Influence of Sewershed Characteristics on Rainfall-Derived Inflow and Infiltration

Basement backups and sewer overflows from rainfall-derived inflow and infiltration (RDII) are a significant threat to human and environmental health; however, reducing inflow and infiltration is a challenge for municipalities due to the difficulty and resources required to accurately identify source areas. This case study seeks to address this challenge by evaluating the influence that sewershed characteristics have on inflow and infiltration into sanitary sewer systems. To do so, we used 4.5 years of monitoring data from 19 sanitary sewer locations in Milwaukee, Wisconsin to explore the relationships between RDII defined using the RTK unit hydrograph method and sewershed physical, land cover, and pipe characteristics. Results demonstrate that inflow, or fast direct flows into the system, is positively correlated to pipe length per acre, number of parcels, and medium intensity land use. Infiltration, or slow inputs from groundwater sources, is negatively correlated with imperviousness, pipe length per acre, low intensity, and medium intensity land use. Multivariable linear regression using these parameters explained between 55% and 72% of the variance in normalized inflow and infiltration. These findings demonstrate a way in which collection system managers may be able to narrow the search areas for RDII sources within their sanitary sewer systems by evaluating sewershed characteristics.

Automated summary

This study examined the impact of sewershed characteristics on inflow and infiltration in sanitary sewer systems. The findings showed that inflow is positively correlated with pipe length per acre, number of parcels, and medium intensity land use, while infiltration is negatively correlated with imperviousness, pipe length per acre, low intensity, and medium intensity land use.