Jomas Consulting undertook a Drainage Strategy and Flood Risk Assessment for the construction of a new multi-unit residential development in Purley.
The site is located in an area where surface water infiltration is likely to be possible. However, until a Site Investigation (and soakaway tests) has confirmed this, both infiltration and attenuation need to be considered. Thus, as part of the assessment, two drainage options have been prepared. Option 1 considers the use of infiltration. This is a relatively simple design, with the front driveway area of the site proposed to be permeable paved and all surface water infiltrated via this area. The benefits of this option are both the sustainability (preferable to the Lead Local Flood Authority) and also having the paving at the front of the site, in the case of drainage system failure, overflow will be to the sewers in the street. Option 2 considers the case where the surface water discharges to the sewer. This is further complicated by the steeply sloping site and the site layout having 2 separate buildings. To overcome these complications, a multi stage drainage system has been designed. This includes an area of green roof for the rear building at the high point of the site. The discharge from this roof is restricted and connects into the surface water drainage system towards the front of the site, where an attenuation tank and area of tanked permeable paving ensure there is sufficient attenuation to store the 100 year + climate change storm event with discharge restricted to greenfield rates. Effective Solutions This strategy ensures that the quality of surface water discharge is of an acceptable level to the LLFA through the use of permeable paving and a green roof and also satisfies the requirement for a maximum discharge of no greater than the greenfield rate. By providing a detailed design for both possibilities for the drainage of this site (infiltration and attenuation), Jomas have satisfied the requirements of the LLFA and ensured a smooth and quick path through the planning process.