A DESIGN METHODOLOGY FOR COMBINED SEWER SYSTEM ELEMENTS WITH OVERFLOWS IN COASTAL ZONES

The European Community Directive (ECD) referring to bathing water qual ity establishes its admissibility as a function of the results obtaine d in control samples. This procedure is convenient to establish a moni toring and control program but presents serious difficulties in design ing elements of combined sewer systems with overflow discharges in the coastal zones. These difficulties appear when it is necessary to desi gn according to unknown future random control results which are obtain ed with also unknown sampling frequency. This uncertainty is usually s olved by identifying the percentage of samples that do not comply with water quality criteria with the percentage of time that water quality does not satisfy the regulation which, strictly speaking, is not corr ect from the point of view of ECD. In this paper, a methodology is pre sented to analyze, in probabilistic terms, the influence of overflow f rom sewer systems on the quality of bathing water and on the design of the capacity of sewer system elements. This methodology includes some concepts as the return period of no compliance time and shows the pos sibility of using regression equations between overflow parameters and no compliance time, this being useful to handle the large amount of d ata for the probabilistic study. All this being in strict accordance w ith 76/160/EEC. Finally, the design procedure for the coastal intercep tor of Gijon (Spain) is presented as an example of the application of this methodology.