Flood Risk Assessment Surrey

Flood Risk Assessment Services in Surrey

Surrey is one of England’s most active development markets, with sustained demand for residential and commercial property across the county’s towns and rural areas. The county’s rivers, including the Wey, Mole, Thames, and dozens of smaller watercourses, combined with its complex geology and high groundwater vulnerability, mean that flood risk assessment is a routine requirement for development proposals across virtually every borough and district.

Aegaea provides specialist flood risk assessment, hydraulic modelling, and drainage consultancy services throughout Surrey. Our team has detailed knowledge of Surrey’s watercourses, the Environment Agency’s flood defence infrastructure, Surrey County Council’s LLFA requirements, and the specific flood risk policies of each of the county’s 11 boroughs and districts.

Surrey’s Flood Risk Environment

The River Thames

The River Thames forms Surrey’s northern boundary, flowing from Staines through Shepperton, Walton-on-Thames, Weybridge, Esher, Kingston, and into Greater London. The Thames floodplain through Surrey is extensive, and several of the county’s largest towns are built partly within Flood Zones 2 and 3.

The winter 2013-14 floods brought the Thames to levels not seen since 1947 in many locations along the Surrey reach. Prolonged rainfall from December 2013 through February 2014 saturated the vast Thames catchment, and the river remained at flood levels for several weeks. Communities in Staines-upon-Thames, Chertsey, Shepperton, Walton-on-Thames, and the Dittons experienced severe flooding, with hundreds of properties inundated and thousands more at risk.

The River Thames Scheme, a major Environment Agency and Surrey County Council initiative, aims to provide improved flood protection along the Thames from Datchet to Teddington. The scheme, which has been in development for many years, proposes a flood channel and associated defences to reduce flood risk to around 11,000 properties. Until the scheme is delivered, existing levels of protection remain below the 1 in 100 year standard in many locations.

Development along the Thames corridor in Surrey requires careful assessment of fluvial flood risk, including climate change impacts and the consequences of defence overtopping or failure where defences exist. The extensive Flood Zone 2 and 3 areas mean that the Sequential Test is a critical consideration for all but the lowest vulnerability development.

The River Wey

The River Wey flows through the heart of Surrey, from Alton in Hampshire through Farnham, Godalming, Guildford, and Woking before joining the Thames near Weybridge. The Wey Navigation, a canalised section from Godalming to the Thames, adds complexity to the river’s hydrology and flood risk.

Guildford has experienced repeated flooding from the Wey, most significantly in the winters of 1968, 2000, and 2013-14. The town centre is partly built on the Wey floodplain, and areas including Millmead, Shalford Road, and Woodbridge Road are at significant flood risk. The Guildford Flood Alleviation Scheme, currently being developed by the Environment Agency, aims to reduce flood risk to the town through a combination of upstream storage, channel improvements, and property-level protection.

Godalming, upstream of Guildford, is similarly affected by Wey flooding, and the town has experienced significant events including the winter 2013-14 floods. The River Wey’s catchment includes large areas of greensand and clay, which generate rapid runoff during sustained rainfall, and the relatively narrow valley through Godalming concentrates flood flows.

Farnham, at the upper end of the Wey in Surrey, faces flood risk from both the Wey and its tributary the River Bourne. The town has a well-documented flood history, and the Farnham Flood Alleviation Scheme provides partial protection. Development in Farnham requires detailed assessment of fluvial risk from both watercourses and consideration of groundwater flood risk from the underlying chalk and greensand.

The River Mole

The River Mole flows northward through the centre of Surrey, from Gatwick through Crawley (in West Sussex), Dorking, Leatherhead, Cobham, and Esher before joining the Thames near Hampton Court. The Mole valley through Dorking and Leatherhead is one of Surrey’s most attractive residential areas, but it is also one of the most flood-prone.

The Mole is known for its unusual hydrological behaviour. In its upper reaches near Dorking, the river flows over chalk, and during dry periods, significant flow is lost to the aquifer through swallow holes. During wet periods, however, the chalk becomes saturated, the swallow holes are unable to absorb the flow, and the river can rise rapidly to flood levels.

The winter 2013-14 floods caused severe flooding along the Mole, particularly in Dorking, Leatherhead, and Cobham. The prolonged wet weather saturated the chalk aquifer, eliminating the river’s natural attenuation and generating sustained high flows. Groundwater flooding, caused by the rising water table, compounded the fluvial flooding in many locations.

Development in the Mole valley requires assessment of both fluvial and groundwater flood risk, and the interaction between the two. The river’s unusual hydrology means that standard hydrological assessment methods may not adequately capture the Mole’s flood behaviour, and site-specific assessment is frequently required.

Groundwater Flooding

Groundwater flooding is a distinctive and significant flood risk across Surrey. The county’s geology includes extensive areas of chalk (the North Downs), greensand (the Lower Greensand ridge), and alluvial deposits in the river valleys. All of these can be susceptible to groundwater flooding under the right conditions.

The winter 2013-14 event was primarily a groundwater flood event in many parts of Surrey. Prolonged rainfall caused groundwater levels to rise to record heights, and springs, winterbournes, and rising groundwater affected communities across the chalk and greensand. Areas around Dorking, Leatherhead, Guildford, Farnham, and Reigate were particularly affected.

Groundwater flooding has significant implications for development. Basements and below-ground construction may be vulnerable to groundwater ingress, and infiltration-based SuDS may be inappropriate where groundwater levels are high. Infiltration testing, combined with assessment of seasonal groundwater level variation, is essential for determining appropriate drainage strategies in Surrey.

Surface Water Flooding

Surface water flooding affects widespread areas across Surrey, driven by the county’s varied topography and the intensity of rainfall events. The steep scarp slopes of the North Downs and the greensand ridge generate rapid surface water runoff during intense storms, and the low-lying clay vales between can accumulate surface water.

Surrey County Council’s Surface Water Management Plan identifies areas at greatest surface water risk across the county, including locations in Guildford, Woking, Epsom, Reigate, and numerous smaller settlements. The plan prioritises investment in surface water management and informs planning decisions.

Planning and Regulatory Framework

NPPF and Borough Planning Policies

Surrey’s 11 boroughs and districts 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.

Some Surrey boroughs have particularly detailed flood risk policies. Spelthorne and Runnymede, the boroughs most affected by Thames flooding, have specific policies on development in the floodplain, finished floor levels, and flood resilience. Guildford Borough Council’s policies address the specific challenges of the Wey floodplain and groundwater flooding. Mole Valley’s policies reflect the river’s unusual hydrology and the widespread groundwater risk.

Surrey County Council as LLFA

Surrey County Council is the Lead Local Flood Authority for the county, 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 detailed SuDS guidance.

The county council’s requirements include restricting discharge rates to greenfield runoff rates for all events up to the 1 in 100 year plus climate change event, providing appropriate SuDS treatment in accordance with the SuDS treatment hierarchy, demonstrating that infiltration has been considered and tested where ground conditions may be suitable, and securing long-term maintenance arrangements for SuDS features.

Surrey’s geology means that infiltration is feasible in many locations, particularly on the chalk and greensand, but site-specific infiltration testing is essential. The county council will not accept assumed infiltration rates without field testing evidence. Aegaea provides infiltration testing services, including BRE365 soakaway tests and borehole permeability testing, to support drainage design.

Environment Agency Requirements

The Environment Agency manages flood risk from main rivers (the Thames, Wey, Mole, and others) and is a statutory consultee on planning applications in flood risk areas. The EA holds extensive modelling data for Surrey’s main rivers and provides this data to support site-specific flood risk assessments.

The EA’s requirements for Surrey FRAs include detailed assessment of the design flood event plus climate change, safe finished floor levels above the predicted flood level plus freeboard, demonstration of safe access and egress during a design flood event, and evidence that the development will not increase flood risk elsewhere. Where the site benefits from flood defences, breach and overtopping analysis is required to assess residual risk.

Key Development Areas in Surrey

Guildford Town Centre

Guildford town centre faces significant development pressure, with the local plan allocating sites for housing, commercial, and mixed-use development. The River Wey constrains development in the town centre, and the emerging Flood Alleviation Scheme will change the flood risk context for key sites. Development proposals must address Wey fluvial risk, surface water management in the constrained urban centre, and groundwater from the underlying chalk.

Woking Town Centre

Woking’s ambitious town centre regeneration programme includes tall buildings and high-density mixed-use development. The Basingstoke Canal and Hoe Stream present flood risk in parts of the town, and surface water management for major urban schemes requires innovative SuDS solutions. The Sheerwater regeneration project has required comprehensive flood risk and drainage assessment.

Thames Corridor: Staines, Walton, and Weybridge

The Thames corridor communities face the county’s most significant fluvial flood risk, with extensive areas in Flood Zones 2 and 3. Development pressure is high, particularly for riverside residential, and the Sequential Test represents a significant hurdle. The River Thames Scheme, when delivered, will change the flood risk context for many sites, but until that time, development must be assessed against current undefended risk levels in many locations.

Epsom, Leatherhead, and the Mole Valley

The Mole Valley area, including the market towns of Dorking and Leatherhead, faces combined fluvial and groundwater flood risk. The area’s desirability for residential development means that demand for flood risk assessment services is consistently high. The Mole’s unusual hydrology and the groundwater flooding experienced in 2013-14 require specialist assessment approaches.

Farnham and Waverley

Farnham and the wider Waverley borough face flood risk from the Wey, its tributaries, and groundwater. The borough’s attractive rural and semi-rural character drives residential development, and sites adjacent to watercourses or in groundwater-vulnerable areas require detailed assessment. Nutrient neutrality requirements in parts of the wider catchment may also be relevant.

Aegaea’s Surrey Services

Aegaea provides comprehensive flood risk and drainage services across Surrey, including desktop and detailed flood risk assessments for planning applications, hydraulic modelling of the Wey, Mole, Thames, and smaller watercourses, groundwater flood risk assessment, infiltration testing (BRE365 and borehole permeability tests), surface water drainage strategies and SuDS design to Surrey County Council standards, Sequential Test and Exception Test assessments, Environment Agency and LLFA pre-application consultation, and expert witness services for planning appeals.

Our detailed knowledge of Surrey’s diverse flood risk environment, from the Thames floodplain to the Mole’s unusual hydrology and the county’s groundwater vulnerability, means we deliver authoritative assessments tailored to each site’s specific challenges. We work with developers, landowners, architects, and planning consultants across all 11 Surrey boroughs and districts.

Contact Aegaea

If you are planning a development in Surrey and need a flood risk assessment, drainage strategy, infiltration testing, or SuDS design, contact Aegaea for a no-obligation discussion. Our team can advise on the likely scope of work and help you navigate Surrey’s flood risk requirements.