Flood Risk Assessment & Surface Water Drainage Strategy in Bedford
Nov 1, 2019


A repeat client approached RAB Consultants to prepare a Flood Risk Assessment and surface water Drainage Strategy to support a planning application. The application involved the development of 20 dwellings with associated parking, hard/soft landscaping and amenity space.

The development site is located in Flood Zone 1 according to the Environment Agency’s Flood Map for Planning (Rivers and Sea) and at low risk of surface water flooding with geology that did not support infiltration.

The proposed drainage system was developed in line with the Bedford Borough Council SuDS SPD 2018 document.

The challenge:

Draining the site was certainly challenging given the ground levels and existing flow paths. In addition, the impeded drainage potential of the soil in combination with the absence of a surface water sewer in the area, dictated the selection of the ultimate discharge location and the proposed layout.

The solution:

The proposed drainage scheme was modelled in Mircodrainage Source Control to fully understand the evolving flow regime under flood conditions and the potential for flooding. The Environment Agency’s flood maps along with relevant documents published by the Bedford Borough Council were used to assess the risk of flooding to the site from all sources. Local SuDS policy was applied to inform the proposed drainage scheme and ensure conformity.

The drainage solution included the installation of an open SuDS feature (wet pond) along with the appropriate piped network. The discharge rate was limited, in line with recommendations made by the Bedford Borough Council, using an ACO Q-Brake flow control chamber prior to discharging surface water to the adjacent open channel. The scheme was designed not to flood during the 1% AEP (1 in 100 year) plus 40% Climate Change while an urban creep factor of 10% was also tested.

The drainage scheme will reduce greenfield runoff rates and thus provide significant betterment to others downstream. In addition, it will offer water quality, biodiversity and amenity benefits in line with the four pillars of SuDS.

The result

The proposed development went successfully through planning in October 2019. RAB Consultants are very proud that the Bedford office was able to support the client, while actively contributing to ensuring sustainable development within the Bedford Borough.