Inflow and Infiltration Study

The City of Duvall is developing a comprehensive program to reduce infiltration and inflow (I&I)—the stormwater and groundwater that can enter the sanitary sewer system through cracks, joints, or faulty connections. Managing I&I helps preserve sewer capacity, reduce treatment costs, and prevent backups or overflows during heavy rainfall.

Program Background

The City’s 2021 Sewer Utility Capital Improvement Program (CIP) identified the need for a long-term I&I program, structured around two coordinated efforts:

• CP-1: I&I Program Planning – an ongoing citywide effort to monitor and inspect the sewer collection system to locate areas where excess water enters.

• CS-1: I&I Repairs – construction projects to fix identified issues through methods such as pipe lining, grouting, or manhole rehabilitation.

The City Council approved $615,000 in the 2023–2024 budget to establish this program and complete the initial maintenance cycle, with a focus on Duvall’s Old Town area where older concrete sewer mains are most prevalent.

Monitoring and Field Work

To build the foundation of the program, the City began flow monitoring in early 2023 at four key sites in Old Town. Once the monitoring period ended, the City’s consultant reviewed the data and produced a report identifying potential I&I problem areas. To confirm these findings, the City performed smoke testing in the affected neighborhoods and successfully located several sources of infiltration and inflow.

Building on that early work, in October 2024, the City installed five additional flow meters at strategic points throughout the sewer system, along with a rain gauge at the wastewater treatment plant. These instruments tracked sewer flow during rainfall events to help determine where rainwater may be entering the system. Monitoring concluded in April 2025, and the collected data is currently being analyzed.

Next Steps

Once data analysis is complete, the results will guide a targeted maintenance and rehabilitation plan to further reduce I&I. This plan will identify the highest-priority areas for repair and will serve as the basis for future design and construction projects aimed at improving the efficiency and reliability of Duvall’s wastewater collection system.