Effective, Sustainable, Long-Term GWI and RDI/I Source Reduction: RTK and Design Storm Analysis of 7 Years of Post-Rehabilitation Monitoring

Understanding and documenting the effectiveness of source flow reduction methodologies in separate sanitary sewers and deriving parameters that can be input into predictive hydrologic and hydraulic models to reliably simulate these alternatives remains a challenge. This paper presents findings on analysis of 26 months (Pre-Construction) and 7 years (Post-Construction) of flow monitoring data for a source flow reduction project. The intent here is to establish peak flow rate and volumetric flow reduction effectiveness of the source flow reduction work as well as to establish the impact of the completed project on various predictive parameters. The project was a nighttime flow isolation study based, targeted, partial parallel replacement program. The project successfully eliminated chronic basement flooding and multiple sanitary sewer overflow (SSO) structures in a 56,000 l.f. mainline, 70-year-old suburban residential separate sanitary sewer system. Approximately 28% of the mainline sewer was replaced utilizing a parallel replacement approach that converted the existing mains to a subsurface drain. Engineering approach and analysis of rainfall derived infiltration and inflow (RDI/I) parameters are presented. Methodologies include wet weather hydrograph deconstruction regression analyses, as well as unit hydrograph (RTK) and groundwater infiltration (GWI) analysis. Extrapolation of RTK findings to a 10-year design storm is presented.