Flood Risk Assessment Kent

Flood Risk Assessment Services in Kent

Kent’s geography places it at the intersection of multiple flood risk challenges. The county’s extensive coastline along the English Channel and Thames Estuary, the River Medway and its substantial catchment, low-lying marshland areas, and chalk downland create a diverse range of flood risks that developers must navigate. As one of England’s fastest-growing counties, with significant housing targets in the Local Plans of its 12 district and borough councils, the demand for flood risk assessment is consistently high.

Aegaea provides specialist flood risk assessment, hydraulic modelling, and drainage consultancy services across Kent. Our team has detailed knowledge of the county’s coastal, fluvial, and groundwater flood risks, the Environment Agency’s flood defence infrastructure, Kent County Council’s LLFA requirements, and the specific planning policies of each district and borough.

Kent’s Flood Risk Landscape

The Thames Estuary

The Thames Estuary defines the northern boundary of Kent, from Dartford in the west to the Isle of Sheppey in the east. The estuary’s tidal influence exposes extensive areas of North Kent to tidal flood risk, particularly the low-lying marshlands of the Hoo Peninsula, the Isle of Grain, and the northern coast of the Isle of Sheppey.

The Thames Estuary 2100 (TE2100) Plan, published by the Environment Agency, sets out the long-term strategy for managing tidal flood risk in the estuary over the next century. The plan identifies different management approaches for different sections of the estuary, ranging from maintaining existing defences to raising defence levels and, in some areas, managed realignment. For Kent, the TE2100 Plan has significant implications for development in the North Kent Marshes and along the Medway Estuary.

The 1953 North Sea flood remains the most significant tidal flood event in the Thames Estuary’s modern history. The storm surge on the night of 31 January to 1 February 1953 overwhelmed sea defences across the East Coast and Thames Estuary, killing over 300 people in England and causing devastating damage. In Kent, the Isle of Sheppey, Whitstable, and the North Kent Marshes were severely affected. The 1953 event led directly to the construction of the Thames Barrier and the upgrade of tidal defences throughout the estuary.

More recently, the December 2013 tidal surge event tested the estuary’s defences, and while the Thames Barrier and upgraded defences performed well, the event demonstrated the continued vulnerability of low-lying areas to extreme tidal events. Climate change and sea level rise will progressively increase tidal flood risk in the estuary, and the TE2100 Plan anticipates the need for continued investment in defences and, potentially, a new Thames Barrier later this century.

Development in the Thames Estuary zone requires detailed assessment of tidal flood risk, including the consequences of defence breach and overtopping. The Environment Agency’s breach modelling for the Thames Estuary provides data on predicted flood depths, velocities, and timing in defence failure scenarios, and this data is essential for setting finished floor levels and designing flood resilience measures.

The River Medway

The River Medway is Kent’s principal river, with a catchment of approximately 1,700 square kilometres extending from the High Weald through Tonbridge, Maidstone, and the Medway Towns to the Medway Estuary. The Medway and its major tributaries, the River Beult, River Teise, and River Eden, create an extensive fluvial flood risk across central and western Kent.

The autumn 2000 floods were a defining event for the Medway catchment. Prolonged October rainfall saturated the heavy clay soils of the Weald, and intense rainfall on 11-12 October generated devastating flooding along the Medway, Beult, and Teise. Tonbridge, Yalding, Collier Street, and Maidstone were severely affected, with Yalding experiencing some of the worst flooding seen anywhere in England during the event. The total damages in the Medway catchment exceeded 100 million pounds.

The Leigh Flood Storage Area, located upstream of Tonbridge, provides significant flood attenuation by storing floodwater on agricultural land behind an embankment. The storage area, managed by the Environment Agency, can hold approximately 5.5 million cubic metres of water, reducing peak flows through Tonbridge by up to 35 percent. The Leigh FSA was instrumental in reducing flooding during the December 2013 event, which would otherwise have caused significant damage in Tonbridge.

The Tonbridge flood defences, upgraded following the 2000 floods, provide protection to the town centre against approximately a 1 in 100 year event when combined with the Leigh FSA. However, areas outside the defended zone, including much of the Beult and Teise floodplain, remain at undefended risk.

Maidstone, the county town, sits on the Medway and is partly within the river’s floodplain. The town centre, including areas around the River Medway at the Archbishop’s Palace and the Len Valley, has experienced flooding, and development proposals in these locations require detailed fluvial assessment.

Coastal Flood Risk: Channel Coast

Kent’s English Channel coastline, from Folkestone through Dover, Deal, Ramsgate, Margate, and Whitstable to the Isle of Sheppey, faces a range of coastal flood risks including tidal flooding, wave action, wave overtopping, and coastal erosion. The coastal towns have experienced significant flood events, and climate change with sea level rise will increase coastal risk over time.

The low-lying areas of Romney Marsh, protected by the historic Dymchurch sea wall and associated defences, represent one of Kent’s most extensive coastal flood risk areas. The marsh extends inland from the coast and lies below sea level in places, making it entirely dependent on sea defences for protection. Development on Romney Marsh requires detailed assessment of coastal flood risk including defence breach scenarios.

The Thanet coast, including Margate, Broadstairs, and Ramsgate, faces wave overtopping risk during storm events. Regeneration proposals for these coastal towns must incorporate coastal flood risk assessment and appropriate mitigation, including finished floor level setting and flood resilience measures.

The River Stour and East Kent

The River Stour flows through East Kent from its sources near Ashford to the sea at Pegwell Bay near Ramsgate. The Stour valley, including Canterbury and the low-lying farmland of the Stour Levels, has a well-documented flood history. Canterbury experienced significant flooding from the Great Stour in November 2000 and January 2003, affecting properties in the city centre.

The Stour’s floodplain through Canterbury is a significant constraint on development, and Canterbury City Council’s local plan policies reflect this. Ashford, at the upper end of the Stour catchment, has experienced significant growth, and managing the flood risk and drainage implications of this growth has been a major planning challenge.

Groundwater Flooding

The North Downs chalk ridge, which runs across Kent from the White Cliffs of Dover to the Surrey border, is susceptible to groundwater flooding. After prolonged wet weather, groundwater levels in the chalk can rise above the ground surface, causing flooding from springs, winterbournes, and rising groundwater.

The Nailbourne in the Elham Valley is one of Kent’s most notable groundwater flood features. This chalk stream is normally dry but activates after sustained rainfall, flowing through villages including Lyminge, Elham, and Barham. When the Nailbourne flows, it can cause significant flooding to properties and roads along its course. The stream activated in the winters of 2000-01, 2013-14, and 2023-24, causing damage on each occasion.

Groundwater flooding has implications for development across the chalk areas of Kent. Basements and below-ground construction may be vulnerable to groundwater ingress, and infiltration-based SuDS must be designed with reference to peak groundwater levels to ensure adequate unsaturated zone depth. FRAs for sites on the chalk must assess seasonal groundwater level variation and the potential for groundwater flooding.

Surface Water Flooding

Surface water flooding affects urban and rural areas across Kent. The county’s varied topography, from the steep scarp of the North Downs to the flat marshlands of the Thames Estuary and Romney Marsh, creates diverse surface water flood risk conditions. The heavy clay soils of the Low Weald generate rapid runoff during intense rainfall, and urban areas with limited drainage capacity are particularly vulnerable.

Kent County Council’s Surface Water Management Plan identifies areas at greatest surface water risk, including locations in Maidstone, Ashford, Canterbury, and the Medway Towns. The plan informs planning decisions and drainage design requirements.

Planning and Regulatory Framework

NPPF and District Planning Policies

Kent’s 12 district and borough councils each maintain their own local plan and planning policies, and flood risk requirements can vary between authorities. However, all operate within the NPPF framework and apply the Sequential Test for development in Flood Zones 2 and 3.

Key districts with significant flood risk policies include Tonbridge and Malling, which has detailed policies reflecting the Medway floodplain; Maidstone, which addresses Medway and Len Valley flood risk; Canterbury, which addresses Stour floodplain development; and Ashford, which has extensive policies on drainage and SuDS for the borough’s growth areas.

Kent County Council as LLFA

Kent County Council is the Lead Local Flood Authority, responsible for managing local flood risk from surface water, groundwater, and ordinary watercourses. The county council is a statutory consultee on surface water drainage for major planning applications and has published a comprehensive drainage and planning policy statement.

The county council’s requirements include restricting discharge rates to greenfield rates, providing SuDS treatment appropriate to the pollution hazard level, demonstrating that infiltration has been considered and tested, and securing long-term management and maintenance arrangements. The county council provides detailed technical guidance for drainage strategy preparation, and pre-application consultation with the LLFA is recommended for complex schemes.

Environment Agency Coastal and Fluvial Requirements

The Environment Agency manages flood risk from main rivers and the sea across Kent. The EA holds extensive modelling data for the Medway, Stour, Thames Estuary, and coastal areas, and provides this data to support site-specific flood risk assessments. For coastal developments, the EA may require specific assessments of wave action, wave overtopping, and the consequences of sea defence breach.

Key Development Areas in Kent

Ebbsfleet Garden City

Ebbsfleet Garden City, one of England’s most significant new settlements, is located in the Thames Estuary zone in Dartford and Gravesham. The site’s position in the marshland behind Thames tidal defences means that flood risk assessment, including defence breach scenarios, is a critical element of the development framework. Individual phases and plots require site-specific FRAs within the overarching flood risk strategy.

Medway Towns Regeneration

The Medway Towns, including Rochester, Chatham, and Gillingham, face both tidal risk from the Medway Estuary and fluvial risk from the Medway and its tributaries. Major regeneration sites at Rochester Riverside and Chatham Maritime have required comprehensive flood risk assessment, and ongoing development in the Medway area continues to generate demand for specialist FRA services.

Ashford Growth Area

Ashford has experienced significant housing growth under successive local plans, with major developments at Finberry, Chilmington Green, and Park Farm. Managing the flood risk and drainage implications of this growth, including the impact on the River Stour and its tributaries, has required extensive flood risk and drainage consultancy. The SuDS requirements for Ashford developments are among the most detailed in Kent.

Canterbury and East Kent

Canterbury’s local plan allocates sites for housing and commercial development, some of which fall within or adjacent to the Stour floodplain. The city’s heritage status adds complexity to flood mitigation design. In the wider East Kent area, coastal regeneration at Margate, Ramsgate, and Folkestone generates demand for coastal flood risk assessment services.

Tonbridge and the Medway Corridor

Development in Tonbridge and the Medway corridor must navigate the complex interaction between the river, the Leigh FSA, and the town’s flood defences. The Sequential Test is particularly challenging in Tonbridge, where much of the town centre falls within Flood Zones 2 and 3. Upstream sites in the Beult and Teise catchments face undefended fluvial risk.

Aegaea’s Kent Services

Aegaea provides comprehensive flood risk and drainage services across Kent, including desktop and detailed flood risk assessments for planning applications, hydraulic modelling of the Medway, Stour, and smaller watercourses, coastal and tidal flood risk assessment including breach analysis, groundwater flood risk assessment, infiltration testing (BRE365 and borehole permeability tests), surface water drainage strategies and SuDS design, Sequential Test and Exception Test assessments, Environment Agency and LLFA pre-application consultation, and expert witness services for planning appeals.

Our detailed knowledge of Kent’s diverse flood risk environment ensures we deliver targeted, efficient assessments for each site. We work with developers, landowners, architects, and planning consultants across all 12 Kent districts.

Contact Aegaea

If you are planning a development in Kent and need a flood risk assessment, drainage strategy, or coastal flood risk analysis, contact Aegaea. Our team can advise on the likely scope of work and help you navigate Kent’s flood risk requirements from the earliest stages of your project.