Housing SuDS Scheme.

The following case study discusses a residential development for eight units within Crew Green. 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  

In addition to the above, we completed the design and adoptable works approvals for the Section 104 (S104) foul sewer and Section 38 (S38) estate roads. 

How We Approached The Design:

The main constraints for the site were the placement of an existing foul sewer to the front of the site, local surface water flood risk, and topography due to the placement of an adjacent ordinary watercourse at the highest point of the site. Furthermore, the site lays on impermeable clay ground meaning that surface water had to be treated, attenuation and discharged at a controlled rate to the ordinary watercourse.  

With the main aim to remove the need for surface water pumping, Berrys reviewed the site levels and developed a design which achieved gravity falls for both the SuDS and foul systems. This proved particularly challenging to overcome, so we advised our client that some additional land was required to achieve a SAB compliant SuDS network. The design solution included a combination of on-plot upstream source control SuDS, along with downstream site control components. The SuDS included a treatment train of permeable paving, rain gardens, swales, with a final ‘dry’ attenuation basin. Rates of discharge to the watercourse were controlled to at or below equivalent greenfield runoff rates by a Hydrobrake flow control device.  

Surface water drainage and SuDS features were hydraulically modelled using the latest Causeway Flow software to convey a maximum storm return period of 1 in 100 year plus 40% climate change (or 1% annual event probability).  

Exceedance flow routing was also a key consideration during design, due to a catchment laying above the site. To reduce the risk for overland flows flooding property, Berrys designed the site levels to include an exceedance channel routed towards the adjacent fields to maintain the pre-existing flow paths through the site.