Assessing SWMM 5 Hydrologic Parameter Benefits for Model Calibration

Abstract

A 33 acre (13.2 ha) sewershed in Cincinnati, Ohio that is regulated by sanitary sewer overflow (SSO) structure 228 experiences high amounts of rain derived inflow and infiltration (RDII), which leads to overflows. The amount of overflow that SSO 228 discharges during a typical rainfall year must be significantly reduced per a federal consent decree between the United States Environmental Protection Agency (USEPA) and the Metropolitan Sewer District of Greater Cincinnati (MSDGC). As part of the SSO reduction alternatives development and planning process, a system wide model (SWM) representation of the SSO 228 sewershed system was developed and subsequently calibrated and validated in 2012 within the USEPA Storm Water Management Model, Version 5 (SWMM5) model framework. As the SSO 228 overflow reduction preferred alternative development progressed from planning to detailed design, the SSO 228 SWM received additional refinement, as well as recalibration and validation. As a part of the model refinement effort, the SSO 228 SWM was evaluated as a pilot study to test additional modeling techniques available in the SWMM5 program that may be useful in other areas of the MSDGC system. These additional techniques included the use of initial abstraction and drying time, seasonally varied RDII parameters, and application of the Aquifer module to simulate groundwater impacts. Results and conclusions drawn from this pilot study were used to reevaluate the MSDGC approach to calibration and validation, assess the benefit of the additional hydrologic parameters, and develop guidelines for determining model complexity based on monitored flow data trends.