In evaluating the present or future state of integrated urban water systems, sewer drainage models are often used in calculating the expected return periods of given detrimental acute pollution events and the uncertainty thereof. In the present study an urban runoff pollution model is developed for a Dutch combined sewer catchment. The water quantity model consists, in its final shape, of a combination of linear reservoirs in series and some physical characteristics of the sewer system. Attached to the water quantity model, a model was developed to simulate the suspended chemical oxygen demand (COD) concentration at the point of combined sewer overflow (CSO). The model structure has been developed on the basis of a rainfall input series and a water level and a suspended COD concentration output series. Physically interpretable parameters have been included where this could be statistically justified by the available observation. Model development has been based on an approximate likelihood ratio test, tests for parameter significance, parameter correlation and information criteria.

It has been possible to formulate a model to describe the rainfall-runoff process at the Loenen catchment with a coefficient of determination (R2) of 0.83 for the water level at the overflow weir. The best model contained eight estimated parameters of which six were directly interpretable in traditional urban drainage terms. A three parameter suspended COD concentration model was developed with an R2 of 0.44. Though considerably less successful than the water quantity modelling, the results are interesting when viewed in relation to the small number of model parameters. Furthermore the model structure can be linked to existing resuspension theory involving a critical shear stress.

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