CALIBRATION TO A 50-YEAR STORM VALIDATES MODELING TO SUPPORT CSO RELOCATION DESIGN

Hydraulic/hydrologic modeling was conducted using the EPA Stormwater Management Model (SWMM) in support of the design of a CSO consolidation/relocation project in South Boston. A detailed SWMM model previously developed during CSO facilities planning predicted the peak flow to the consolidation/relocation conduit to be on the order of a 25-year, 24-hour storm. The original SWMM model, however, was only calibrated to a 3-month, 24-hour storm. For design, the SWMM model used in facilities planning was refined and recalibrated to provide greater confidence in the design flows. A 50-year storm that occurred during the flow monitoring period provided confidence that the recalibrated flows were appropriate for use as design criteria for the CSO consolidation/relocation conduit. Predicted peak flows from the updated model prior to recalibration with the 50-year storm were within eight percent of the final predicted peak flows following recalibration. This finding indicated that the original facilities planning model, with certain enhancements in detail, was reasonably accurate in predicting peak flows in extreme storm events, despite only being calibrated to a 3-month storm. Key factors contributing to this finding were that the collection system had the capacity to convey runoff from a storm on the order of a 25-year storm, and that limitations on catchbasin inlet capacity were partially offset by runoff being conveyed by street gutters to areas with greater inlet capacity.