If city leaders replace the cracked West Seattle Bridge, the new span might need to rise higher than the existing 140-foot-tall span above the Duwamish Waterway.
Ship-channel height is one of the top five risks that could add costs and time to a new bridge project, the city’s engineering consultants believe.
The Coast Guard says it will require a navigation impact statement for any new bridge there, as is done for thousands of corridors across the country.
“It’s meant to be difficult because whatever they build is going to have to be there for the next 100 years,” Steven Fischer, the Coast Guard’s regional bridge administrator, said in an interview. “So we have to project: What’s the need of Seattle maritime business going to be 100 years from now?”
But if Seattle Department of Transportation (SDOT) contractors repair the cracked 36-year-old concrete bridge, allowing traffic to return in 2022, the city can duck the clearance question for 15 to 40 years. Mayor Jenny Durkan is scheduled to make a recommendation Oct. 21 about whether to try repairs, or proceed to demolition and a new bridge.
In a preliminary chart this month, engineering firm WSP listed a 70% chance the Coast Guard will require more height, and that doing so would add $146 million in construction costs.
Heather Marx, SDOT mobility director, said those numbers were little more than placeholders, to show a 31-member advisory task force how cost-benefit studies are being conducted, to guide the city’s repair-or-replacement decision.
Still, the federal government will have its say.
“Yes, there is a chance it could be higher,” Fischer said. “We have anecdotal evidence the [existing] bridge is constricted, and businesses have had to turn away customers because of the height of the bridge.” He mentioned Delta Marine, a company upstream that sells and services luxury yachts. Delta executives declined an interview request.
The Coast Guard hasn’t yet suggested height estimates for a new bridge.
If a taller bridge is required, contractors can’t just do a limited replacement by tearing out the cracked central span and its two connected side spans. Approach decks would be replaced and new columns might be needed so the overall slope is gradual enough for traffic.
The existing bridge, which SDOT closed March 23, has a 6% slope.
“You have to have more length added to the project,” WSP project manager Greg Banks told the task force. “You can’t just tie back in right within the high-rise, which means that you have approach work that needs to be done as well, so there’s a cost associated with that.”
If politicians choose a structure that combines motor vehicles with Sound Transit light rail, that would “require a complete corridor replacement due to grade considerations,” the preliminary WSP report says.
When they seek permits, both Sound Transit and SDOT must deal with the Coast Guard’s checklist question, “Does the proposed bridge(s) match (or is greater than) the navigational clearance of existing structures on the waterway?”
Yazmin Mehdi, an aide to U.S. Rep. Pramila Jayapal, a Seattle Democrat, wondered if the risk of a taller bridge requirement can be resolved early. “Is there a way to eliminate it by going to the Coast Guard at the front end, rather than waiting?” she asked Wednesday in a task force session.
Transportation staffers say they’ve begun talks with the Coast Guard, to save time.
“As far as I’m concerned, it’s all the more reason to support a repair,” Kevin Broveleit, co-founder of the West Seattle Bridge Now coalition, said Friday. “The fact that a full replacement could be required, if the mainspan is replaced at all, is a game changer.”
Marx emphasized the risk figures are still being refined, and any additional cost to build higher than 140 feet is only one criterion. SDOT’s top priority will be to assure public safety for any repair or replacement option, she said.
Alaska Marine Lines, which barges food and building materials to communities along the Alaskan coast, will occasionally transport high loads such as cranes, but says the existing clearance is sufficient.
The existing West Seattle Bridge is the seventh-tallest road bridge over a maritime channel in Washington state. The 1950 Tacoma Narrows Bridge, the Lewis and Clark Bridge from Longview to Oregon, and the Highway 101 bridge from Ilwaco, Pacific County, to Oregon are the tallest, tied at 198 feet, according to the National Bridge Inventory.
The West Seattle Bridge rises 140 feet because the Port of Seattle aspired to construct a big container terminal upstream, requiring more clearance than previous river ships needed. It was never built.
That’s no longer a factor for the port, which will reopen Terminal 5 in Elliott Bay next year, providing crane lifts exceeding 146 feet to serve the world’s ultra-large container ships.
“There is no ship, that we have any conceivable prospect of going up the Duwamish, that would require a bridge that high,” said Port Commissioner Peter Steinbrueck, when asked about a taller clearance.
Sound Transit hasn’t filed its navigation impact statement yet, spokesperson Rachelle Cunningham said. In community forums from 2017-19, its diagrams assumed 140-foot-high fixed bridges to West Seattle and Ballard, or a Ballard drawbridge as a last resort.
Tough questions lie ahead about the engineering, operations and seismic needs for a higher bridge, especially if it would carry 500,000-pound four-car trains. Sound Transit amended its Link Design Manual this year to allow climbs of up to 8% grade in future trackways.
“Four percent is generally our max. But we can do 6% for short distances and in some instances, for very short distances, can do 7%,” Cathal Ridge, Sound Transit project director, said by email.
A shallow immersed-tube tunnel, promoted by Seattle megaproject historian Bob Ortblad, remains among WSP study options.
A drawbridge would accommodate any ship mast or cargo, but create traffic jams. Throughout the 1970s, West Seattle residents clamored for a fixed high-rise bridge, to relieve their aggravation of twin drawbridge openings.
“The Coast Guard doesn’t like drawbridges either,” Fischer said. “You have to monitor it and we have to inspect it. We have to enforce a federal rule on the operations of it.”
For the sake of traffic mobility, tall boats are already restricted or banned six hours a day at Seattle drawbridges, he said. Yet the Coast Guard is bombarded with requests to further limit marine openings — especially for the city’s swing bridge across the Duwamish, at lower Spokane Street.