Planners: What if 520 span were lost?

It carries more than 115,000 vehicles each day between Seattle and the Eastside, is vulnerable to strong winds and major earthquakes and is nearing the end of its useful life.

But with a new Evergreen Point Floating Bridge still years away, transportation officials say it's time to develop a detailed strategy for how the region would respond to the loss of one of its most vital bridges.

The state Department of Transportation (DOT) said it is collaborating with communities on either side of Lake Washington, King County Metro and the University of Washington to craft what it calls a catastrophic failure plan.

DOT officials will brief Seattle's council Monday on their progress and will meet with other local leaders and transportation departments in coming weeks.

Over the past few years, the state has worked with Seattle officials to assemble a similar plan for the Alaskan Way Viaduct, which engineers have deemed vulnerable to earthquakes. An updated plan released in July now includes detailed information — from alternate routes to where construction cones will be stored.

"A lot of the focus has been on the viaduct," said Richard Conlin, chairman of the Seattle City Council's transportation committee. "We've got to make sure 520's on the table."

The state already has response plans should disaster strike the region's major roadways, including 520 and the viaduct, said Melanie Coon, a DOT spokeswoman.

This time around, the department wants to incorporate the needs and ideas of the communities that rely on the bridge and map out how it might respond to dozens of scenarios — one sunken pontoon, for instance, or a snapped column, or a swamped section.

"We need to have plans that are developed for the event of any sort of failure," said John Milton, acting director of the state's 520 replacement project.

While standards now call for bridges to withstand winds stronger than 90 mph, the 42-year-old span must be closed whenever winds whipping across Lake Washington top 57 mph. There's a 1-in-20 chance the hollow columns that support its approach spans could collapse in an earthquake.

The new plan will include traffic-management strategies for short- and long-term bridge closures, funding scenarios and emergency response. The state hopes to have a draft completed in a couple of months and then will gather public comments. Traffic-control plans for each community probably will take longer, Milton said.

Karen Gaudette: 206-515-5618 or kgaudette@seattletimes.com