Seattle’s getting a new Elliott Bay seawall, and a very small part of this very big job is to give migrating juvenile salmon a safer route to the sea.

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Seattle’s $330 million replacement seawall is taking shape — and local fish are taking notice.

Workers have completed the first section of the wall, including a migratory corridor for juvenile salmon that will eventually run the entire length of the downtown waterfront.

The finished section sits between South Washington Street and Yesler Way, near the Colman Dock ferry terminal at the southern end of the waterfront. It features a suite of design elements meant to attract fish and other aquatic animals and plants.

“Before the old seawall was built on the waterfront, we had tidal flats with birds, plants and salmon,” said project manager Jessica Murphy, who works for the Seattle Department of Transportation. “The tide came in every day as far as Western Avenue. Then the seawall was built. They filled in the area behind it.”

The new wall is made from a concrete slab, about 5 feet tall and 25 feet thick, supported by grout injected into the ground up to 40 feet from the water and fronted with a concrete face up to 19 feet tall and a foot thick.

The wall’s face, where it meets Elliott Bay, is studded with grooves and shelves to promote algae growth and help critters like starfish and mussels take hold.

Workers have stacked mesh bags stuffed with rocks on the seabed next to the completed section of the wall to build a shallow-water environment where salmon and other critters can forage and hide. The same is planned along the rest of the structure, as well, creating what Murphy calls a “habitat bench.”

In addition to shallow water, juvenile salmon prefer swimming in sunlight.

When they leave the Duwamish/Green River during their migration to the open ocean, about half turn left, toward West Seattle. The other half turn north, toward downtown, and when they reach the waterfront piers, they encounter trouble.

They don’t like passing below the piers; their eyes need a long time to adjust to the dark. Many instead head into deeper water — and into the jaws of predator fish.

To keep the salmon safe as they hug the shore, workers are installing a sidewalk cantilevered out from the new wall. Straddling the space between the wall and the piers — and edging out over the water between piers — the sidewalk’s translucent panes will allow sunlight through, down to the water.

The panes aren’t transparent, so pedestrians won’t be able to watch salmon swimming underfoot. But the sunlight reaching the habitat bench should cause the fish to treat the length of the wall like an underwater highway, said Jeff Cordell, a research biologist from the University of Washington’s School of Aquatic and Fishery Sciences.

“The illumination is going to be really interesting, because it’s never been done on this scale before,” said Cordell, one of several UW scientists who worked on the design. “There have been some examples of light-penetrating surfaces used in smaller applications.”

The space for the habitat bench and sidewalk is available to work with because the replacement structure will stand about 10 to 12 feet inland from the old wall.

The last touch will come when workers build a rocky beach into Elliott Bay just south of the ferry terminal. With shoreline-appropriate vegetation and tide pools, the outcropping should give passers-by a window into Seattle’s wild past, the project director said.

“We’re trying to replace some of what was lost,” said Murphy. “We’ve already seen some salmon. Last fall, shortly after we got the marine mattresses and wall face installed, we started to see activity. I saw an entire school, one day. That’s really promising. We’ve only completed a short section, but they’re finding it.”

The nature-friendly design elements represent 4 percent of the project’s $330 million price tag, Murphy said. The new seawall — from South Washington Street to Virginia Street — is mostly funded by a 30-year, $290 million bond measure that voters approved in 2012. SDOT must perform habitat restoration in order to meet U.S. Army Corps of Engineers permit requirements.

Grants and local money make up the rest of the funding, Murphy said.

The old seawall, which workers are demolishing, was built between 1916 and 1936 with a concrete face supported from behind by landfill and timbers. It stretches about 7,000 feet, more than a mile, from South Washington Street to Broad Street. Replacing the seawall from Virginia to Broad streets hasn’t yet been funded.

The City Council and then-Mayor Mike McGinn asked voters to greenlight the project in 2012 due to stability concerns. In 2002, engineers assessing damage from the Nisqually earthquake had discovered that the wall’s timbers were being eaten away by gribbles — small crustaceans that chomp into wood.

The city estimated that about half the old wall was kaput and warned voters that the structure had a 1-in-10 chance of failing during an earthquake within 10 years.

Staging work began in 2013 and construction on the new wall started in early 2014.

The project is proceeding in segments; the work zone currently stretches from Yesler Way to the Seattle Aquarium. For each segment, workers install a temporary metal barrier to seal the site off from Elliott Bay. Next they take apart the old wall. Finally, they build the new structure, which includes injecting grout into the ground.

Because pumping groundwater out of the construction site could cause the Alaskan Way Viaduct and nearby buildings to sink, SDOT circulates a super-cold brine through underground pipes on the inland edge of the work zone, Murphy said. It freezes the water in the soil, creating a wall of ice that keeps groundwater from flowing into the site.

Workers have completed the first section of Seattle’s $330 million replacement seawall south of the Colman Dock ferry terminal. The new wall is made from a concrete slab about five feet tall and 25 feet thick. (Ellen Banner / The Seattle Times)

The project was supposed to cost $300 million but on Jan. 1, 2014, Murray’s first day in office, The Seattle Times reported that the price tag had reached $330?million.

In July, crews will complete a second section of the new wall, between Madison Street and Pike Street. The project is planned to end in mid-2016 and is currently on schedule, Mayor Ed Murray noted during a recent tour of the work zone.

The city had spent about $125 million on the project through 2014, Murphy said.

But the outlook wasn’t always so rosy, due to complications with another mega-project, Murray said, raising his voice over the roar of construction equipment.

“The day of the Seahawks parade (Feb. 5, 2014), I got a call and was told there was a problem moving forward with the seawall,” the mayor said.

Bertha, the drill boring a tunnel under downtown to replace the viaduct, had broken down in December 2013, and the state officials responsible for the project were concerned about the seawall project conflicting with plans to restart the machine.

“The proposal that was presented to me was that the danger to Bertha and the viaduct was such that we needed to stop working on the seawall,” Murray said.

“The decision I struggled with was the danger to Bertha (versus) the danger of an earthquake causing part of downtown to slide into Elliott Bay. It was the scariest decision I’ve made in 20 years of elected office,” the mayor said.

The mayor had the city’s “technical people” work out a monitoring plan with the state’s experts. Then he told SDOT to move ahead with the seawall project, he said.

Bertha has moved about 20 feet since then, reaching a 120-foot-deep pit where it is to be repaired.