The application of design storms in the analysis of large sewer systems typically assumes that an intense precipitation scenario applies uniformly across the modeled sewer service area. However, spatially uniform precipitation across large areas is a modeling assumption that fails to
acknowledge the significant spatial variability that is generally observed. Yet there is little basis in the literature for the use of design storms that account, directly or indirectly, for spatial variability. Using spatially constant design storm rainfall can cause over-estimation of wet
weather flow and storage volumes. Taking into consideration the spatial variability of precipitation in the design storm analysis of sewer system capacity is the subject of this paper. In order to support use of their System Wide Model (SWM) for sewer system capacity analysis, the Metropolitan
Sewer District of Greater Cincinnati (MSDGC) project team performed spatial analysis of the local rain gauge data collected during model calibration, plotting rainfall depths for basin areas of varying size for selected events. This analysis revealed that relatively little variability occurs
across areas less than 2 km2 (494 acres). However, for basin areas between roughly 2 km2 and 10 km2, basin precipitation decreases to approximately 85% of the rainfall depths observed for the smaller basin areas. For areas larger than roughly 10 km2,
no clear trend applies as areas increase in size. As a result of this analysis, MSDGC developed a set of spatial adjustment factors that enable the appropriate reduction in design storm hyetographs to be applied based on the size of the basin area modeled. For areas of 2 km2
or less, no adjustment is required. For larger areas, an adjustment factor is applied to each discrete point on the selected design storm hyetograph to produce a precipitation input time series that accounts for spatial variability. Once an appropriate design storm hyetograph (or set of
hyetographs) is developed that accounts for spatial variability; other considerations that affect wet-weather sanitary sewer flow conditions are storm duration, and inter-storm periods. Three aspects of the design storm protocol addressed herein are: 1) the range of design storms to be applied;
(2) the approach to accounting for spatial variability in those storms; and (3) the assumed conditions for antecedent soil moisture to be applied with those storms. Finally, guidance on developing a design storm protocol addressing the appropriate range of design storms that account for spatial
variability along with recommendations for future considerations are proposed.
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