Continuous simulation of rainfall-derived inflow/infiltration (RDI/I) into wastewater collection systems has been widely applied to assess wet-weather system performance. SWMM 5 allows up to 18 empirically-derived unit hydrograph parameters to be defined on a monthly basis for continuous simulation. In order to define monthly values, a sufficiently long duration of flow monitoring data must be available; e.g. a one-year dataset is not likely to provide reliable monthly values. Long-duration wastewater flow records are often available only at a relatively few critical flow monitoring locations. This can create trade-offs between spatial and temporal resolutions in the definition of the various unit hydrograph parameters.
Using the Sewer System Capacity Model Update 2006 project for the City of Columbus, Ohio as a case study, use of a one-year flow monitoring dataset to define the initial abstraction (IA) parameters is demonstrated. With this dataset, the IA parameters were characterized seasonally (not monthly) at the finer spatial resolution available for this dataset (over 200 flow monitors within the 450 square-mile area). Alternatively, the use of monthly values for the IA parameters could be achieved by using the longer (13-year) flow monitoring dataset available at a coarser spatial scale (roughly 30 flow monitors). Due to the amount of effort involved and time constraints, only a pilot study was performed to demonstrate the approach. The pilot area was selected and several RDI/I unit hydrograph approaches were compared to develop monthly-varied values for the IA parameters. In order to provide an efficient means for adapting the seasonally-varied values to the monthly level, a relationship of monthly-varied IA values versus seasonally-varied IA values was investigated. The pros and cons of this approach are discussed.