Characteristic Width and Infiltration for Continuous SWMM.
A SWMM computer model was developed for the Detroit Water and Sewerage Department (DWSD) collection system to gain an understanding of overflows of combined and wet sanitary sewage (Camp, Dresser and McKee, 1993). This system serves both the City of Detroit and surrounding communities, with a service area of 800 square miles (2067 km2), which includes both combined and separated service areas. It contains a number of complex interconnections, loops, and overflow points within the system. Both SWMM EXTRAN and SWMM TRANSPORT were used to provide estimates of overflows within the system, while SWMM RUNOFF was used to compute the wet-weather inflows from the combined sewer service areas.
The SWMM EXTRAN model was developed for specific calibration and design events. The use of EXTRAN enabled a more detailed analysis of the complex hydraulic interactions within the large collection system. The information gained from this analysis was used as a basis for the continuous modelling. A continuous SWMM model was developed to provide estimates on annual overflows volumes and frequencies that could be expected for existing conditions and for a variety of alternative control strategies. For the continuous modelling, the SWMM TRANSPORT model was used for analysis of the collection system. During the calibration of the continuous model, it was determined that the conventional approach of accounting for a subarea’s internal routing by reducing the subarea's characteristic width resulted in underpredicting runoff volumes from pervious areas. This chapter deals with the problem of underpredicting runoff from pervious areas when the width parameter is used to account for internal routing and attenuation, and how this problem was addressed in the study of the DWSD collection system.
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