Flood Risk Assessment - Challenging the Flood Map in Cambridgeshire
Aug 28, 2015

When our client found out his proposed new dwelling was in Flood Zone 3 (high probability flooding) he asked RAB to undertake a flood risk assessment.

The site is adjacent to the River Shep, a non-main river.  The Environment Agency (EA) was unable to supply modelled flood levels and maps as the flood zones were produced using a national generalised 2D modelling approach, not suitable for a detailed assessment of flood risk.  The EA Flood Map showed an extensive area of flooding in the area particularly upstream of a railway line, suggesting that the railway culverts were under capacity.

EA Flood Map

RAB undertook a hydrological analysis using the Flood Estimation Handbook to determine flows in the River Shep for a range of flood fequencies.  A channel and culvert survey was then undertaken to allow a 1D computer model of the waterourse to be constructed as well as a survey of the adjacent watercourse some 400m to the east.

Our model and calculations proved that the railway culverts provided sufficient capacity to allow flood events to pass up to and including the 1 in 100 year plus an allowance for climate change without overtopping the banks at the site.  Even when 50% blocked as shown below.

Long section 1 in 100yr plus CC and blockage. 

RAB was then able to make recommendations to the client for minimum floor levels to ensure that the new dwelling would be safe for its lifetime.