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Protecting residential schemes from flood risk

Civil Engineering By Paul De Vere, Associate Director – 28 June 2023

Flood street with garages and housing with people in the distance deep in water


Paul De Vere in a navy jacket and white shirt standing in front of a glass meeting room

Paul De Vere

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Riverside Commercial is a simple but elegant building, providing flexible light industrial and office space, and forming the first phase of a larger planned development on the site. It is located immediately east of the River Wandle.

From the outset, Cundall’s civil engineering team was presented with a number of challenges – not least the fact that the site is located within National Flood Zone 2.

Early engagement with the Environment Agency (EA) proved highly beneficial in establishing what development would be permitted next to the river. The Cundall team used flood level data provided by the EA to analyse the elevated river levels during flood events to establish if a change of use from commercial to residential use was viable on the site.

A proposal was therefore made to elevate floor levels for the ‘more vulnerable’ part of the development such that dwellings would sit above the 1 in 100-year fluvial flood event with a freeboard allowance. This ensured that all proposed residential development would be safe from flooding events up to this event and well into the future with the inclusion of a climate change allowance.

Particular attention was required for the main building entrance and lobby areas where external site levels were to remain unchanged – lower than the elevated residential floor levels. After consultation with the Environment Agency, Cundall worked with Rolfe Judd Architects to establish lower floor levels for the lobby areas of the building. These areas could still flood in an extreme event, but residents could be safely evacuated from their dwellings, located above the flood level. This produced an optimal outcome whereby residential accommodation could still be offered in the desired location with views of the river whilst ensuring residents are safe from fluvial flooding significantly into the future.

Another area of consideration was flood volume analysis. Our team used volumetric analysis software to assess the potential impact on neighbouring properties of any flood water displaced from the proposed development. We devised a strategy in conjunction with the architect and the mechanical and electrical teams to allow certain sections of the new basement areas to flood. These were low-risk areas, such as cycle stores, which were designed to sequentially flood when flood water reaches a certain level. Other areas of the site such as the commercial one was allowed to flood given the shallow nature of the flood water in this location of the site as well as the ‘less vulnerable’ nature of this type of development. Appropriate flood resilience measures to account for this have also been incorporated in these areas.

The civils design also incorporates sustainable drainage systems (SuDS) in the form of permeable paving and below ground attenuation storage below the service yard of the commercial area. Sections of green/blue roof have also been designed to manage rainwater run-off. The remaining zone of the site around the residential areas incorporates a series of shallow, below ground storage features upstream of which are other SuDS such as permeable paving and filter trenches to enhance surface water quality.

The whole of the surface water system discharges to the River Wandle at greenfield run-off rate, taking surface water away from the Thames Water Combined Sewers which are already overloaded, particularly during wetter periods.

The whole flooding strategy was captured in a comprehensive Flood Risk Assessment presented as part of a planning application which demonstrates the sustainable and safe development of a site in a medium risk fluvial flooding area.

Creating safe residential developments near rivers or flood zones can be achieved, with advice from experienced civil engineers. If you would like to find out more, please get in touch.