Optimisation of control of river and sewage flows in the Great Culvert, England

In response to the European Union Urban Waste Water Directive, primary and secondary treatment is being installed at Birkenhead Treatment Works on the Wirral Peninsula in the North West of England. The 3.26 m diameter, 3.5 km long Great Culvert conduit currently conveys both river and sewage flow, and discharges these into the Mersey estuary. To avoid unnecessarily treating river flow, River Birket flow will be diverted directly into the Mersey estuary at the upstream end of the Great Culvert. Sewage flows remaining in the Great Culvert will be pumped to the Birkenhead Treatment Works, and treated before discharge to the Mersey. During extreme storm events, excessive river flows will be released into the Great Culvert and into the Mersey by weirs and penstocks. An innovative approach was taken to investigate, develop, and optimise a control strategy of various pumps, weirs, and penstocks in the Great Culvert, with the aim of minimising unscreened discharges and flooding. By combining hydraulic and dynamic control modelling into one study, implementation and commissioning times were reduced. In addition, by exhaustive testing, it was possible to demonstrate to the Environment Agency that all reasonable measures were being taken to minimise flooding in the upstream catchment and discharges to the Mersey.