Spatial and Seasonal Characterization of Infiltration/Inflow for a Regional Sewer System Model
The most recent release of the City of Detroit regional sewer collection system SWMM model includes improvements made to the previous characterization of rainfall-dependent inflow/infiltration (RDII). These improvements included a method to account for seasonal variation in RDII C factors, initial abstractions, and initial abstraction recovery rates. Furthermore, a correlation was developed which describes spatial variation of the seasonally varied RDII C factors as a function of construction age. The GIS tools and the correlations were used to establish RDII C factors throughout the separate sewer areas of the system. The initial abstraction term showed significant seasonal variation; however, spatial variation was not discernible. To support the RDII characterization method and to quantify a reasonable value for the initial abstraction recovery rate, a RUNOFF model was developed for three separate sewer service areas for which flow and rainfall data had been collected. The calibration results for the initial abstraction recovery rates support seasonal variation in this parameter.
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