Drainage plan paramount to housing development delivery

The developer of a small residential scheme in Ash, Surrey, realised that there was no obvious means to drain surface water from the site. Motion was contracted to investigate the options and, subsequently, carry out the detailed design of the system. Aspen Homes already had planning permission. However, the site was only for nine dwellings and was not classified as a major development, so including a drainage strategy with the proposal had not been a requirement. Motion’s remit was to design a drainage strategy that would ensure that surface water run off could be discharged effectively, even in extreme events, without increasing flood risk in and around the site.

Motion Technical Director Neil Jaques explains the problem: “Usually there is a hierarchy for how a site can be drained. The best solution is infiltration to ground but the soil was clay based and, therefore, unsuitable. There were no adjacent water courses and a surface water sewer was also not available. As the roof of the existing large house on the site drained into the foul water sewer, this was the only practical solution.”

The challenge for Motion’s team of drainage experts was to produce a plan to replicate the existing surface water discharge rate for the nine new houses. Considering the increase in hard standing and roof covering, Thames Water needed to be persuaded that there would be no detriment to the system from continuing to discharge the surface water into the foul water drain. Normally, when surface and foul water flow into the same drain, it is classified as ‘combined’. However, in this case, the sewer was classified as ‘foul’ on Thames Water’s mapping system and there was no record of surface water drainage being discharged in this manner.

Unusual situation resolved

Neil Jaques continues: “We had to demonstrate the current situation to Thames Water and Surrey County Council, as Lead Local Flood Authority (LLFA), and prove that the proposed plan wouldn’t increase the surface water flow into the sewer. Gaining approval from the LLFA involved infiltration testing onsite. It was an unusual situation; with a major development the initial strategy would have identified the problem and a minor development usually has enough around it to provide other options.”

Joe Jelley, Managing Director, Aspen Homes concludes, “We knew what we wanted to achieve but needed Motion’s expertise and knowledge to create a workable plan. The company’s involvement was paramount; without a surface drainage system that was acceptable to Thames Water, we could not deliver the site. We were absolutely delighted with the service and clever design Motion provided for us.”

The plan included a hydrobrake to control the flow of surface water into the foul sewer. To ensure the integrity of the system was not compromised, cellular storage was incorporated under the gardens of the houses which holds surface water in an extreme event, allowing the flow rate to be maintained.


An abridged version of this article first appeared in the Winter 2018/19 issue of Insight.

Image credit: © Paul Hewett. RIBA Chartered Architect

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