“Puget Sound will be greatly impacted by a tsunami from a Cascadia Subduction Zone earthquake,” says Maximilian Dixon, hazards and outreach program supervisor at the Washington State Military Department’s Emergency Management Division. “The tsunami will severely damage or destroy most of the maritime infrastructure, including ports, marinas, boats [and] ferries. It will also inundate low-lying areas. With up to five or six minutes of shaking, all of Washington will be impacted.”

Actually surviving this once-in-several-lifetimes event requires rehearsal that goes beyond a single day. For people near the tsunami inundation zone, that means knowing how long it takes to outrun a wall of water as soon as tsunami sirens go off.

The Washington state Department of Natural Resources (DNR) Washington Geological Survey is visualizing that evacuation calculation in a first-of-its-kind collection of tsunami evacuation walk maps. Funded through the National Tsunami Hazard Mitigation Program, DNR's maps show how much time it will take people on foot to evacuate cities in the inundation zone, and the best routes to take, depending on where they are.

The walk maps serve both outer-coastal cities like Westport and inner-coastal cities along Puget Sound like Anacortes. They’re a huge step up from previous maps, says Dixon, who manages federal grants in Washington state that fund initiatives like the walk maps.

Based on walking speeds timed from when the ground starts shaking, the maps include the most efficient routes to high ground from dozens of locations within each city. Beyond aiding in disaster preparedness, they drive home the urgency needed when the Big One strikes our 3,026 miles of coastline.

Designing tsunami evacuation walk maps

Staying alive during an earthquake or a tsunami often necessitates escaping on foot. The U.S. Geological Survey’s evacuation guidance focuses on pedestrians because of copious research into the hazards of driving, says DNR lead project scientist Dan Eungard.

“After a major earthquake event [like] Cascadia, your road network is going to be largely unusable,” Eungard says, noting that liquefaction, ground shaking and landslides will destroy roads and bridges. Where they don’t fall apart, downed trees and power lines may make them impassable. “[In] other major earthquake events around the world, all it took was one car crash and it's blocked a road, and then everyone else couldn't evacuate out off of that road in their vehicle.”

In agreement is Dr. Alireza Mostafizi, postdoctoral scholar at Oregon State University with expertise in transportation engineering. He's unaffiliated with the maps, and has worked on theoretical tsunami evacuation modeling scenarios and behavioral surveys. “According to one of our case studies on the coast of Oregon, vehicular evacuation results in eight times higher mortality compared to 100% on-foot evacuation,” he says.

DNR used the conservative “crosswalk” speed of 24 minutes per mile to account for people with restricted mobility. “Many of them will not be able to evacuate at a higher rate of speed than a slow walk and so it's kind of a baseline for the entire population,” Eungard says.

The walk maps also incorporate information from various agencies’ inundation models, land cover imagery and existing evacuation maps, which DNR fine-tunes with input from local safety officials.

State databases and land covery imagery do “not get down to the level of detail of knowing where all the fences are in the community, or blackberry bushes, or things of that nature,” says Eungard. Only local reconnaissance can tell him which bridges are questionable, or where power lines might fall into routes.

“We can't predict what sort of damage will occur from the earthquake but we can certainly kind of look around before the earthquake and have a guess,” Eungard says.

His team visits locations at least once but largely doesn’t test or time out their routes. In some cases, being on the ground has been critical: Eungard says an initial route model for Port Angeles suggested people walk straight up a cliff, because USGS route-making guidance didn’t note the point at which slopes are no longer considered “walkable.” It wasn’t until he was on the ground in the city that this became apparent.

Hannah Cleverly, deputy director of emergency management with Grays Harbor County, says her department hasn’t time-tested the routes yet but is planning to. Grays Harbor’s largest city is Aberdeen, where more than 12,000 people live in the tsunami inundation zone.

“We are working with our local stakeholders to work toward these assembly areas and these routes. It’s an ongoing project for us. I wish I could wave a magic wand, and it would all be completed in five seconds, but unfortunately it takes a little time,” she says.

Tsunami evacuation on foot is slower in practice

Calculating around slow walk times might seem overzealous, since many people can move more quickly than those with mobility issues. But in a disaster, Eungard says, people may face physical and emotional hurdles that prevent them from going full speed.

“By using that slow walk pace we are hopefully capturing the pace that most of the population will be able to sustain over a distance, including those sort of delays,” Eungard says.

That’s especially true for older residents, who make up a large percentage of Washington coastal population. “We have an elderly population that could have mobility issues, [and] that could slow down the walk time quite a bit,” Cleverly says. “[So] the message that we convey to them is to be prepared to just get out of your house as fast and as easily as you can and don't wait for the [tsunami] sirens to go off. Don't wait after the ground stops shaking for a localized event to evacuate immediately.”

Mostafizi says “milling time”— the period between when people realize an earthquake is happening and when they start evacuating — is the most important survival factor. “Even [a] 10-minute increase in the preparation time can significantly reduce the chance of survival, even … doubling the mortality rate of the evacuation,” he says.

Beyond that, survival is a function of many factors: from familiarity with a route or the ability to follow evacuation signage to the time of day and beyond.

“Considering all of these, it is rather likely to not do the right move if you are not prepared and not familiar with the area,” Mostafizi says, making preventative multichannel communication important.”

Eungard recommends people do timed test runs from home, work and other frequented locations to gauge their pace in ideal conditions. Some outer-coastal communities conduct timed test runs together. Eungard says the Shoalwater Bay Tribe near Westport hosts ‘Yellow Brick Road’ evacuation test events, which have shown “they've been able to make it out within the minimum time specified on the map.”

That’s one reason the state hosts the Great ShakeOut, the largest earthquake drill in the world. “[There’s] a huge tsunami component,” Dixon says. Washington schools in inundation zones have to perform at least one tsunami drill a year, and all schools are required to practice one earthquake drill.

Worst-case tsunami evacuation scenarios

For some populations, these maps deliver a scary message: Without changes to infrastructure, some people in the most vulnerable areas of the coastline likely won’t be able to reach high ground in time to beat wave landfall.

Long Beach’s evacuation walk map shows waves hitting 15 minutes after the first tremors; most areas in town require 45 minutes to evacuate. All of Long Beach's population and businesses are in the inundation zone.

“You kind of do the math,” Eungard says. “These are the locations where we need to think about alternate evacuation options.”