Social Housing Detailed Drainage Strategy.

This case study explores a surface water SuDS and foul design for a social housing development, consisting of 14 apartments within a single block. The proposed layout is to be situated on a steeply sloping site laying on heavy clay impermeable ground. Due to the site being located in Wales, Sustainable Urban Drainage System (SuDS) are a mandatory requirement and SuDS Approval Body (SAB) approval was required. The drainage scheme design corresponded to all four pillars of SuDS: 

• Water Quantity 
• Water Quality 
• Amenity 
• Biodiversity  

How We Approached The Design:

In addition to designing on a steep slope, Berrys had to contend with a local flood risk issue at the lowest corner of the site, where it was reported that surface water pluvial flooding affects several neighbouring properties along the line of an ordinary watercourse. This issue meant that the site was sensitive to local residents during planning, which meant we had to approach the design and consultation process sensitively.  

In line with the SuDS discharge hierarchy, listed within Welsh Statutory SuDS Standards, Berrys reviewed the infiltration data which returned poor rates due to clay ground conditions. Following this we met with the client on site to locate possible discharge routes, where we located the drainage ditch on the northern boundary and had not been maintained for many years. Therefore, we worked with and advised the developer to ensure that they exercised there riparian ownership duties to carry out the required maintenance works.  

Berrys then commenced preliminary design with outline hydraulic modelling and scheme drawings for pre-SAB application. The SAB returned favourable comments, agreeing with the principles of the scheme and wanted to see further detail. At the early stages of the site, we also considered level design thoroughly as we wanted to ensure slopes were not too steep, to ensure that residents had space available for amenity and leisure purposes. Gradients and levels were also a critical consideration for the development buildability during the construction stage, along with safe future maintenance. The level design also presented challenged for the sewer design, as the block contained a lower ground floor. Berrys were able to achieve a gravity foul sewer connection through liaison with our in-house architect and the client’s site team for existing surveyed sewer connection data. 

Due to the site constraints the Berrys engineering team adopted an innovative approach to the SuDS design which adopted a true management train approach, to maximise source control and upstream storage and treatment. The site was therefore split into sub-catchments, with a number flow control devices utilised to maximise storage opportunities within rain gardens and permeable paving. With space at a premium on the site, this approach was required to ensure that the downstream storage volume was kept to a minimum. A final storage swale was designed with a vortex flow control device discharging to the ordinary watercourse. The Berrys design lowered the bed of the watercourse, allowing a sump to be formed, which will reduce the risk of blockage to the pipe outlet which currently causes flooding. This measure will therefore protect the adjacent properties from future flooding.