Hitchin Road Flood Risk Assessment and Drainage Strategy
Oct 23, 2018


A flood risk assessment and drainage strategy has recently been undertaken by RAB Consultants to support the development of 150 new homes and a new lower school on the land near Hitchin Road, Shefford.

The development site is located in Flood Zone 1 according to the Environment Agency’s Flood Map for Planning (Rivers and Sea). An FRA was completed due to the site being over 1ha, in line with NPPF.

The challenge:

Draining the site was really challenging given the site topography and geology; infiltration was not an option given the underlying bedrock of mudstone and the poor infiltration rates. The site was split into two catchments West and East, due to the high elevation towards the south-central part of the site. This meant that the West Catchment would flow to the north, and discharge to the ditch at the north boundary, whereas the East Catchment would discharge to the ditch at the north-east of the site.

The solution:

The proposed drainage scheme was modelled in Mircodrainage Network to fully understand the evolving flow regime under flood conditions and the potential for flooding. The Environment Agency’s flood maps were used to access the fluvial, surface water and reservoir flood risk.

The West Catchment includes a swale which conveys surface water to a dry detention pond. Another separate wet pond would receive surface water runoff, via a piped network, serving the east part of the West Catchment. Linear drainage features will be installed to manage road runoff.

The East Catchment includes a dry detention pond, two cellular storage tanks, which will manage runoff from the school, and linear drainage along the road.

The proposed development was deemed appropriate given the flood risk, integrating multiple SuDS features in line with the 2015 SuDS Manual recommendations, and providing significant betterment to others downstream.