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.