DRAINAGE



Flooding and drainage

The vast majority of the proposed Waterbeach New Town Settlement site is located within Flood Zone 1 (low probability of fluvial flooding, less than 1 in 1000 annual probability of flooding), with localised small areas in Flood Zone 2 (medium probability) and Flood Zone 3 (high probability), based on the EA undefended fluvial extents flood maps.

The site is protected by the River Cam left (west) bank defence, protection above the 1% annual exceedance probability (AEP) flood level (based on the detailed topographical survey of the flood defences carried out in 2002 and the hydraulic model).

Based on the extensive technical detailed work done to date on flood risk with the key Stakeholders (EA, IDB, CCC, SCDC, AW) in the form of a comprehensive water cycle study, the risk of flooding is considered to be low. In the very unlikely event of a breach in the River Cam during the 100-year return period rainfall event, a small proportion of the total site area is susceptible to shallow flooding. As agreed with the EA in the WCS, mitigation measures and appropriate land uses are proposed to safeguard against this unlikely scenario.

All the potential sources of flooding will be assessed in further detail as part of the Flood Risk Assessment for the site, and adequate flood mitigation measures identified and proposed. These will be discussed with the key Stakeholders.

The proposed development area lies within the District Boundary of the Waterbeach Level Internal Drainage Board (IDB). The area is a pumped catchment reliant on the IDB for operation of pumping plant and drain maintenance for all surface water drainage discharge.

The existing drainage system at the site consists of a network of open channel gravity surface water drains which discharges into the River Cam via Bottisham Locks Electric Pumping Station and the Cam Pumping Station. The pumping stations have been sized to discharge surface water runoff from agricultural land at the rate of 1.1 l/s/ha.

The proposed surface water drainage strategy will be designed to replicate, as close as possible, the existing drainage regime and provide betterment over and above the existing hydrological conditions, where feasible and practicable. In order to maintain the current runoff rate from the pumping stations into the River Cam and ensure that flood risk is not increased elsewhere, the surface water drainage network for the proposed development will need to be designed to limit discharge to 1.1 l/s/ha.

The proposed method of surface water disposal comprises strategic surface water balancing ponds/basins, each designed to limit surface water runoff rates and volumes to the existing rates for up to and including the 100-year rainfall event plus an allowance for climate change.

The development of the site will not increase the groundwater flows and surface water runoff flows will be controlled above ground prior to discharging into the existing IDB system and the River Cam.

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