RECEIVING WATER PROTECTION DURING WET WEATHER

It is proposed that in the future only site-specific and problem-related measures should be applied to solve the combined sewer overflow problem. Based on experience in Switzerland a procedure is developed to plan technically and ecologically optimised measures to protect urban receiving waters during wet weather. This procedure is based on results of a field study (the Fehraltorf/Switzerland case) carried out by a multidisciplinary research team. The most important stages of the procedure can be described by the definition of site-specific ecological objectives, the determination of a few key variables that have significant influence on the ecological state of the receiving water and that can be modified with some of the site-specific measures below, measurement, observation, and calculation of these variables, choice of site-specific measures against wet weather pollution problems (in catchment, sewer system, and receiving water), and recommendation of measures and their priorities, based on predicted cost and effectiveness with respect to the variables above. The results of the case study demonstrate the necessity of the multidisciplinary approach that treats the urban catchment, the urban drainage system, and the receiving water as one single system. Since urbanised areas continue to grow and since the reconstruction of old depreciated sewer systems is a major technical and economical challenge, the proposed modification of the planning concept may lead towards improved drainage systems, requiring appropriate investment, and causing less serious damage to the environment.