When developer Barratt wanted to build 350 homes at Alma Dettingen Barracks, Deepcut, Surrey, Thames Water imposed a strict maximum surface water discharge rate of 5 litres/ second to avoid overloading the current system in times of heavy rainfall. Consultant Hemsley Orrell Partnership was called in and, together with Thames Water, developed a sustainable rainwater proposal to manage the run off. One natural advantage was the gravelly sand matrix substrate which permitted natural percolation.
The project was zoned into three catchment areas; primary roads, secondary roads and roofs/drives. The primary roads were positively drained. A conventional gully and pipework system runs into a large (400m 3) storm cell manufactured by Hydro in the centre of the site, concealed under a grassed amenity area. This provides a 'buffer' against a 20-year storm, and incorporates a regulated outfall. Additional storage was designed in by allowing back up through the pipework in the event of the tank reaching capacity.
The secondary roads and drives were constructed from FormPave permeable concrete paving, which allows rainwater to percolate through and re-charge the water table. The roof water was collected via a conventional gutter system and piped into the granular bedding under the FormPave structure. An additional benefit was that the roads could be laid virtually flat. The scheme has now been adopted.