Fylde Coast Bathing Water Improvements – Storm Water Management for Compliance with Bathing Water Directive

As part of the investigations to determine the most appropriate scheme to bring the Fylde Coast bathing waters up to the quality required by the EC Bathing Water Directive, mathematical models have been used to examine the hydraulic behaviour of the sewerage system and the subsequent dispersion of effluent in the receiving waters. The discharge characteristics of the existing sewerage systems were examined by means of an extensive WASSP model, validated by means of flow measurements gathered from critical points of the sewer network. This model was then used, in conjunction with a time series rainfall record for the area, to investigate the effect of the intensity and duration of rainfall events on the volume of storm sewage to be discharged. Because the area is drained by a combined system, the management of storm water is of utmost importance and the output of the WASSP model was used to determine the input to the coastal dispersion model for rainfall events. A preliminary examination of the possible sites for sewage treatment works and sea outfalls, for sewage and storm water, suggested four possible schemes involving either inland treatment, marine treatment or a combination of the two. All options required the discharge of storm water to the sea. Extensive dispersion modelling was carried out to examine the probable effects of various storm-water management strategies on bacterial concentrations in the receiving waters. From this it was apparent that storage of storm flows, with controlled discharge, at the state of tide when environmental impact would be minimal, was required to achieve compliance with the provisions of the Directive. By integrating the WASSP modelling of various base-flow and storm-water storage options with dispersion modelling, it was possible to establish appropriate design criteria.