General Sewer Plan Update
Project Details
The City’s sewer service area encompasses the entirety of Mercer Island and serves a population of approximately 24,000. The system includes approximately 105 miles of gravity mains, 2,400 manholes, 17 pump stations, 1 flush station, and 12 miles of low-pressure Lakeline system.
Background
A General Sewer Plan (GSP) provides a comprehensive, long-range plan for the development, operation, maintenance, and improvement of a municipality’s sanitary sewer system. In Washington State, it is a key planning document required by the Department of Ecology that is typically updated every 6 to 10 years. The City’s current GSP was last approved in January of 2019 and is available here: https://www.mercerisland.gov/publicworks/page/general-sewer-plan
In addition to updating the General Sewer Plan (GSP), the City plans to develop its first comprehensive, systemwide hydraulic model. The current model—developed as part of the 2019 GSP—was limited in scope, focusing primarily on specific infrastructure within the Town Center area. A complete system model will enable the City to more accurately assess sewer capacity, evaluate the impacts of future development and land use changes, and prioritize capital reinvestment and rehabilitation efforts.
Description
The purpose of this project is to update the City’s General Sewer Plan (GSP) in compliance with Washington Administrative Code (WAC) 173-240-050 and Revised Code of Washington (RCW) 90.48.110. This forward-looking plan will guide sewer system investments and improvements over the next 20 years. In conjunction with the update, the City will expand its current hydraulic model to encompass the entire Island, enabling a comprehensive evaluation of system capacity, performance, and future needs.
Project Timeline
Planning/Design: August 2025 – December 2027
Estimated Budget
$500,000
Contact:
Christopher Marks, Utilities Engineer, 206-677-1027; chris.marks@mercerisland.gov
Status Updates
October 30, 2025
A project kickoff meeting was held on September 23 with Carollo Engineers and City staff to establish key project elements and schedule. Since then, Carollo has begun reviewing background documents and integrating GIS data to develop a comprehensive hydraulic model of the sewer system. The next major step is installing 20 flow meters at locations across Mercer Island. These meters will collect dry and wet weather flow data over the next three months, providing critical information to calibrate the model.
June 13, 2025
On May 5, the City issued a Request for Qualifications (RFQ) seeking statements of qualifications (SOQs) from engineering firms to update the City’s GSP and develop a comprehensive hydraulic model. Last week, following a thorough evaluation of the submitted SOQs, the City identified Carollo Engineers as the most qualified firm. Contract negotiations are currently underway.