Tsunami conundrum: How to escape Oregon town if bridge collapses

Tsunami conundrum: How to escape Oregon town if bridge collapses

CANNON BEACH, Ore. (AP) — The goal: evacuate downtown Cannon Beach before a tsunami hits.

The problem: The existing bridge across Ecola Creek will collapse, stranding students at Cannon Beach Elementary School and those at the Cannon Beach Conference Center who need to reach the higher north end of town across the creek.

The solution: a stationary pedestrian bridge made of concrete or a floating bridge made of wood.

And therein lies the conflict.

The city's emergency preparedness committee supports a 12-foot wide concrete pedestrian bridge across the creek in addition to the current bridge. The cost could range between $1.6 million to $3 million, depending on the bridge's alignment.

The committee is working with a proposal from OBEC Consulting Engineers, of Eugene, that suggests two bridge alignments. Although the proposal, requested by Public Works Director Mark See, was submitted to the committee in February, it wasn't until this month that some members of the City Council learned about it.

Meanwhile, Mayor Mike Morgan began talking to architect and former mayor Jay Raskin about a wooden stationary pedestrian bridge or a floating bridge that would cost between $300,000 and $600,000.

Discussion over the two concepts has already become heated, especially now that the city has an opportunity to seek a grant from the Oregon Department of Transportation to do a geological study of the creek bank. The grant application is due this month.

"I think it's great that the city got OBEC involved and came up with a proposal for a fixed bridge," Morgan said at a City Council meeting earlier this month. "But a fixed bridge will cost $1.5 million to $3 million.

"The option Jay is proposing is to have a flexible bridge that will withstand an earthquake and allow the evacuation of children and the conference center and then be destroyed by a tsunami.

"I think both approaches have merit, but it would be hard to pass a bond measure for a $3 million bridge, where $300,000 to $500,000 is more doable," Morgan said.

If the city were to spend $3 million on a bridge that is wide enough for pedestrians and possibly an emergency vehicle, Morgan added, it might as well replace the existing two-lane bridge.

State geologists have said that the wooden pilings under the existing Ecola Creek Bridge wouldn't withstand either an earthquake or a tsunami. The wooden bridge that spanned the creek during the 1964 tsunami collapsed when a house and other debris hit it. It took several months before that bridge was replaced with the bridge that is there now.

Bill Brehm, chairman of the emergency preparation committee, asked the Council how anyone would know if the wooden bridge proposed by Raskin would withstand an earthquake. He noted that "quite a bit of work" has already gone into studying a proposed concrete bridge.

"The (ODOT) grant application ought to go in for the OBEC bridge," Brehm told the Council.

The OBEC proposal suggests a 12-foot-wide deck that would carry the load of an emergency vehicle and withstand a 1,000-year interval Cascadia earthquake and a "higher level" tsunami wave.

According to the preliminary design, the bridge would be constructed of precast slab spans and anchored to concrete-filled steel pipe pilings.

The bridge's cost would depend on its location. One option would have the 650-foot-long bridge aligned with an extension of Spruce Street north to Fifth Street west of Cannon Beach Elementary School.

A second option would have the bridge alignment parallel to and downstream from the existing bridge. It would begin at the northeast corner of the school and end directly west of the Fir Street-Fifth Street intersection to the north. The bridge would be 250 to 300 feet long but would have less tsunami resistance than the first option. Some private land also might need to be purchased for the bridge's right-of-way.

But, Raskin said this week, "doing a big, expensive concrete bridge (for pedestrians) doesn't make a lot of sense."

What happens, he asked, if the concrete bridge is built, an earthquake occurs and the ground sinks three to six feet? "Where's the tidal zone then, where's the downtown? How does it work?"

Why not build a cheaper bridge that pedestrians can use to evacuate before the tsunami, Raskin asked. Any bridge that is built might very well disappear under 10 feet of water when the tsunami waves come down Ecola Creek, he said.

"We're trying to solve the problem of when the existing bridge collapses and we have 20 minutes to get people out of town," Raskin said.

Les Wierson, a retired engineer and member of the emergency preparedness committee, said the idea of a pedestrian bridge was in response to Raskin's proposal for a tsunami evacuation building that would also act as a city hall. The building in the midtown area of Cannon Beach is estimated to cost $4 million.

Wierson advocates that, instead of building the tsunami evacuation building, residents and visitors in midtown be directed to head for higher ground east of town and that evacuation pathways be cleared.

"We expect to see small tsunamis rather than earthquakes," Wierson said. "The bridge (would cost) $1.5 million, but I don't think we would build both the (evacuation building) and the bridge."

The pedestrian bridge proposal supported by the emergency committee would allow emergency vehicles access to the north side and would provide a "viable alternative" for evacuation at that end of town, Wierson said.

Morgan suggested that the city's grant application could request funding to study both proposals. The city already allocated $25,000 to study the geology of the City Hall site to determine if an evacuation building would withstand an earthquake there. Another $25,000 is being spent on an evacuation study to find out which evacuation routes people most likely would use if an earthquake occurred in the ocean nearby and started a tsunami.

Scientists have predicted such an earthquake, known as a "Cascadia" quake, and a tsunami with waves reaching beyond Hemlock Street could occur within the next 30 to 50 years.

But the City Council expressed concerns about both bridge proposals and suggested that the public should weigh in on the question.

"I have a big problem with this," said City Councilor Sam Steidel. "I think (applying for) the grant is valid, but we have a lot of discussion to do yet. My own personal feeling is that depending on a bridge is extremely dangerous. It gives a false sense of security. A bridge is the worst possible place to go in a tsunami. Why head toward water when there's a tsunami?"

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Information from: The Daily Astorian, http://www.dailyastorian.com

 

Copyright 2010 The Associated Press.