The influence of Combined Sewer Overflows (CSOs) on receiving water quality must be considered for comprehensive river basin management. Receiving water impact models are tools to assess the influence of intermittent discharges on water quality. Owing to the difficulty and expense of collecting data and a perceived requirement for a computationally fast model this relatively simple fast dynamic flow and pollutant routing model was developed. In this paper the flow and pollutant routing techniques are detailed for the construction of a dendritic model to represent many aspects of an urban watercourse. The flow routing is achieved with a combination of hydraulic and hydrologic techniques. The pollutant routing is undertaken using a variation of a Lagrangian routing technique to enable routing on a widely spaced flow grid. Results of the flow routing model and details of the complete models performance in a simulation of an English urban river system are given. The assessment of the model showed the pollutant routing technique to require some further refinement for real-time simulation and the Water Quality Index (WQI) was also found to be successful but required more sensitivity. As a whole the model was found to be too slow to be operated as part of a real time control scheme but was a useful tool for the assessment of multiple CSO impacts on a dendritic urban river system. The future development of RATS will involve a close association with the continuing development of radar based real-time control procedures for controlling strategic urban drainage systems in a climate of uncertainty.

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