Since 1995, in the Netherlands guidelines have been used by practitioners in the field of urban drainage to determine the hydraulic and environmental performance of combined sewer systems, as required by the water boards responsible for quality control of surface waters. These guidelines should guarantee a certain quality level of master plans, generally produced by consultants. We checked ten representative master plans issued over the last six years on quality, verifiability and completeness. We also checked on computed output differences in relation to the allowable mass balance error of the software used. From this sample study it may be concluded that the quality of model application is generally speaking very poor; at least one cannot be sure of the quality as no data whatsoever to access the quality is presented in most cases. None of the parties involved (municipalities, consultants, water boards) seem to take accountability for the large investments based on modelling results of unknown quality. This situation should be altered by renewed interest in responsible model application, more budget for monitoring and model calibration and enforcement of water quality demands based on factual system performance and not only simulation results.

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