Flood Risk Assessment in Stratford, London

Stratford, in the London Borough of Newham (not to be confused with Stratford-upon-Avon in Warwickshire), sits low in the Lower Lea Valley, where the River Lea's back channels once flooded so badly that flood defence justified a lock of its own. A flood risk assessment in Stratford, London has to weigh surface water on some of the capital's tightest sewer capacity, the fluvial and formerly tidal River Lea, and a development pipeline still rising on remediated marshland. Unda prepares flood risk assessments in Stratford for planning, property purchase, mortgage and insurance, and BREEAM, alongside sequential and exception tests and surface-water drainage strategies.

Speak to one of our experienced flood risk consultants on 01293 214444 or enquiries@unda.co.uk for a free, no-obligation quote.

Newham has the lowest sewer capacity of any London borough, and Stratford's Queen Elizabeth Olympic Park was built on reclaimed marshland that flooded so severely in 1947 it justified a flood relief channel running all the way to Ware in Hertfordshire.

• Dominant risk: surface water, driven by Newham's Victorian combined sewer network, assessed as the lowest-capacity system of any London borough.
• Main river: the River Lea (Lee), plus the heavily engineered Bow Back Rivers running through the Queen Elizabeth Olympic Park.
• Tidal history: the lower Lea was tidal as far as Stratford until Three Mills Lock closed the tide out in 2009.
• Lead Local Flood Authority: the London Borough of Newham, for surface water, groundwater and ordinary watercourses.
• Development context: one of Europe's largest urban regeneration areas, still building out on remediated former industrial and marsh land.

Flood risk and hydrology in Stratford, London

Flood risk in Stratford is shaped by three overlapping sources: surface water across a dense, historically marshy urban area; the fluvial and formerly tidal River Lea and its distributaries; and shallow groundwater and made ground beneath a site with a long industrial past. A flood risk assessment in Stratford has to work through all three, because the Environment Agency's Flood Map for Planning captures the river and tidal picture but not the surface water and groundwater risk that dominates day-to-day flooding here. Our guides to surface water (pluvial) flooding, fluvial flooding and tidal flooding set out how each source is assessed, and our wider guide to the types of flood risk covers sewer and reservoir flooding too.

The River Lea (also spelled Lee) runs along Stratford's western edge, and through the Queen Elizabeth Olympic Park it splits into the Bow Back Rivers, a complex of engineered channels including the City Mill River, the Waterworks River, the Channelsea River, the Pudding Mill River and the Prescott Channel. These waterways were historically tidal as far upstream as Stratford, but since Three Mills Lock (also known as Prescott Lock) closed near the House Mill in June 2009, the tide is now locked out of most of the Bow Back Rivers, leaving a largely still, canalised network managed for amenity, flood storage and navigation.

Is Stratford still affected by the tidal Thames?

Yes, indirectly. Stratford falls within the Thames Estuary 2100 (TE2100) "Isle of Dogs and Lea Valley" policy unit, which extends north along the tidal Lea as far as Stratford and Hackney Wick. The unit carries a P5 policy, meaning defences need further upgrading to keep pace with future sea level rise, and without the Thames Barrier and the secondary tidal defences along the Thames and Lea frontages, much of the unit would carry a 0.1% or greater annual chance of tidal flooding. A flood risk assessment in Stratford still has to reference this residual tidal risk even though Three Mills Lock keeps day-to-day tides off the Bow Back Rivers themselves.

Groundwater and made ground beneath Stratford

Groundwater does not appear on the Environment Agency's national flood maps, but it matters in Stratford because much of the Queen Elizabeth Olympic Park was built on river terrace gravels and reclaimed industrial land. The site was contaminated throughout by decades of industrial use, and pre-development remediation processed hundreds of thousands of tonnes of soil through on-site "soil hospitals" using soil washing, bioremediation and stabilisation. Residual contamination remains in places, and shallow groundwater within made ground and gravel can behave differently from the wider water table, which is why a flood risk assessment for a basement, deep foundation or infiltration drainage scheme in this part of Stratford needs a proper groundwater section rather than an assumption carried over from the surface water picture. Our guide to how groundwater flood risk is mapped explains why national tools miss it and what evidence a site-specific assessment needs instead.

Historical flooding around Stratford

Stratford's flood history explains much of its modern infrastructure. The worst flood on record in the Lea Valley struck in March 1947, when rapid snowmelt combined with heavy rain on frozen ground sent the River Lea's flow at Lea Bridge up to around twenty times its normal rate, roughly 156 cubic metres per second. Land equivalent to some 4,000 football pitches flooded across the Lea Valley, Lea Bridge Road stood under a metre of water, and hundreds of properties were flooded, with some residents evacuated by boat. Water levels did not return to normal until 19 March.

• March 1947: the River Lea's flow at Lea Bridge reached around 20 times its normal rate; roughly 4,000 football pitches' worth of land flooded across the Lea Valley.
• 1947–1976: construction of the Lee Flood Relief Channel, running some 20 kilometres from Ware in Hertfordshire down to Stratford, in direct response to the 1947 flood.
• Since 1976: no major flood event in the Lea Valley, though the system ran close to full capacity in 1987, 1993 and 2000.
• July 2021: two flash floods two weeks apart hit London; Pudding Mill Lane DLR station, on the edge of the Olympic Park, flooded to a depth of around 0.4–0.5 metres.
• February 2026: UCL researchers highlighted Newham, Tower Hamlets and Hackney as former marshland where more than 85% of London's historic marshland has been lost to development, leaving surface water as one of east London's greatest current flood risks.

That 1947 flood is the reason the Lee Flood Relief Channel exists at all: construction began the same year and the channel became fully operational in 1976. Since then there has been no major flood event in the Lea Valley, though the system has come close to capacity three times, in 1987, 1993 and 2000. That defence record matters for a flood risk assessment in Stratford, but it also means many local drainage assumptions were set decades ago, before the July 2021 flash floods showed how quickly London's Victorian sewers (built for a city of around four million people and now serving roughly nine million, running at about 80% capacity even in dry weather) can be overwhelmed by short, intense summer downpours. Our explainer on flood return periods and annual probability and our guide to residual flood risk both help explain why a well-defended river can still sit alongside a site facing real surface water risk.

What happened in the July 2021 floods?

In July 2021, two separate flash floods hit London within two weeks of each other, with some areas receiving more than twice their average monthly rainfall in around two hours. Pudding Mill Lane DLR station, on the Bow Back Rivers at the edge of the Queen Elizabeth Olympic Park, flooded to a depth of roughly 0.4 to 0.5 metres. Across London the two events flooded more than 1,000 properties and caused over £281 million in insurance losses. That scale of disruption is now a standing reference point for surface water risk in east London, and recent academic commentary from UCL, published in February 2026, has flagged it as a growing rather than a one-off concern.

Development in Stratford and the Queen Elizabeth Olympic Park

Stratford has been one of Europe's largest urban regeneration areas since hosting the 2012 Olympic and Paralympic Games, and the pace has not slowed. The Queen Elizabeth Olympic Park now includes East Village (the former Athletes' Village), Chobham Manor, East Wick and Sweetwater, the International Quarter London business district, Westfield Stratford City, and a growing cultural and education quarter bringing V&A East, Sadler's Wells East, the BBC and UCL East to the Park. Nearly all of it sits on land reclaimed from decades of heavy industry and former marshland, so flood risk and ground conditions sit at the centre of almost every planning application in the area. Our guide to surface water flooding and planning and our piece on why urban flood risk in England is now dominated by surface water both apply directly to sites like these.

The original masterplanning for the Park set a design flood event of the 1% annual probability standard plus a 20% climate change allowance, with mitigation intended to reduce flood risk to more than 4,000 properties across the wider area. The north of the Park uses natural flood management features, among them swales and reed beds, to slow and store water rather than relying solely on piped drainage, a pattern that newer phases of development are expected to continue and that a drainage strategy for a site here should engage with directly.

Who decides planning applications on the Olympic Park now?

Since 1 December 2024, the London Legacy Development Corporation (LLDC) has no longer been the local planning authority for the Queen Elizabeth Olympic Park. Planning powers passed back to the four host boroughs (Newham, Hackney, Tower Hamlets and Waltham Forest), each now determining applications on the part of the Park within its own boundary, while the LLDC's existing Local Plan, supplementary planning documents and Community Infrastructure Levy charge remain in force until each borough adopts its own replacements. For a site anywhere near the Park boundary, working out which policy set and which authority currently applies is a genuine, easy-to-miss complication, and one a flood risk assessment or drainage strategy needs to get right from the outset. Most of Stratford itself falls within the London Borough of Newham, which is also Stratford's Lead Local Flood Authority.

Planning and drainage policy for a flood risk assessment in Stratford

Newham is both the local planning authority for most of Stratford and its Lead Local Flood Authority, responsible for surface water, groundwater and ordinary watercourse flood risk. Our guide to what a Lead Local Flood Authority does and to the Environment Agency's role in flood risk and planning set out how the two bodies work together. The council's evidence base includes a Level 1 and Level 2 Strategic Flood Risk Assessment (SFRA), last substantively updated in 2017 to cover the current Local Plan period of 2018–2033; the Level 2 SFRA alone screened 61 proposed development sites, identified 44 with significant flood risk or access and egress issues, and produced 43 detailed site summaries. Newham's Local Flood Risk Management Strategy was refreshed again in October 2024.

• Newham Local Plan policies SC1, SC2, SC3, SC4 and SP9 set out the borough's flood risk and surface water drainage requirements that a planning application must satisfy.
• SuDS requirement: major development must restrict post-development runoff to greenfield rates unless it can be demonstrated that site conditions genuinely prevent this.
• TE2100 policy unit: the Isle of Dogs and Lea Valley policy unit (P5) covers the tidal Lea as far as Stratford and sets the framework for future tidal defence upgrades.
• Sewer capacity: Thames Water has assessed Newham as having the lowest sewer capacity of any of London's 33 boroughs, projected to be over capacity within 20 to 30 years, sharpening the case for source control and SuDS on every scheme.
• London-wide policy: the London Plan and the 2025–30 London Surface Water Strategy both push new development toward sustainable drainage and away from adding load to an already stretched sewer network.

Every London borough, Newham included, also requires the London Sustainable Drainage Proforma alongside a drainage strategy at validation, and it pays to understand how foul, surface water and combined sewers differ before deciding where a scheme can discharge. Our guides to what drainage information a planning application needs, why applications get invalidated over missing flood risk or drainage documents, and flood risk and drainage for retrospective planning applications cover the process pitfalls we see most often on sites like these, alongside the national framework set out in our NPPF flood risk guidance.

A flood risk assessment in Stratford therefore has to work on two levels at once: the site-specific technical case (which flood sources actually affect this plot, and how) and the local policy case (which of Newham's, or on parts of the Olympic Park a neighbouring borough's, Local Plan policies the scheme must satisfy). Getting the LLFA's evidence base and the current policy references right the first time avoids the validation delays and holding objections that a mismatched or generic report tends to attract.

How Unda helps with flood risk assessments in Stratford

Unda has prepared more than 5,000 flood risk assessments and drainage strategies across England and Wales since 2014, working directly with the Environment Agency, Lead Local Flood Authorities and insurers on sites ranging from single homes to major mixed-use regeneration schemes. Our flood risk consultants bring that experience to Stratford's particular mix of surface water, formerly tidal watercourses and remediated ground, rather than reusing a generic London template.

A Senior Flood Risk Consultant signs off every report Unda produces, and we reply to every quote request within 60 minutes. Read more about our flood risk consultancy services or the team behind them, and see what else we've been saying about flood risk in the media.

Flood risk assessments for every situation in Stratford

We prepare flood risk assessments in Stratford for planning, property purchase, mortgage and insurance, and BREEAM certification, alongside sequential and exception tests and surface-water drainage strategies. 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 core report submitted with a planning application to satisfy the NPPF and its Planning Practice Guidance.
• Flood risk survey for property purchase. An independent, site-specific survey for homebuyers, solicitors, surveyors and lenders.
• Flood risk assessment for insurance. Property-specific evidence when an insurer, broker or underwriter has raised concerns.
• Flood risk assessment for BREEAM. Structured around BREEAM Pol 03 flood and surface water management credits.
• Sequential and Exception Test reports. The locational tests that sit alongside an FRA where flood risk affects site choice.
• Surface water drainage strategy and SuDS design. How runoff from a site in Stratford is managed to meet Newham's, and where relevant a neighbouring borough's, drainage requirements.
• Groundwater flood risk assessment. Specialist assessment of the shallow groundwater and made ground beneath much of the Olympic Park.
• Flood modelling. Detailed hydraulic modelling where standard mapping is not enough to define flood levels around the Lea and the Bow Back Rivers.

Where drainage design needs to go further than a strategy, we also handle the discharge of drainage planning conditions and preliminary drainage design at outline stage. Our free Flood Risk Map, Live River Levels Map and Geology & Ground Conditions checker are a useful first look at a Stratford site before commissioning a full report. Where infiltration drainage is being considered, our drainage calculators and BRE 365 infiltration rate calculator give a fast first-pass check, though made ground across much of the Olympic Park often rules infiltration out and points toward attenuation and controlled discharge instead.

Local flood risk management and emergency contacts in Stratford

If flooding does affect a property or site in Stratford, several bodies share responsibility for managing the risk and responding on the day.

• Report a flood emergency: call 999 if there is a danger to life; call 105 to report a power cut caused by flooding.
• Sign up for flood warnings: the Environment Agency's free flood warning service covers the River Lea and the tidal Thames.
• Check current risk: use GOV.UK's check for flooding service for live conditions, or Unda's own Flood Risk Map for a fuller postcode-level screening view.

Two further pieces of context are worth knowing if you live or work near the Lea: the Mayor of London's basement flash flood warning applies directly to the below-ground residential units increasingly built across east London, and the Thames Tideway Tunnel "super sewer" is now cutting the sewage overflows that once fed directly into the tidal Thames and Lea system. If a property has struggled to get affordable cover, our guide to Flood Re explains how the scheme works.

Frequently asked questions about a flood risk assessment in Stratford

Working on a site outside Stratford? Browse Unda's full flood risk assessments by area, or see how flood risk is shifting the wider housing market in our piece on whether flood risk affects house prices and our guide to flood risk and mortgage lending.