Long-Term Water Infrastructure Improvement Project

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SPU Water Transmission Line Work


Project Overview

The City is considering replacing and moving Seattle Public Utilities (SPU) main water transmission line to improve resiliency of the water supply to Mercer Island.


The Project

Seattle Public Utilities (SPU) is the sole provider of potable water to Mercer Island. Water is provided by SPU via 24-inch water transmission line traveling across the East Channel, through the Mercer Island Boat Launch and East Mercer Way, then west along SE 40th Street, through steep and difficult-to-access terrain.

The existing critical transmission line is comprised of a steel cylinder wrapped with a concrete casing. Areas where the transmission line is located have a history of slope instability, requiring multiple repairs to joints in the pipeline since shortly after its installation in 1956. Access to areas of the pipeline is very difficult, as the 24-inch water main is located under a steep slope, and in close proximity to high- and low-pressure natural gas pipelines.

The City is reviewing potential actions to improve the resilience of the water transmission line through the replacement of 3,500 feet of concrete cylinder pipe with 4,800 feet of earthquake resistant ductile iron pipe in a new location.

The City is responsible for 97.2% of the costs associated with this transmission line, with the balance of cost responsibilities belonging to the Shorewood Apartments. Given the overwhelming cost responsibility held by the City, and the importance of this water transmission line, the City began exploring options to replace this segment of pipe with a stronger, earthquake resistant transmission line in an alternative alignment. The estimated planning level cost for this replacement, together with other related improvements, is $19 million.


Background

Recent Leak: 2024 SPU Water Main Leak, Conservation, and Repair

In April 2024, the City learned that the SPU 24- inch water transmission line to Mercer Island was leaking in the steep sloped area in the SE 40th Street right-of-way, just north of where Mercerwood Drive meets SE 40th Street. Consequently, SPU crews shut off flow to the transmission line, and Mercer Island received water through a smaller backup pipeline until the repair was complete in August 2024.

During that time, Island residents and businesses lived and worked under emergency water conservation actions due to the constrained supply.

The section of the transmission line where the leak occurred, was along steep, difficult-to-access terrain. The repaired section was eventually slip-lined with approximately 1,300 feet of structural liner, reducing its diameter from 24-inches to 17.4-inches. This loss of carrying capacity was analyzed to understand the impact on the water supply and deemed acceptable. Read more about the SPU Water Main Leak here.


SPU Water Transmission Line Alternatives to Consider

“Do Nothing”

SPU’s recent repair mitigated the immediate adverse impact on the City’s water supply and has an estimated “useful” life projection of approximately 50 years. Challenges remain, however, due to the transmission line’s location in an area with known slope movement, difficult access in some portions, and being constructed of materials that are not seismically resilient. Additionally, the City is responsible for the entire cost of the transmission line segment, despite having only secondary and negotiated control over infrastructure decisions.

“New City Owned Pipeline”

Moving the critical 24-inch transmission line out of the SE 40th Street corridor, and converting to a City-owned asset would consolidate control with responsibility, provide a more resilient and reliable water supply line, and reduce risk in a vulnerable area. Multiple routes for construction of a replacement transmission line were reviewed, but topography and rights-of-way alignments constrain viable options.

The alignment from the Boat Ramp area, north on E. Mercer Way, west on SE 36th Street, and southwest up Gallagher Hill Road to SE 40th Street (see map) is the most feasible route. Hydraulic and planning level studies were conducted, evaluating different pipe sizes and materials. The alternative alignment is longer than the current alignment but is a more reliable route and will meet supply needs with a 24-inch transmission line. This alignment also has the advantage of simultaneously replacing an old asbestos-cement pipeline in Gallagher Hill Road that was scheduled for replacement in 2025.

If approved, this project will design and construct a new, City-owned 24-inch transmission line that connects to the current SPU infrastructure on 92nd Avenue SE, and west on SE 43rd Street where the SPU meter is located. These remaining segments of pipe (92nd Avenue SE and SE 43rd Street) are currently owned and operated by SPU and would necessitate negotiations with SPU to transfer this existing pipeline segment, for which the City is currently 100% financially responsible, to the City. The City and SPU have had preliminary discussions about this option and this option is attenable.

Other Considerations: Shorewood Apartments Supply

The SPU transmission line includes a combination of “reaches” or sections of pipe that are owned and operated by SPU. Currently, Shorewood Apartments is a direct customer of SPU. In the most recent emergency, along with a previous water supply emergency in 2023, temporary connections to the Shorewood Apartments from the City’s distribution system were needed to maintain water supply. Fire flow protection during these periods was significantly impaired.

City engineering staff, with consultant support, evaluated several feasible options to provide service to the Shorewood Apartments in the future. Should this proposal move forward, Shorewood Apartments would need to become a direct customer of the City of Mercer Island. The primary advantages of this proposal would be the elimination of the need to mobilize City personnel to provide temporary connections whenever SPU supply is interrupted, and reduction of risk posed by Shorewood Apartment’s aging asbestos cement and cast-iron transmission pipelines on SE 40th Street, 90th Avenue SE, SE 37th Street, and 88th Avenue SE.

Alternatively, Shorewood Apartments can remain connected to SPU’s 24-inch transmission line and remain a SPU wholesale customer; however, the existing 24-inch transmission line would remain in service.

Any future solution will require additional discussion and negotiation between the City, SPU, and the owners of the Shorewood Apartments.


Next Steps

Upon approval from the City Council, City staff would prepare a project to design and construct a new 24-inch ductile iron transmission line in the 2025-2030 Capital Improvement Plan.

If approved, staff would evaluate the optimal project delivery and funding approaches to ensure efficiency and effectiveness, and return to the City Council with a recommendation and appropriation request to commence design. Staff would also begin negotiations with SPU to address ownership issues and further explore alternatives for service to the Shorewood Apartments. Finally, an amended Wholesale Service Contract and construction contract would be brought back to the City Council for review and approval.

Given the size, scope, and importance of this project, the City Manager would also recommend this project be included in a capital funding request to the Washington State Legislature.

This project page is dedicated to providing information and updates about the water transmission improvement project.

Transmission line, water main, 24-inch water line, and pipe are used interchangeably throughout the project page to refer to the main SPU water pipe that brings potable water onto Mercer Island.


Project Overview

The City is considering replacing and moving Seattle Public Utilities (SPU) main water transmission line to improve resiliency of the water supply to Mercer Island.


The Project

Seattle Public Utilities (SPU) is the sole provider of potable water to Mercer Island. Water is provided by SPU via 24-inch water transmission line traveling across the East Channel, through the Mercer Island Boat Launch and East Mercer Way, then west along SE 40th Street, through steep and difficult-to-access terrain.

The existing critical transmission line is comprised of a steel cylinder wrapped with a concrete casing. Areas where the transmission line is located have a history of slope instability, requiring multiple repairs to joints in the pipeline since shortly after its installation in 1956. Access to areas of the pipeline is very difficult, as the 24-inch water main is located under a steep slope, and in close proximity to high- and low-pressure natural gas pipelines.

The City is reviewing potential actions to improve the resilience of the water transmission line through the replacement of 3,500 feet of concrete cylinder pipe with 4,800 feet of earthquake resistant ductile iron pipe in a new location.

The City is responsible for 97.2% of the costs associated with this transmission line, with the balance of cost responsibilities belonging to the Shorewood Apartments. Given the overwhelming cost responsibility held by the City, and the importance of this water transmission line, the City began exploring options to replace this segment of pipe with a stronger, earthquake resistant transmission line in an alternative alignment. The estimated planning level cost for this replacement, together with other related improvements, is $19 million.


Background

Recent Leak: 2024 SPU Water Main Leak, Conservation, and Repair

In April 2024, the City learned that the SPU 24- inch water transmission line to Mercer Island was leaking in the steep sloped area in the SE 40th Street right-of-way, just north of where Mercerwood Drive meets SE 40th Street. Consequently, SPU crews shut off flow to the transmission line, and Mercer Island received water through a smaller backup pipeline until the repair was complete in August 2024.

During that time, Island residents and businesses lived and worked under emergency water conservation actions due to the constrained supply.

The section of the transmission line where the leak occurred, was along steep, difficult-to-access terrain. The repaired section was eventually slip-lined with approximately 1,300 feet of structural liner, reducing its diameter from 24-inches to 17.4-inches. This loss of carrying capacity was analyzed to understand the impact on the water supply and deemed acceptable. Read more about the SPU Water Main Leak here.


SPU Water Transmission Line Alternatives to Consider

“Do Nothing”

SPU’s recent repair mitigated the immediate adverse impact on the City’s water supply and has an estimated “useful” life projection of approximately 50 years. Challenges remain, however, due to the transmission line’s location in an area with known slope movement, difficult access in some portions, and being constructed of materials that are not seismically resilient. Additionally, the City is responsible for the entire cost of the transmission line segment, despite having only secondary and negotiated control over infrastructure decisions.

“New City Owned Pipeline”

Moving the critical 24-inch transmission line out of the SE 40th Street corridor, and converting to a City-owned asset would consolidate control with responsibility, provide a more resilient and reliable water supply line, and reduce risk in a vulnerable area. Multiple routes for construction of a replacement transmission line were reviewed, but topography and rights-of-way alignments constrain viable options.

The alignment from the Boat Ramp area, north on E. Mercer Way, west on SE 36th Street, and southwest up Gallagher Hill Road to SE 40th Street (see map) is the most feasible route. Hydraulic and planning level studies were conducted, evaluating different pipe sizes and materials. The alternative alignment is longer than the current alignment but is a more reliable route and will meet supply needs with a 24-inch transmission line. This alignment also has the advantage of simultaneously replacing an old asbestos-cement pipeline in Gallagher Hill Road that was scheduled for replacement in 2025.

If approved, this project will design and construct a new, City-owned 24-inch transmission line that connects to the current SPU infrastructure on 92nd Avenue SE, and west on SE 43rd Street where the SPU meter is located. These remaining segments of pipe (92nd Avenue SE and SE 43rd Street) are currently owned and operated by SPU and would necessitate negotiations with SPU to transfer this existing pipeline segment, for which the City is currently 100% financially responsible, to the City. The City and SPU have had preliminary discussions about this option and this option is attenable.

Other Considerations: Shorewood Apartments Supply

The SPU transmission line includes a combination of “reaches” or sections of pipe that are owned and operated by SPU. Currently, Shorewood Apartments is a direct customer of SPU. In the most recent emergency, along with a previous water supply emergency in 2023, temporary connections to the Shorewood Apartments from the City’s distribution system were needed to maintain water supply. Fire flow protection during these periods was significantly impaired.

City engineering staff, with consultant support, evaluated several feasible options to provide service to the Shorewood Apartments in the future. Should this proposal move forward, Shorewood Apartments would need to become a direct customer of the City of Mercer Island. The primary advantages of this proposal would be the elimination of the need to mobilize City personnel to provide temporary connections whenever SPU supply is interrupted, and reduction of risk posed by Shorewood Apartment’s aging asbestos cement and cast-iron transmission pipelines on SE 40th Street, 90th Avenue SE, SE 37th Street, and 88th Avenue SE.

Alternatively, Shorewood Apartments can remain connected to SPU’s 24-inch transmission line and remain a SPU wholesale customer; however, the existing 24-inch transmission line would remain in service.

Any future solution will require additional discussion and negotiation between the City, SPU, and the owners of the Shorewood Apartments.


Next Steps

Upon approval from the City Council, City staff would prepare a project to design and construct a new 24-inch ductile iron transmission line in the 2025-2030 Capital Improvement Plan.

If approved, staff would evaluate the optimal project delivery and funding approaches to ensure efficiency and effectiveness, and return to the City Council with a recommendation and appropriation request to commence design. Staff would also begin negotiations with SPU to address ownership issues and further explore alternatives for service to the Shorewood Apartments. Finally, an amended Wholesale Service Contract and construction contract would be brought back to the City Council for review and approval.

Given the size, scope, and importance of this project, the City Manager would also recommend this project be included in a capital funding request to the Washington State Legislature.

This project page is dedicated to providing information and updates about the water transmission improvement project.

Transmission line, water main, 24-inch water line, and pipe are used interchangeably throughout the project page to refer to the main SPU water pipe that brings potable water onto Mercer Island.

Page published: 02 Dec 2024, 12:35 PM