At present, dimensioning of combined sewer detention basins in Germany is based actually on the rational formula and on the idea of the existence of the so-called “first flush”. New measurements and recent investigations have led to the questioning of this dimensioning procedure.

An alternative developed in this paper is based on the application a storm-water-runoff-model (hydrological and/or hydrodynamic) in each special case. Input to the model is a period of at least one decade of continuous rainfall data (long term simulation). The sanitary waste water component is superimposed to the simulated runoff-hydrograph. The first flush-effect is neglected. The dimensioning of a detention basin is based on the determination of several quantities such as number, duration, peak discharge and volume of overflow events in relation to the total water detention volume of the system. The decision on final dimensioning has to be made with respect to the characteristics of the receiving water.

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