The performance of combined sewer systems is often assessed on the basis of a typical year approach. In this approach, the typical year is defined as a one-year period for which system performance is representative of the long-term average. “Performance” is often defined by overflow frequencies and/or volumes. The use of the typical year is a modeling convenience which avoids the need to run multi-decade simulations to find the long-term averages.
In wastewater collection systems, flow is composed of dry weather flow (DWF) and wet weather flow (WWF). DWF can be separated into base wastewater flow (BWF) and groundwater infiltration (GWI). Flow hydrographs, especially during small storms, and total annual overflow volume can be significantly affected by DWF rates and volumes. GWI is a significant component of DWF, therefore estimation of GWI is critical for characterizing system performance with typical year simulations.
As demonstrated in other studies by the City of Columbus, long-term wastewater treatment plant effluent data and long-term precipitation data at the Columbus CMH airport rain gage could be used to estimate GWI. As part of the Sewer System Capacity Model Update 2006 project for the City of Columbus, Ohio, the relationship of GWI and rainfall was further investigated. The estimation of typical year GWI rates was achieved by applying that relationship to the typical year rainfall. Finally typical year GWI was applied to simulate wastewater collection system performance under typical year conditions.