CANNON BEACH — Bent over maps during a work session last week, Cannon Beach Emergency Preparedness Committee members and Public Works Director Dan Grassick considered the best options for a safe crossing at the north end of town in case of an earthquake and tsunami.
Because of the likelihood of a bridge collapse over Ecola Creek, the city’s evacuation map guides residents and visitors south, toward midtown. This is a longer route that could eat up valuable time, committee Vice-Chairman Les Wierson said.
OBEC Consulting Engineers conducted a bridge study in 2011, warning the Fir Street bridge at Ecola Creek is “vulnerable to failure” even in a “relatively small earthquake” because of its age and construction.
“Saving lives is my top priority,” committee Chairwoman Karolyn Adamson said.
The group studied options to replacing the traffic bridge or building a pedestrian bridge in the same location or elsewhere.
Whatever the committee decides, Grassick said, 75-foot-deep pilings would likely be needed because of sand liquefaction during a quake.
The committee focused on the most affordable pedestrian alternatives, particularly a bridge over Ecola Creek near NeCus’ Park. The plan calls for two separate bridges to meet on a small island in the creek, providing solid ground to build on.
A second option at that location would require more costly permits to float equipment down the creek, Grassick said.
The third pedestrian option would lead people up Second Street before hitting a yet-to-be-built trail connecting to U.S. Highway 101.
According to the 2011 study, replacement of the existing bridge would cost $4.8 million, while a 6-foot-wide pedestrian bridge could cost up to $1.6 million.
If Cannon Beach does go with a pedestrian bridge, Grassick said, options are suspension or truss bridge construction. The latter would more likely survive, he added, as cables can break with suspension bridges.
Wierson suggested taking a closer look at Washington County’s bridge standards, because they are designed to withstand earthquakes.
“The structural aspect is big,” Grassick said. He added the bridge would need to be at least 10-feet wide with 1,000 people crossing it.
An Oregon State University engineering professor made computer models of potential evacuation routes in the city in 2011, estimating at least 1,080 people would head toward the bridge on a typical summer day after an earthquake.
Grassick said there are a number of ways to fund the bridge, including grants such as a predisaster mitigation grant, sharing costs with the Federal Emergency Management Agency and a Connect Oregon grant.
Committee members agreed they would seek a bond measure if grants don’t provide the money needed.
“To do this project, to get a final design, an in-depth geotech study needs to be done,” Grassick added.
The committee will have to go through the City Council before taking any further steps. The topic could be reviewed this month.