Not All Projects Are Created Equal—The King County North Mercer Enatai Project Goes from 945 Alternatives to 1

King County’s Mercer and Enatai interceptors were built in the 1960s and extend over 14,000 ft from Mercer Island into the Enatai neighborhood of Bellevue, Washington. Some parts of the system are reaching the end of their useful lives, and future peak flows are projected to exceed the system’s capacity. To address these deficiencies, the Mercer Enatai project team completed a complex alternatives analysis, which began its evaluation with over 945 potential pipeline alternatives and ultimately ended with one selected alternative. The alternatives analysis process began with the brainstorming of conceptual alignment options across the three distinct areas of the project: Mercer Island, East Channel (Lake Washington), and Bellevue. The existing North Mercer interceptor is primarily a subaqueous pipeline located within Lake Washington along the northern shoreline of Mercer Island. Any new pipeline on Mercer Island would have either significant challenges in constructing in the water in Lake Washington or on land that has experienced significant development since the 1960s, including the construction of Interstate 90 through the northern part of Mercer Island. The East Channel in Lake Washington is 1400 ft wide and separates Mercer Island from Bellevue. The existing Enatai siphon carries flows across the channel and traverses along the lake bottom and under the East Channel I-90 bridge. Any new pipeline across the channel would need to meet the complex constructability, hydraulic, and Army Corps 404 and WSDOT permitting requirements. The last pipeline segment is located in Bellevue in the Enatai neighborhood, a Native American term meaning “across the water”. Aptly named, the existing Enatai interceptor carries flows from across the channel and from Bellevue, along the southern end of the Enatai neighborhood, and through the environmentally sensitive Mercer Slough. The last 2,000 ft of this pipeline segment is also supported on piles due to the soft ground, and runs under the I-90 corridor, which is also supported on piles in this area. A new pipeline alignment through Bellevue would have significant challenges in constructing in the water through the Mercer Slough or on land through the Enatai hillside. This presentation explores the complex challenges of this project, and the extensive alternatives analysis process that started with over 945 potential alternatives.