Flood Risk Assessment Cambridge

Cambridge occupies a low-lying position in the valley of the River Cam, where the chalk hills of the Gog Magog range meet the flat fenland landscape to the north. This transitional geography creates a flood risk profile defined by slow-flowing rivers, a shallow groundwater table and flat terrain where surface water accumulates rather than draining away. The River Cam and its tributaries have shaped Cambridge’s development for centuries, but they also present persistent flood risk that must be carefully managed as the city grows.

The city’s flood risk is dominated by the interaction between the River Cam and the underlying geology. Cambridge sits on river terrace gravels overlying the chalk bedrock, and both formations are aquifers. The shallow gravel aquifer is hydraulically connected to the Cam, meaning that when the river rises, groundwater levels rise correspondingly. This mechanism extends the effective area of flood risk beyond the mapped fluvial floodplain, as groundwater can emerge at the surface in low-lying areas even where direct river flooding does not reach.

Cambridge is one of the fastest-growing cities in England, with major development planned on the city’s fringes and significant infill and intensification within the existing urban area. The North East Cambridge development area, the southern fringe and multiple sites across the city all require robust flood risk assessment and drainage design. The flat topography, shallow groundwater and limited drainage gradients create distinctive challenges for sustainable drainage that require specialist expertise.

Flood Risk Sources in Cambridge

Fluvial Flooding

The River Cam is Cambridge’s principal watercourse, flowing through the city centre past the famous Backs and college grounds. Upstream of Cambridge, the river is known as the River Granta, and its catchment extends south into the chalk hills. The Cam’s gradient through Cambridge is exceptionally low, meaning that flood flows move slowly and the floodplain remains inundated for extended periods during high-water events.

Cherry Hinton Brook and Coldham’s Brook drain the eastern parts of the city and have significant flood risk zones along their corridors. Bin Brook flows through the western suburbs, and Hobson’s Conduit, a historic artificial watercourse built in the 17th century, can contribute to flood risk in the city centre. Several other minor watercourses and drainage ditches add to the network.

Environment Agency flood mapping shows extensive Flood Zone 2 and 3 along the Cam corridor and its tributaries, with significant development land affected, particularly in the south and east of the city.

Groundwater Flooding

Cambridge’s shallow gravel aquifer creates widespread groundwater flood risk. The river terrace gravels that underlie much of the city are typically 2-5 metres thick and rest on the chalk bedrock. Both formations store and transmit groundwater, and the water table in the gravels responds to Cam river levels and to direct rainfall recharge.

During prolonged wet periods, the water table can rise to within a metre of the ground surface across large areas of Cambridge. In the lowest-lying areas — along the Cam corridor, in south Cambridge and in parts of the east — groundwater can emerge at the surface. This creates flooding that is slow in onset but can persist for weeks as the aquifer drains gradually.

Groundwater flood risk assessment in Cambridge requires understanding of the gravel aquifer’s response to both river levels and direct recharge. Development involving below-ground construction, including basements and underground parking, must carefully consider groundwater levels and the implications for waterproofing and structural design.

Surface Water Flooding

Cambridge’s flat topography makes surface water flooding a widespread concern. Unlike hilly cities where surface water flows rapidly to rivers, Cambridge’s minimal gradient means that rainfall ponding is extensive. Surface water collects in topographic depressions, at road junctions, in underpasses and in areas where drainage infrastructure has limited capacity. The city’s drainage network includes both Anglian Water’s piped systems and a network of open ditches and watercourses that provide some conveyance but have limited capacity.

When the River Cam is in flood, surface water outfalls cannot discharge freely, causing backing up through the drainage system. This interaction between surface water drainage and river levels is a critical factor in Cambridge’s flood risk assessment.

Sewer Flooding

Anglian Water’s sewer network in Cambridge includes combined sewers in the older parts of the city and separate systems in newer areas. During intense rainfall, the combined system can become overwhelmed, causing surcharging. The flat topography limits the hydraulic gradient available for sewer flows, meaning that the system is inherently slow to drain and susceptible to capacity exceedance.

Planning Requirements

Cambridge’s planning framework for flood risk operates within the NPPF, supported by Cambridge City Council’s (and South Cambridgeshire District Council’s) Local Plan policies, and Cambridgeshire County Council’s role as Lead Local Flood Authority. The Greater Cambridge Shared Planning Service manages planning applications across both authority areas.

The Sequential Test directs development to areas of lowest flood risk. Cambridge’s constrained development land supply means that the Sequential Test is a critical planning consideration, as many available sites are affected by fluvial, groundwater or surface water flood risk. The Exception Test applies where development in higher-risk areas is justified.

Cambridgeshire County Council, as LLFA, is the statutory consultee on surface water drainage for major developments. The county council requires drainage strategies to achieve greenfield runoff rates and to follow the SuDS hierarchy. The council has a strong preference for infiltration-based drainage where ground conditions allow, but requires robust evidence of feasibility including BRE365 testing and groundwater level assessment.

The Environment Agency is consulted on Flood Zone 2 and 3 sites and proposals affecting the Cam and its tributaries. The EA also has an interest in groundwater flood risk matters, particularly where development may affect the shallow gravel aquifer.

Anglian Water’s pre-development enquiry process is required for sewer connections. Capacity constraints in Cambridge’s sewer network are a recurring issue, and the emphasis is on minimising surface water connections to the public sewer through SuDS.

Where development is within catchments subject to nutrient neutrality requirements, the drainage strategy must also address nutrient management. This may influence SuDS design, with features such as constructed wetlands and bioretention providing nutrient removal alongside flood risk management.

Key Flood History

October 2001 Cambridge Floods

In October 2001, prolonged heavy rainfall caused the River Cam and its tributaries to flood across Cambridge. Properties along the Cam corridor, in Newnham, and in the Cherry Hinton area were affected. Groundwater levels rose significantly, causing flooding in low-lying areas away from the river. The event highlighted the interaction between fluvial and groundwater flooding that characterises Cambridge’s flood risk.

July 2021 Surface Water Flooding

In July 2021, intense summer rainfall caused widespread surface water flooding across Cambridge. The flat topography meant that water ponded extensively across the city, with properties and roads in multiple areas affected. The event demonstrated Cambridge’s acute vulnerability to surface water flooding when rainfall intensity exceeds drainage capacity on such flat terrain.

Winter 2000-2001

The winter of 2000-2001 brought prolonged wet weather that caused river levels and groundwater levels across Cambridge to reach high levels. Flooding affected properties along the Cam corridor, and groundwater emergence was reported in south Cambridge and Cherry Hinton. The extended duration of the event — with elevated groundwater persisting for months — demonstrated the slow-draining nature of the gravel aquifer.

Easter 1998

In April 1998, sustained rainfall caused the River Cam to flood through Cambridge. The event affected properties along the river corridor and in areas susceptible to the interaction between river levels and groundwater. The floods contributed to updated flood risk mapping and strategic assessment across the city.

Our Services in Cambridge

Flood Risk Assessments

Aegaea delivers flood risk assessments for development across Cambridge, addressing the city’s combination of fluvial, groundwater, surface water and sewer flood risk. Our assessments are tailored to Cambridge’s specific requirements, including the gravel aquifer groundwater interaction and the implications of the flat topography for drainage. We prepare Sequential and Exception Test evidence, site-specific FRAs, groundwater flood risk assessments and flood emergency plans.

Flood Modelling

Where Environment Agency flood mapping does not provide adequate site-specific detail, Aegaea undertakes bespoke hydraulic modelling. In Cambridge, this includes River Cam and tributary modelling, surface water modelling accounting for the flat topography and ponding behaviour, and combined fluvial-groundwater assessment where these flood mechanisms interact.

Drainage Design and SuDS

Cambridge’s flat topography and shallow groundwater table create distinctive drainage design challenges. Aegaea designs drainage solutions that account for minimal hydraulic gradients, seasonal groundwater variation and the need to manage surface water on flat sites. We incorporate infiltration-based SuDS where ground conditions allow, with attenuation and controlled discharge where infiltration is constrained. Our designs meet Cambridgeshire County Council’s requirements for greenfield rates and SuDS compliance.

Infiltration Testing

BRE365 soakaway testing is essential for infiltration-based drainage in Cambridge. Aegaea undertakes site-specific testing to determine infiltration rates in the gravel and chalk, accounting for groundwater proximity and seasonal variation. Our testing results inform robust soakaway and infiltration basin design that performs reliably throughout the year.

Nutrient Neutrality Assessment

Where development falls within a nutrient neutrality catchment, Aegaea provides integrated nutrient neutrality and drainage design. Our SuDS solutions can incorporate nutrient removal through constructed wetlands, bioretention features and other treatment systems, addressing both flood risk management and nutrient neutrality requirements in a coordinated design.

Why Choose Aegaea for Your Cambridge Project

Cambridge’s flood risk environment — flat topography, shallow groundwater, slow-flowing rivers and the interaction between fluvial and groundwater flooding — requires consultants who understand these specific mechanisms. Aegaea brings detailed knowledge of the Cam catchment, the gravel terrace aquifer and the particular challenges of drainage design on flat, low-lying sites.

Our experience in Cambridge includes residential development on the city fringes, university and college buildings, city centre commercial schemes, and major development areas where the interaction between multiple flood sources must be carefully managed. We understand Cambridgeshire County Council’s approach to flood risk and drainage, the Environment Agency’s requirements for the Cam corridor, and the integration of nutrient neutrality with drainage design where applicable.

For developers and planning consultants working in Cambridge, Aegaea provides the specialist flood risk and drainage expertise that addresses the city’s distinctive challenges. We deliver assessments that satisfy regulatory requirements and enable development to proceed with confidence in a city where flood risk is pervasive but manageable with the right technical approach.