Comparison of RDII Unit Hydrograph Approaches for Continuous Simulation using SWMM 5
Continuous simulation of collection systems allows the modeler to quantify the frequencies of overflows, runtimes of pumps, utilization and dewatering durations of storage, and other important characteristics of wet-weather sys-tem performance. SWMM 5 uses the RTK unit hydrograph approach to calculate rainfall derived inflow and infiltration (RDII). SWMM 5 allows up to eighteen empirically derived unit hydrograph parameters which may be varied monthly. These parameters can be used in different ways through various emphases and parameter combinations including omission of some of the parameters (Gheith, 2009). In some cases similar accuracy can be attained in a particular objective measure, such as peak flow, when using another approach. A consistent approach is recommended to facilitate more equitable comparisons between modeled sanitary sewer service areas, both in terms of the parameters used and the simulation results produced.
Using available flow monitoring data from the Sewer System Capacity Model Update 2006 project for the City of Columbus, Ohio, wet weather flow responses were analyzed seasonally (dormant and growth seasons). A sample flow meter basin was selected and several RDII unit hydrograph approaches were compared using the flow and rainfall data selected for a 16 month period. The analysis indicated that seasonally varied RTK with monthly varied initial abstraction parameters (Dmax, Drec and D0) provide the best simulation results for both large and small storms. However, seasonally varied RTK with seasonally varied initial abstraction parameters (Dmax, Drec and D0) may be more appropriate with limited monitoring data to support planning solutions. This chapter first compares different approaches in terms of the accuracy of the model calibration results, and then further discusses the alternative approach when data availability is limited.
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