Flood Risk Assessment in Kent

Our team at Unda prepares comprehensive flood risk assessments and drainage strategies for residential and commercial sites across Kent. A flood risk assessment in Kent shows your local planning authority that a development will stay safe from flooding for its whole lifetime, without pushing flood risk onto land or property nearby.

Kent carries almost every kind of flood risk England has. The River Medway and its tributaries flood the valleys of the west; the Great Stour winds through Ashford and Canterbury in the east; the Thames and Medway estuaries, the Swale and the vast low-lying expanse of Romney Marsh bring tidal and coastal risk along tens of miles of coastline; and surface water now overwhelms drains in the towns after heavy rain. We assess every source that matters on a given site, then deal directly with the Environment Agency and the relevant Lead Local Flood Authority 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.

If you need a quote or want to talk a Kent site through, contact one of our experienced flood risk consultants on 01293 214444.

Flood risk assessment in Kent at a glance

• The River Medway drives the west. The Medway and its tributaries — the Beult, Teise, Eden and Len — converge around Tonbridge, Yalding and Maidstone, where the county's worst river floods have happened.
• A long tidal coastline. The Thames and Medway estuaries, the Swale, Thanet and Romney Marsh place large parts of north and south-east Kent behind sea and tidal defences rather than out of reach of the water.
• Two Lead Local Flood Authorities. Kent County Council covers the twelve district and borough councils; Medway Council is a separate unitary authority and its own LLFA. We work with both.
• 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.

Check flood risk anywhere in Kent

Kent 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 Kent: rivers, the coast and surface water

Kent faces fluvial, tidal, surface water and groundwater flooding, and many sites face more than one at once. River flooding concentrates along the Medway in the west and the Stour in the east; tidal and coastal risk runs along the Thames and Medway estuaries, the Swale and Romney Marsh; and intense rainfall increasingly overwhelms drains in the towns. Establishing which source governs a site is the heart of a credible assessment.

The Medway is the county's largest river, gathering the Beult, Teise, Eden and Len before it passes Tonbridge and Maidstone and opens into its tidal estuary at the Medway towns. In the east, the Great Stour and East Stour run through Ashford and Canterbury towards Sandwich Bay, while the Darent and Cray drain the north-west towards the Thames. Knowing how each behaves — and where the river gives way to tidal influence — is what separates a robust fluvial flood risk assessment from a generic one.

• Rivers (fluvial). The Medway, Beult, Teise, Stour, Darent and Cray flood their valley floors. Land in Flood Zones 2 and 3 along these corridors almost always needs an assessment.
• Tidal and coastal. A tidal flood from the Thames or Medway estuary, the Swale or the open Channel coast is governed by surge and sea level rather than rainfall, and is rising as the sea does.
• Surface water (pluvial). Now the most widespread flood risk in England. Intense rain overwhelms drains in built-up Maidstone, Ashford, Canterbury and the Medway towns. Since the 2025 PPG update, surface water on the Flood Map for Planning can trigger an assessment even in Flood Zone 1.
• Groundwater. The chalk of the North Downs holds water that rises after a wet winter and floods low ground and basements without showing on any river map. See groundwater flooding.

Coastal and tidal Kent

More of Kent sits behind tidal defences than people expect. The north Kent marshes along the Thames and Medway estuaries, the Swale separating the Isle of Sheppey from the mainland, and the whole of Romney Marsh are low-lying ground kept dry by sea walls, embankments and shingle banks. These places do not screen as low risk: they screen as defended, which is a different thing. A site there has to be assessed for what happens during a tidal surge and as sea levels rise over the development's lifetime, including the residual risk if a defence were overtopped or breached.

Surface water and groundwater

Away from the rivers and the coast, the day-to-day risk in Kent's towns is surface water. Hard surfaces shed rain faster than Victorian and mid-century drainage can carry it, and the run-off collects in the low points of Maidstone, Ashford, Canterbury and the Medway towns. On the dip slope of the North Downs, chalk groundwater adds a second, slower hazard that undermines soakaway assumptions and floods basements. Both are the Lead Local Flood Authority's territory rather than the Environment Agency's — see the role the LLFA plays.

A history of flooding in Kent

Kent's flood history is written in dates: the 1953 North Sea surge along the north coast, the River Medway floods that hit Tonbridge, Maidstone and Yalding in 2000 and again at Christmas 2013, and the Great Stour floods around Ashford and Canterbury in 2024. Each one shows where the water goes when conditions line up.

• 1953 — the North Sea flood. The tidal surge of 31 January to 1 February 1953 overwhelmed sea defences along the north Kent coast and the Isle of Sheppey, the disaster that drove a century of coastal defence investment.
• Autumn 2000 — the Medway. The Medway and its tributaries burst their banks in October 2000, flooding homes in Tonbridge, Maidstone and the villages of the Weald.
• Christmas 2013 — over 900 properties. During winter 2013/14 more than 900 homes and businesses flooded across Tonbridge, Maidstone, Yalding and East Peckham from the Medway and its tributaries; 56 families in Yalding alone spent Christmas in temporary accommodation, and a tidal surge struck the north Kent coast the same December.
• 2024 — the Great Stour. Prolonged rain flooded low-lying land and roads around Ashford and Canterbury in February and October 2024; the Environment Agency estimates around 2,000 properties are at risk from the Stour in Canterbury.

Dated, local flood history is evidence, not background colour: it tells you how a site has behaved before, and a credible assessment uses it.

Flood defences and schemes across Kent

Large parts of Kent rely on engineered defences. The Leigh Flood Storage Area shields Tonbridge on the Medway; the Romney Marshes are kept dry by an Internal Drainage Board and a wall of shingle and sea defences; and the Thames Estuary 2100 plan governs the long-term tidal defence of north Kent. A defended site still needs its residual risk assessed.

• Leigh Flood Storage Area. Between Leigh and Penshurst, this 1.3 km embankment holds back the Medway to protect around 1,200 homes and businesses downstream. A £21 million expansion scheme is raising its capacity by about 25% to better protect more than 1,400 properties in Tonbridge and Hildenborough.
• Romney Marshes Area IDB. The Internal Drainage Board maintains around 220 miles of drainage ditches across roughly 130 square miles of reclaimed marsh that sits below the level of high tides, behind shingle banks and sea walls from Hythe to Dungeness.
• Medway Flood Partnership. The Environment Agency, councils and partners coordinate flood schemes across the Medway catchment, of which the Leigh expansion and Hildenborough embankments are part.
• Thames Estuary 2100. The Environment Agency's long-term plan for managing tidal flood risk along the Thames, including the north Kent frontage at Dartford and Gravesham, as sea levels rise.

Defences change the question rather than remove it. A site behind the Leigh barrier, on Romney Marsh or along the estuary is not free of flood risk; it carries a residual risk that has to be quantified, and any loss of floodplain storage usually has to be compensated level for level.

Planning policy and your Lead Local Flood Authority in Kent

Flood risk in Kent is judged against national policy — the NPPF and its Planning Practice Guidance — applied locally by two Lead Local Flood Authorities, the Environment Agency and twelve district planning authorities, each with its own Strategic Flood Risk Assessment and Local Plan policies. A good report speaks to all of them.

Who is the Lead Local Flood Authority in Kent?

Kent has two. Kent County Council is the LLFA for the twelve districts — Ashford, Canterbury, Dartford, Dover, Folkestone & Hythe, Gravesham, Maidstone, Sevenoaks, Swale, Thanet, Tonbridge & Malling and Tunbridge Wells — and is the statutory drainage consultee on major applications there. Medway Council is a separate unitary authority and acts as its own LLFA across the Medway towns. Both manage surface water, groundwater and ordinary watercourse flooding, while the Environment Agency leads on the main rivers and the sea.

Sustainable drainage (SuDS) requirements in Kent

Kent County Council publishes a Drainage and Planning Policy Statement and its own sustainable drainage guidance, and expects new development to follow the SuDS hierarchy — reuse or store run-off, then infiltrate, then discharge to a watercourse, with sewers a last resort. Under the latest NPPF, paragraph 182 expects proportionate SuDS on applications that affect drainage, regardless of size, while the government has confirmed it will not commence Schedule 3 of the Flood and Water Management Act. In practice that means a credible SuDS strategy remains the route to compliance on Kent sites, usually restricting run-off to greenfield rates.

When you need a flood risk assessment in Kent

You will usually need a flood risk assessment in Kent if your site is in Flood Zone 2 or 3, is a hectare or more in Flood Zone 1, carries surface water risk on the Flood Map for Planning, sits near the estuary or coast, or changes to a more vulnerable use. The table below is a quick guide; the flood zones are defined by the Environment Agency.

The reasons people come to us reflect how varied Kent is. Brownfield regeneration on the Thames riverside — the Ebbsfleet area alone is planned for around 15,000 homes — sits close to tidal risk. Growth around Ashford and Canterbury runs into the Stour floodplain. Schemes in the Weald meet the Medway and its tributaries, and rural and coastal conversions across the marshes meet defended tidal risk. Each setting changes what the assessment has to prove.

How Unda helps with flood risk assessment in Kent

Unda are experts on flood risk and planning in Kent. We have prepared more than 5,000 flood risk assessments since 2014, we are a CIWEM business partner, and a senior flood risk consultant signs off every report. We act as the accurate translation layer between your scheme and the Environment Agency, the LLFA and national policy — and reply to every quote within 60 minutes.

Our flood risk consultants know how Kent behaves: which Medway tributary drives a Weald site, where the Stour floodplain bites around Ashford, how a north Kent estuary site reads once you account for the defences, and when chalk groundwater on the Downs will undermine a soakaway. We do not template reports. We assess the site in front of us, design proportionate mitigation, and stay involved past the report to answer the questions the Environment Agency or local authority come back with.

That same approach — reading the modelled data against the real ground — is how we clarified a high-risk surface water flag so a purchase could proceed, and it is how we approach a Kent planning site. Start a Quote

Flood risk assessments and related reports for every situation in Kent

A flood risk assessment, a sequential or exception test and a drainage strategy are three different reports. They often travel together in a Kent planning submission, but they answer different questions: is the site safe, should the development be here, and where does the surface water go. We prepare each of them, for whatever reason your scheme needs one.

• Flood risk assessment for planning. The site-specific report that satisfies the NPPF and its Planning Practice Guidance across Kent. FRA for planning.
• Flood risk survey for a property purchase. An independent, property-specific view for buyers and their advisers when a search flags risk. Homebuyer flood risk survey.
• Flood risk assessment for insurance. Property-level evidence for brokers and underwriters when cover or premiums are in question. FRA for insurance.
• Flood risk assessment for BREEAM. Structured around BREEAM Pol 03 for project teams pursuing certification. BREEAM FRA.
• Sequential and exception tests. The locational justification that steers development to lower-risk land — a live policy area, as the Kent Faversham appeal showed. Sequential & exception tests.
• Surface-water drainage strategy and SuDS. How run-off is managed to meet Kent County Council and Environment Agency requirements, with detailed design to discharge drainage conditions. Drainage strategy & SuDS.
• Groundwater and flood modelling. A groundwater assessment sits within the FRA where chalk or gravels matter; hydraulic flood modelling derives accurate levels where national maps are not enough. Groundwater flood risk.

Kent flood resources and emergency contacts

If you need to report flooding, check a river level or prepare for a flood in Kent, these are the bodies to contact. In a life-threatening flood emergency always call 999.

Before commissioning an assessment, you can screen a site yourself with Unda's free tools: check modelled flood risk on our Flood Risk Map, watch nearby rivers such as the Medway on the Live River Levels Map, read the geology under a postcode for soakaway feasibility on the Geology & Ground Conditions map, and size drainage with our drainage calculators and BRE 365 infiltration calculator. You can also check the official flood warnings for Kent and the Flood Map for Planning.

Flood risk assessment in Kent: frequently asked questions