Continuous simulation of collection systems allows the modeler to quantify the frequencies of overflows, runtime of pumps, exceedence and dewatering time/duration of storage, and other important characteristics of wet-weather system performance. SWMM 5 allows up to 18 empirically-derived unit hydrograph parameters to be defined and applied on a monthly basis. These parameters can be used in different ways to achieve similar and more or less equally accurate calibration results. However, 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 season). 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 may be more appropriate for multi-year analyses to support planning solutions. To verify these conclusions, the selected approaches were applied to a second flow meter basin and similar results were achieved.