An integrated modelling study to upgrade the sewerage system of a coastal town

Two complementary methods for the design of storage capacity in an upgraded sewerage system in relation to compliance with the Bathing Water Directive are described. A ten year record of hourly rainfall depths was analysed to give the daily maximum depth of rainfall for durations of 1h, 2h, 4h, 6h, 8h and 12h. The rainfall events that exceeded the five year design storm were removed. The first method comprised selecting the largest of the second worst storms in each of the ten years and using those as design storms of compute the storage volume required for the upgraded sewerage system. The second produced an analysis of the probability of compliance against storage volume in the upgraded sewerage system. This involved consideration of the probability of an overflow event occurring on a sampling day, statistical analysis of the daily maximum depths of rainfall, and the determination of rainfall depths and durations for arange of storage volumes. The probabilistic method was considered to provide a useful aid to decision makers.