The Developer Services Team at Dŵr Cymru Welsh Water (DCWW) required the design of an offline attenuation structure and flow control manhole to limit the forward flow rate into the existing public combined sewer at Holmer West, Hereford. This project was essential to protect the existing asset from the resultant flooding from the additional flows from a proposed housing development upstream. Burroughs carried out the hydraulic modelling, civil engineering design, and a value engineering & buildability exercise. This resulted in a design that was simple to construct, cost effective, and minimised the embodied carbon of the project.

The developer proposed to construct 560 new residential dwellings (including a care home) on a green field site near Roman Road / A4103, north of Hereford. Numerous Hydraulic Modelling Assessments carried out by DCWW all came to the same conclusion – An offline attenuation structure and flow control manhole was required at a strategic location to limit the flows into the downstream public combined sewer and to attenuate additional flows during high flow events.

Burroughs received a partial design for this scheme which incorporated a reinforced concrete culvert and manhole design. Through initial liaison with DCWW and Alun Griffiths where it was suggested that road closures and kerb line amendments would be required to get the large 4m diameter concrete manholes to site. To mitigate this, Burroughs contacted a proprietary recycled plastic drainage product supplier to investigate the feasibility of using their products to create the necessary attenuation system and utilising plastic welded elements to reduce the manhole diameters.

Through a collaborative and iterative process been all key stakeholders, a solution which met both operational and buildability requirement was derived.