Crest Nicholson are planning to build 149 houses off the Horsham Road in Cranleigh with proposals for a sustainable drainage system (SuDS) with run-off flowing into Holdhurst Brook.
This proposed drainage system has not been independently verified by the Lead Local Flood Authority (Surrey County Council) to ensure that there is adequate capacity and that it will not increase the flood risk for existing residents nearby. We would also like to know what the ongoing maintenance plan is for this drainage system and what the contingency plan is in event of its failure?
The video below recorded what happened at Leverstock Green, one of Crest Nicholson’s sites in January 2014. Local residents warnings of flood risk had been ignored and new residents only occupied their homes for a matter of weeks before experiencing this flood water with the added risk of sewage contamination.
This incident was referred to by the Cranleigh Society in a recent meeting in the village chaired by Anne Milton MP, who confirmed that it was incidents like this that made residents justifiably concerned about increased flood risk.
SuDS seems the favoured drainage solution at present. The storage volume required is dependent on surface water run-off rates. For the clay soils around Cranleigh these are high. The developers quoted a run-off rate for the Wyphurst Road development (2004) for affordable housing was 10 litres per second per hectare. This, for the Amlets Park development, neighbouring fields and same clay soils, would translate to 138 litres per second for the entire site. Their proposal used a value of 30 litres per second. If this value is the starting point for their SuDS calculations they may be underestimating their SuDS storage requirement by a factor of 4.6.