Are soakaways suitable in clay soils?
Over the years we have seen multiple examples where soakaway systems have been proposed within development schemes situated on clay without consulting a Flood Risk Consultant, and the Local Authority have rightfully raised a holding objection, pending further investigation.
Typically developments situated on soils with a high clay content tend not to be suitable for infiltration SuDS as water is not able to freely drain into the underlying ground. The clay content in the underlying soil impedes percolation meaning that any stored water has nowhere to go. During a storm event water will accumulate within the soakaway until it reaches its capacity and breaches the ground surface causing localised flooding.
In light of this, Surface Water Drainage features such as soakaways or permeable paving may not be best placed within a development proposed on clay soil.
The map below shows areas within England and Wales where the underlying bedrock geology generally comprises an element of clay.
Areas shown to be underlain by bedrock geology comprising clay include:
London
Colchester
South Reading
Woking
Southampton
Based on British Geological Survey data, development in locations shaded red on the above map may need to explore discharge to watercourse or surface water sewer as the ground conditions may not be favourable for soakaway drainage.
However, it is important to note that there is always the potential that infiltration may be possible based on the presence of localised superficial deposits (soils which sit on top of the bedrock layer). Therefore it is always beneficial to undertake a site-specific investigation to get an understanding of the soil conditions at your development site and to inform your Surface Water Drainage Strategy.
Should infiltration to ground be unviable, there are several alterative SuDS components which may be suitable to incorporate as part of a surface water drainage solution in clay soil. These include:
Green Roofs – Planted roof areas which are designed to intercept and retain precipitation reducing the volume of surface water runoff during a storm event and attenuating peak flows.
Rain Gardens – Also referred to as bioretention schemes, rain gardens can take the form of shallow landscaped depressions or raised planters and are used to reduce rainfall runoff rates and volumes whilst also mitigating the impact of pollution.
Rainwater Harvesting – The collection of rainwater runoff for re-use (eg. washing the car, watering the garden or domestic utilities).
Swales – Shallow, vegetated channels with a flat bottom used to convey, treat and attenuate surface water runoff.
Ponds – Features with a permanent pool of water that attenuate and treat surface water runoff.
Lined Permeable Paving – A source control feature suitable for use as pathways, patios or access roads which allows rainwater to infiltrate through the paving surface. Water treatment is provided through the filtration process and the sub-base is used to attenuate runoff.
Attenuation Storage Tanks – Below ground crates with a void space used to attenuate surface water runoff.
If you have a development on clay soil and would like to find out more about the drainage solutions available to you, please get in touch with one of our team who would be happy to discuss your options.