A model is developed, using a unit hydrograph approach for sewer flow synthesis and a simple mixing model to calculate the pollution load from combined sewer overflows, to simulate the long-term behaviour of storm overflows in combined sewerage systems. It is shown that this procedure synthesizes overflow operation characteristics to acceptable engineering accuracy, measured relative to the adopted standard for UK practice, namely predictions from the WALLRUS suite of engineer's software. This is achieved at only a small fraction of the computer runtime and so makes practicable a wide range of overflow performance and river impact studies using local rainfall records of unlimited extent. The model is applied successfully to three drainage networks. Results show that the model, COSSOM, achieves predictions with respect to runoff volume and overflow characteristics well within : 00 % of full. WALLRUS applications. Sensitivity studies demonstrate that performance of the model improves when catchment unit hydrographs are obtained, by preliminary application of WALLRUS using a rainfall intensity close to the maximum in the observed data and for rain duration close to the time of concentration of each sub-catchment under study. It is also shown that with little loss in accuracy relative to application of the WALLRUS approach for unit hydrograph development, COSSOM can be operated with other rainfall/runoff/pipeflow models.