As part of a Section 106 agreement, we delivered the detailed design of surface and foul water drainage networks for direct connection into a proposed combined system discharging to the public combined sewer downstream.
The surface water system was designed to accommodate a 1 in 100-year storm event + 25% climate change allowance, utilising an oversized pipe network to provide in-line self-storage during surcharge conditions. An offline geo-cellular attenuation tank was also designed to capture and store excess runoff during extreme rainfall events.
The external works included design of earthworks and paving layouts to create large areas for vehicle access, parking, and storage, with levels based on site-specific ground investigation data and operational requirements. All external hardstanding areas were graded to achieve compliant crossfalls and longitudinal profiles, directing runoff toward ACO kerb drains that discharged into strategically placed sump outlet boxes with sediment management via manhole sumps.
Surface water quality was managed in accordance with SuDS best practice, incorporating a by-pass fuel interceptor and a downstream defender unit for pollutant removal. A hydro-brake flow control chamber was used to limit discharge to the agreed rate set by the Lead Local Flood Authority (LLFA) and Water Authority.
The piped networks were optimised for gravitational flow, with gradients and diameters selected to achieve self-cleansing velocities and minimise long-term maintenance requirements. Additionally, a dedicated effluent trade drainage network was designed to accommodate carriageway washdown flows, discharging to the foul sewer system in line with regulatory requirements.
