City staff outlines plan for new 24-inch Mercer Island supply main to replace SPU sections and boost resilience
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Summary
City staff presented plans for about 5,000 feet of new 24-inch transmission main, removal of asbestos-cement pipe on Gallagher Hill, connection of Shorewood Apartments to the city system, an estimated utility cost of roughly $12.5–$13.5 million (with bonds and a state grant available), and an anticipated two-year construction window with major paving targeted for summer 2027.
City staff described a multi-year project to install a new 24-inch transmission main on Mercer Island to reduce reliance on a steep, slide-prone SPU alignment and to consolidate ownership and control of island supply infrastructure.
"This is a generational investment in system resilience," said Clint (city public works staff), who led the presentation. He said the 24-inch main has an expected useful life of 50 to 75 years and would serve multiple generations of Mercer Islanders.
The project will install about 5,000 feet of 24-inch ductile iron restrained-joint pipe running up Southeast 36th, Gallagher Hill Road and turning onto Southeast 40th, according to staff. As part of that work the city will replace roughly 1,500 feet of asbestos-cement (AC) pipe on Gallagher Hill and relocate a pressure-reducing valve station at the bottom of the hill.
Clint said the plan also transfers ownership of certain SPU segments to the city: the city would take over Reach 4 and intends to abandon the Reach 3 segment that runs along the steep Southeast 40th right-of-way after construction. Staff noted a separate 16-inch emergency line (the blue line on the presentation) that runs along East Mercer Way will remain available as redundancy.
The presentation addressed the Shorewood Apartments' current connection to SPU. "Shorewood owns that water main," Clint said, describing a 10-inch AC main the complex currently uses. The project would abandon that AC line and connect Shorewood to the city's recently upgraded 8-inch ductile iron mains on 88th and 90th Avenues, providing two meters and additional redundancy. Staff said they will meet with Shorewood and with SPU in the coming months to finalize easements, temporary access agreements and rate implications.
Board members pressed staff on reuse of retired pipe for emergency purposes and on construction impacts. "It's a very good question ... tonight's not the time to have that conversation," staff said, adding that some retired segments may be valved off or retained in place for potential future uses but that detailed evaluation is required.
On traffic and construction sequencing, staff said the work will be phased: replace the small AC main on Gallagher Hill first, then install the 24-inch main on Gallagher Hill, then install the 24-inch along Southeast 36th before making final connections and disconnecting SPU at each end. For much of the small-main work, crews will use alternating one-way traffic; installation of the 24-inch main will require larger trenches and equipment and could include closing one lane with a partial detour during summer months to speed installation. Staff said they are evaluating night work on some commercial stretches to reduce daytime impacts.
Project cost estimates at 60% design put the utility portion in the range of about $12.5 million to $13.5 million. Clint said the city currently has $6 million in the 25/26 capital budget and about $18 million available from recent water-fund bond proceeds (with $13 million scheduled for 2027). The city also reported a bit more than $1 million from a state Department of Commerce grant. "So we are currently fully funded to do the work that we've been talking about here," Clint said, while noting that some design refinements remain and that staff expect to refine cost figures as the plans reach 90% and 100% completion.
Staff outlined next steps: complete 90% design, negotiate construction cost under the general contractor-construction management delivery method, advertise the 10-inch replacement package first, finalize agreements with SPU, Shorewood and WSDOT, return to the board next month, present a similar update to the city council in April and—if approvals proceed on schedule—begin construction in May with major paving ideally in July–September 2027.
Separately, Jason (city staff) updated the board on liaison duties and confirmed that King County will present an update on its sewer project at the March meeting in a Zoom format. Jason also briefed the board on a recent King County sewage overflow, stating the failure occurred within the county system and that signs and coordination with the Department of Health and Ecology are in place.
The board thanked staff for the presentation and discussed outreach plans for residents in construction zones, noting sensitivity to recent multi-year disruptions in the area and the need for clear messaging and multiple engagement channels. The meeting moved on to routine liaison updates and then adjourned.
