The recently developed water quality simulation model FGSM of the German Association for Water Pollution Control (ATV) has been set up to simulate major water quality parameters of a small urban stream. After collecting the necessary input data the model was calibrated for dry weather conditions. Based on the calibrated model typical combined sewage overflow events were simulated. The model was able to simulate dry weather conditions well. For the simulation of combined sewage overflow events a typical event was simulated. The model was able to simulate high peak flows almost without damping. The biological oxygen demand was shown to sharply increase up to 20 mg/l, while the oxygen concentration decreased to less than 5 mg/l. The simulation showed that dissolved oxygen levels stayed during the simulated event above 4 mg/l. The FGSM appears to be a valuable tool for assessing not only the chronic, but also the acute effects of combined sewage overflow events.

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