Infiltration & Inflow – Degradation of Collection Systems Over 9 Years; Results From Monitoring and Modeling 150 Basins

King County (comprising Seattle, Washington and surrounding cities) is the conveyance and treatment provider for 35 component agencies. The county has undertaken 2 major monitoring and modeling efforts to quantify infiltration and inflow into the sewers throughout the separated system service area. The first comprehensive monitoring and modeling effort involved 800 flowmeters installed during 2 wet seasons from 2000 – 2002. Modeling of I/I response in 150 basins was conducted and long-term simulations were run to estimate peak I/I from each basin. The second major monitoring and modeling effort was conducted from 2009 – 2011, where roughly the same 150 model basins were monitored and modeled. Modeling of the I/I response is being completed and comparisons are being made of the peak I/I estimates made from both monitoring periods.Preliminary results show that there is a wide variety of variation in how much the I/I response in basins has changed over the course of 9 years. Basins with little new development were selected for comparison of changes in I/I over the 9 year period. The change in peak I/I estimates range from −50% to +136% in the basins with the highest confidence in flow monitoring and the best model vs. meter matches. The weighted average gain in I/I over the 9 years is about 6%. Possible reasons for the wide variation in peak I/I changes are included in the paper, as are next steps in confirming and evaluating this information.