buyers and businesses through to a decision.

For a quote or to talk a site through, contact one of our experienced flood risk consultants on 01293 214444 or enquiries@unda.co.uk.

Thamesmead's original tower blocks were built with living space raised above ground level and cars parked below — a direct design response to the flooding that inundated this stretch of marsh in 1953 — and the flood risk that shaped that design has not gone away.

Flood risk assessment in Thamesmead at a glance

• Two Lead Local Flood Authorities. Thamesmead is split between the Royal Borough of Greenwich (the west of the town) and the London Borough of Bexley (Belvedere, Erith and the Abbey Wood extensions to the east) — each responsible for surface water, groundwater and drainage in its own part of the town.
• Built on reclaimed marshland. Much of Thamesmead sits on the former Erith and Plumstead Marshes, ground the Environment Agency's own mapping still tests against tidal, fluvial, surface water and groundwater flooding together.
• A defined Environment Agency policy unit. The Thames Estuary 2100 plan designates a Thamesmead Policy Unit running from Woolwich Ferry Terminal to Erith Pier, covering the residential town, the Belvedere industrial and retail estates, and Crossness Sewage Treatment Works.
• Flood Zone 3 across much of the built area. Bexley's own Strategic Flood Risk Assessment maps large parts of Thamesmead, Abbey Wood and Belvedere within Flood Zone 3, the Environment Agency's highest-probability flood zone.
• A live regeneration area. Peabody's phased masterplan is delivering thousands of new homes across Thamesmead, which keeps flood risk and drainage evidence squarely in view for every application that follows.

Understanding flood risk in Thamesmead

Flood risk in Thamesmead comes from several sources at once: tidal flooding from the Thames, fluvial flooding from the historic Erith and Plumstead Marshes drainage system, surface water where heavy rain outpaces a low, flat drainage network, groundwater rising through reclaimed marsh ground, and the wider urban drainage and sewer network around Crossness. The Environment Agency names all of these for its Thamesmead Policy Unit, and a credible assessment has to read them together rather than screen against just one.

• Tidal (the Thames). The defining risk for the riverside. Thamesmead is defended by river walls downriver of the Thames Barrier, and a surge on a high spring tide is the scenario those defences — and every tidal flood risk assessment behind them — is built around.
• Fluvial (the marsh drainage system). The Environment Agency specifically names the Marsh Dykes and the wider Plumstead and Erith Marshes drainage system as a fluvial source — the network of channels and outfalls built to drain reclaimed land, which can overflow its banks in heavy rain.
• Surface water (pluvial). Now the most widespread source of flood risk in England, and named by the Environment Agency as a serious risk for Thamesmead specifically because the ground is "low lying and very flat", so local drainage capacity is quickly outpaced. See our note on surface water flooding.
• Groundwater. Reclaimed marsh ground holds a high water table close to the surface, which matters for basements, undercrofts and any soakaway. See groundwater flooding.
• Sewers and urban drainage. Crossness Sewage Treatment Works sits inside the Thamesmead Policy Unit itself, at the end of Joseph Bazalgette's Southern Outfall Sewer, and surcharging drains add to the picture in extreme rain. See our guide to sewer flooding and to foul, surface and combined sewers.

A remnant of the marsh drainage era survives as the Broadwater, a section of a convict-built canal that once moved materials from the Thames to the Royal Arsenal; the Southern Outfall Sewer itself is landscaped into an elevated footpath, the Ridgeway, that forms much of Thamesmead's southern boundary from Plumstead to Crossness. Both are a reminder that the ground here has been engineered to drain for two centuries, and that engineering — not the absence of risk — is what keeps it dry.

A history of flooding on the Thamesmead marshes

Thamesmead's flood defences exist because this stretch of the Thames has flooded before, twice within living memory in the twentieth century, and both events still shape how flood risk is assessed here today.

• 7 January 1928. A high spring tide met a North Sea storm surge and the Thames poured over its embankments; 14 people drowned across riverside London, with flooding recorded downriver as well as in the capital.
• 31 January–1 February 1953. The Thames breached its bank at Erith near the Callender Cable Company's jetty as the sea wall collapsed, and burst its banks at Belvedere in the early hours; more than 400 people in Belvedere alone were left homeless by high tide and sheltered in local church halls, the floodwater cut the railway line at Belvedere station and flooded Picardy Street. Queen Elizabeth II visited the flooded streets, including Crabtree Manor Way, Belvedere, less than a fortnight later. A new sea wall, built four feet above the 1953 tide line, followed. Nationally, the North Sea flood killed more than 300 people along the English coast and estuary.
• The Thames Barrier, 1982. The scale of the 1953 disaster led the Waverley Committee to recommend a closable barrier rather than ever-higher walls; it became operational in 1982 and was formally opened in 1984.

Because Thamesmead was designed and largely built after 1953, its planners answered the flood risk directly: the original masterplan raised living accommodation to first-floor level and above, linked by elevated walkways, with ground-floor space left for garaging rather than homes. That is not a historical curiosity — it is the clearest evidence anywhere in London that flood risk on this stretch of marsh has always driven how a site is designed, and it is exactly the same question a flood risk assessment answers on a Thamesmead site today.

Thames Estuary 2100 and flood defence in Thamesmead

Thamesmead sits downriver of the Thames Barrier rather than behind it, so its tidal protection comes from raised river walls and a system of channels with outfalls into the Thames, not from the Barrier's gates directly. The Environment Agency's Thames Estuary 2100 (TE2100) plan is the adaptive, hundred-year strategy that manages tidal flood risk across the whole estuary, and it assigns Thamesmead a "P4" policy: further action is expected over coming decades to keep pace with climate and land-use change, working with other risk management authorities so flood risk does not increase. The same P4 policy runs from Thamesmead through Tilbury, Canvey Island, Southend-on-Sea and the Isle of Grain, in contrast to the "P5" policy that applies immediately upstream around Greenwich and Woolwich.

The Environment Agency has also flagged that the Erith Marshes drainage system will need improving as sea levels rise and rainfall intensifies. That means controlling run-off, enlarging drainage channels, increasing flood storage and improving outfall capacity. There is currently no agreed riverside strategy for this stretch either, with councils expected to work with communities on one over the coming years — a live gap for anyone planning here for the long term, not a settled position.

Crossness, at the eastern edge of the policy unit, is where Joseph Bazalgette's Southern Outfall Sewer has terminated since the 1860s; it was upgraded at a cost of around £220 million between 2010 and 2014, and now also connects to the Thames Tideway Tunnel, London's new "super sewer", which intercepts many of the combined sewer overflows that used to spill into the Thames in heavy rain. Upstream, delays to the wider River Thames Scheme are a useful reminder for any Thamesmead site: a planning application cannot rely on flood defences that have not yet been delivered. Read more on the Thames Barrier and on why a defended site still shows as Flood Zone 3 on the statutory map.

Planning and drainage requirements for developers in Thamesmead

A flood risk assessment in Thamesmead has to satisfy national planning policy, the London Plan, and whichever borough's requirements apply to the site — and because Thamesmead spans two boroughs, working out which authority is which comes first.

• Royal Borough of Greenwich (west Thamesmead). The Lead Local Flood Authority for surface water, groundwater and ordinary watercourses in its part of the town, and the statutory consultee on drainage for major development. Its Strategic Flood Risk Assessment and Local Flood Risk Management Strategy set the local evidence base, alongside its adopted Core Strategy flood risk policy (Policy E(a)), with a new Local Plan currently in preparation.
• London Borough of Bexley (Belvedere, Erith & Abbey Wood). The LLFA for the eastern part of Thamesmead. Its Local Plan, adopted 26 April 2023, sets flood risk and drainage requirements through Policy DP32 (flood risk management) and Policy DP33 (sustainable drainage systems), backed by its own Strategic Flood Risk Assessment, including a dedicated appendix for the Thamesmead & Abbey Wood and Belvedere growth areas.
• The London Plan. Policy SI 12 requires development to manage flood risk and work with the Thames Estuary 2100 approach; Policy SI 13 sets the London drainage hierarchy — reuse, then infiltration, then green attenuation, then a watercourse, and only then a sewer. Our summary of the 2025-30 London Surface Water Strategy sets out what this means in practice, including that London has around 320,000 properties at high surface water risk and roughly 56,000 vulnerable basement dwellings.
• The London Sustainable Drainage Proforma. Every London borough, including both Greenwich and Bexley, requires this Greater London Authority proforma alongside a drainage strategy for major development — so a Thamesmead site that straddles the boundary needs to check which council's version applies. Our explainer on the SuDS proforma and our dedicated page on the London Sustainable Drainage Proforma cover what it needs.
• Sequential and exception tests. With so much of Thamesmead in Flood Zone 3, higher-vulnerability schemes often need to pass the sequential and exception tests, showing why the site is the most appropriate location available and that it can be made safe without increasing risk elsewhere.

Because the ground across Thamesmead is reclaimed marsh and largely impermeable, infiltration drainage is rarely straightforward, so most schemes attenuate and discharge at a controlled rate under the London hierarchy. Ground conditions can be screened early with our geology and ground conditions map, and where infiltration is proposed it is tested to BRE Digest 365 using our BRE 365 infiltration calculator and sized with our drainage calculators. Our guides to avoiding planning invalidation over flood risk and drainage documents and to fixing an LLFA drainage objection cover the most common ways a Thamesmead submission stalls.

Flood risk assessments for every situation in Thamesmead

A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct reports. They often travel together in a Thamesmead planning submission, but they answer different questions: is the site safe, should the development be here, and where does the surface water go. We produce all three, along with the assessments people need outside the planning system.

• Flood risk assessment for planning. The core report for a planning application in Thamesmead, showing a site is safe for its lifetime without increasing risk elsewhere. See FRA for planning.
• Flood risk assessment for property purchase. A site-specific answer for buyers and lenders when a search flags flood risk on a Thamesmead property. See flood risk for property purchase.
• Flood risk assessment for insurance. Evidence for brokers and underwriters where cover on a Thamesmead property is restricted, declined or priced on broad national data. See FRA for insurance.
• Flood risk assessment for BREEAM. Structured around BREEAM Pol 03, framed differently from a planning FRA for certified schemes in the regeneration area. See BREEAM flood risk assessment.
• Sequential and exception tests. The locational justification for building on higher-risk land. See sequential and exception tests.
• Surface-water drainage strategy and SuDS. Managing run-off under the London drainage hierarchy, with preliminary drainage design early and discharge of drainage conditions later. See drainage strategy.
• Groundwater assessment. Groundwater is assessed within the FRA as one of its sources, and matters especially on reclaimed marsh ground. See groundwater.

Regeneration in Thamesmead and why flood evidence matters now

Thamesmead is one of London's largest live regeneration programmes, and almost all of it sits on the same defended, reclaimed marshland this page describes — which is exactly the combination that puts flood risk and drainage evidence at the centre of getting permission.

• South Thamesmead. Peabody and Lovell Partnerships are delivering a masterplan of around 2,800 homes close to Abbey Wood station in phases, including an initial phase of 534 homes (55% affordable) and a second phase of 329 homes (42% affordable).
• Lesnes (Thamesmead East). Bexley Council has approved plans for up to 1,950 further homes, with a minimum of 35% affordable, as later phases of the wider regeneration.
• The long-term vision. Local ambitions for the whole Thamesmead regeneration area point to up to 20,000 new homes over the coming decades — a separate, wider figure from any single phase.
• Southmere. Peabody has invested around £2.5 million improving Southmere Lake's water quality, biodiversity and access, alongside new housing beside the regenerated lake.
• Tump 53. A former wartime ammunition store, now a nature reserve of woodland, meadow and pond, sits among the safeguarded natural assets the regeneration is built around.
• The proposed DLR extension. Transport for London has consulted on a Docklands Light Railway extension from Gallions Reach via Beckton Riverside to Thamesmead, under a new Thames tunnel — a scheme TfL says could support up to 30,000 new homes along its wider corridor, distinct from Thamesmead's own masterplan total.

High-density residential development on Flood Zone 3 reclaimed marsh is precisely the case flood risk planning policy was written for: finished floor levels, safe access and escape, residual breach risk and a compliant drainage strategy all have to be demonstrated before a scheme like this can proceed, whichever phase or borough it falls in.

How Unda helps with flood risk assessment in Thamesmead

We are flood risk and drainage specialists who know the tidal Thames and its planning framework well. We have prepared flood risk assessments and drainage strategies for defended riverside and reclaimed marsh sites across London and the estuary, and we deal with the Environment Agency and the relevant Lead Local Flood Authority directly so you do not have to.

For a Thamesmead site that usually means confirming which borough's requirements apply, establishing the tidal flood levels and their climate-change allowance, reading the marsh drainage system alongside surface water and groundwater, setting finished floor levels and safe access and escape, and building a drainage strategy that follows the London hierarchy and the right council's SuDS proforma. Where the national maps are not enough, we model the site; where infiltration is doubtful, we test and design around it.

Our flood risk consultants work for architects, developers, planning consultants, home buyers and businesses, and you can see the wider areas we cover on our flood risk assessment areas hub, including neighbouring Thames-side areas such as Greenwich, Woolwich, Dartford and Gravesend further down the estuary.

Local flood risk management and emergency contacts in Thamesmead

Several bodies share responsibility for flooding in Thamesmead, split between two boroughs. The Environment Agency manages the tidal Thames and issues flood warnings; the Royal Borough of Greenwich and the London Borough of Bexley each act as Lead Local Flood Authority for their own part of the town, handling surface water, groundwater and ordinary watercourses; and Thames Water is responsible for the public sewers, including the Crossness catchment. In an emergency, the routes below are the ones that matter.

If you are checking a specific property, start with the Environment Agency's free flood warning sign-up service and the Thamesmead Policy Unit page for the wider Thames Estuary 2100 picture, and keep an eye on levels through our live river levels map. For the planning framework London-wide drainage sits in, see the London Sustainable Drainage Proforma guidance. Our notes on London's basement flood risk and on urban surface water risk across England put Thamesmead's position in a wider context.

Prefer to talk it through first? Call one of our experienced flood risk consultants on 01293 214444 or email enquiries@unda.co.uk, and we will help you work out exactly what your Thamesmead site needs.