Indeed, the British Geological Survey (BGS) Maps show that Wadhurst Clay Formation underlies the site. This means the flood risk from groundwater is low, as Wadhurst Clay will prevent the transfer of high concentrations of groundwater. But conversely it means that infiltration techniques such as soakaways are not suitable for surface water drainage.

We appointed civil and structural engineering practice Price & Myers to undertake a detailed flood risk and drainage assessment and prepare an initial drainage design and maintenance strategy. The proposed design will improve drainage and protect against flooding even in rare 1-in-100-year events. We’ve also applied a ‘climate change factor’ to ensure the worst possible case has been considered.

The possibility of implementing Sustainable Drainage Systems (SuDS) at the site was assessed using a standard hierarchy of preferred surface water management methods:

  1. Rainwater harvesting promotes the storage and re-use of rainwater collected from roofs and hard surfaced areas. Individual rainwater butts are proposed for each property. These contribute to the reduction of runoff rates and volumes within a development.
  2. Because infiltration is not possible, surface water will be attenuated and slowly released into the existing drainage ditch to the southeast of the site. New SuDS features such as raingardens, permeable paving, a basin, and below ground stores are included to effectively attenuate the water.
  3. The basin will not typically have water in it, it’s designed to only be needed after large storm events when the water has built up. During a rare 1 in 100 year plus climate change event, the water may reach the top of the basin. In the event of an even greater storm, the basin may overflow. In which case it’s been designed to also flow into the existing ditch (which should no longer typically fill with water due to the new basin).

More detail is available in the Drainage Strategy submitted as part of our planning application. A maintenance strategy is also included to ensure everything continues to work as it should.