Increasing stormwater outfall duration, magnitude, and volume through combined sewer separation

Combined sewer separation projects designed for water quality benefits will change pipe networks and alter stormwater outfall hydrographs. Separation induced outfall hydrograph changes were explored for the 12.14 ha (0.05 mi(2)) combined sewer overflow (CSO) 050 service and its 29 stormwater catchbasins in Syracuse, N.Y. Topographic and infrastructure analysis together with catchbasin monitoring of stormwater runoff were used to parametrize and validate the Storm Water Management Model for two events in 2003. Sewer separation impacts on outfall hydrographs were then analyzed for 50 historical and 28 design precipitation events, ranging from 15 min to 24 h duration, 2-25 y frequency. Separation of combined sewers increased historic storm median outfall hydrograph duration by 2,044%, peak magnitude by 113%, and volume by 617%, whereas increases in design storm median outfall duration was 879%, peak magnitude was 27%, and volume was 54%. Changes in outfall hydrograph characteristics were explained by system infrastructure, boil infiltration, and storm properties of intensity and duration. CSO 050 and adjoining outfall sub-areas comprise less than 1% of the Onondaga Creek receiving water drainage area, yet for an extreme rain event the separated systems generated outfalls peaks 18% greater than the 51 year average Creek flow, even though volumetric contributions to the Creek were relatively small. Innovative terrestrial stormwater management is needed to reduce aquatic impacts of separation projects.