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Moving design storms for combined sewer systems

Abstract:

Design storms for combined sewer system design are generally applied as a uniform input. However, the spatial variability of the rainfall can play an important role. This becomes more and more important, because more larger systems are modelled. Not only is the spatial variability of the rainfall over the catchment as a static phenomena important, but also the dynamic influence of a storm moving over the catchment can result in a significant difference in peak water levels and peak discharges in the system. When a storm is moving over an urban catchment in the same direction as the main flow direction in the combined sewer system, higher peak discharges and water levels often result. If there is a predominant wind direction during wet weather, this can cause a bias in the calculation results. This means that combined sewer systems with a mean flow direction equal to the predominant wind direction during wet weather will have a lower safety level than pbkp_redicted with the uniform rainfall input. For this reason, a procedure is worked out to simulate moving design storms over a combined sewer system in different directions. The largest challenge in applying moving design storms is the proper statistical analysis of the simulation results. One can not just average for each return period the results of the different directions. A whole range of design storms must be simulated for a wide range of return periods. For each peak discharge or water level the global return period T can be determined based on the return periods of that discharge or water level for the different directions.