Divers connect 2,000 feet of pipe under Lake Washington to improve wastewater treatment, restore habitat, enhance I-90 trail

News

King County Wastewater Treatment Division is replacing aging pipes along the bottom of Lake Washington that connect Mercer Island to Bellevue to provide reliable sewer service for decades to come and using the safest methods to protect fish and aquatic wildlife. 

As part of King County’s North Mercer-Enatai Sewer Upgrade Project, crews with unique expertise in underwater construction are working from barges on the East Channel of the lake, near the Interstate 90 bridge, to lay down pipe up to 40 feet deep along the bottom of the lake. Crews have been conducting the “in-water” stage of construction during this past summer and fall, within the window of time when migrating fish are least likely to be impacted.

“The North Mercer-Enatai Sewer Upgrade Project is an important part of the largescale upgrades we are making to our regional wastewater system and demonstrates the unique and challenging environments we work in to ensure reliable sewer service across our 420-square-mile service area for decades to come,” said Kamuron Gurol, Director of the King County Wastewater Treatment Division. “As we build our wastewater infrastructure projects, we seek to protect the environment and look for ways to restore sensitive habitats so that we can leave the area better than when we started.”

When the project is completed in 2026, it will replace 4 miles of sewer pipe located across Mercer Island, underneath Lake Washington, and through the Enatai hillside. The new pipe will connect to a pump station near Mercer Slough in south Bellevue that transports wastewater to King County’s South Treatment Plant in Renton.

The new pipeline consists of three parallel pipes that are U-shaped with the high ends on the shoreline and the lowest point along the lake bottom on the north side of the I-90 bridge. To install the pipeline, a crane with a clamshell dredger on one of the barges digs a 10-foot trench along the lakebed. Next, 300-foot segments of pipe are floated across the water surface. The weight of the water, along with concrete collars, sinks the pipe into the trench, and divers then bolt together the individual segments and anchor the pipe to the lake bottom. Finally, the clamshell dredge caps the trench with a rock-and-pebble gravel type selected to provide a good habitat for fish and other aquatic life.

The $179 million project includes restoration of shoreline habitats between Enatai Beach Park and the Mercer Island Boat Launch by placing large logs and other improvements to create a better habitat for fish and wildlife. Restoring native plants to other areas of the project site, including Mercer Slough Nature Park, will also occur, among other improvements.

The popular I-90 Trail will be improved to meet multi-use trail needs, including the Americans with Disabilities Act standards.