RES Environmental Ltd were commissioned by Mott McDonald Bentley to assess an ongoing pollution issue in the Castle Eden Dene, Peterlee, and the potential solutions to the issue.

The area of concern was a combined sewer running through the Castle Eden Dene National Nature Reserve.  The frequent flooding of the sewer was attributed to a lack of capacity in the sewer, due to excessive infiltration and misconnections of surface run off into the foul sewer system upstream.  An existing hydraulic model of the area was updated with survey data the connectivity of the properties in the upstream area.  The model was then verified against flow survey data.  No single source of surface water was found, so the infiltration problem was deemed to be catchment wide.

Several solutions were modelled to reduce flows through the combined sewer in the Castle Eden Dene.  The flows during storm events were complex, as surface runoff to the sewer was creating a flashy flow response and the excessive infiltration created long storm flows, greatly increasing the volume of sewer flows.  The solutions were carefully tailored to address both these issues.  The solutions modelled included: a number of large offline storage units (efforts were made to minimise the volumes required), upsizing the sewer in the Castle Eden Dene and redirecting a major sewer pump station.

The solutions were optimised to; reduce construction and maintenance costs, stop pollution to the Castle Eden Dene, reduce spils from CSO’s and emergency overflows and limit flooding from the rest of the sewer system.