Environmental impact assessment of sewerage plant releases into a bight on the North Sea with a 2DH model

The city of Btisum, situated at the northwest corner of Meldorf Bight on the North Sea (about 100 km from Hamburg), is a popular coastal resort with a population increase from 4000 to 30,000 inhabitants in summer. The daily volume of released wastewater varies between 1500 and 5000 m3. The effluent discharges are usually treated to good standards. Measurements have shown however, that in the nearby bathing water high coliform concentrations not compliant with the limits for total coliforms set by the EEC directive for clean and safe beaches have been observed sporadically. As a consequence, the beaches had to be closed temporarily for any bathing activity. Due to the costs involved in carrying out measurements covering a wide range of conditions, a detailed diagnose of the problem was not possible. An alternative approach, involving the development and application of a numerical model to the problem in question, was then undertaken.