CANNON BEACH — Higher ground may be only five minutes away on the north side of Cannon Beach, but the 1,000 people – including elementary school students – expected to seek refuge from an oncoming tsunami won’t be able to cross the Ecola Creek Bridge to get there.

A report released recently predicts that the bridge will collapse in an earthquake. And even if it remains standing, a tsunami will more than likely destroy it.

Ecola Creek separates the north side of Cannon Beach from the elementary school and the rest of the town.

The report, written by OBEC Consulting Engineers, of Eugene, at the city’s request, analyzes six alternative bridge designs. They are rated according to their ability to withstand mild to extreme earthquakes and distant and nearby tsunamis.

 Costs for the bridges range from $796,000 for a 6-foot wide floating timber pontoon bridge to $4.8 million for a concrete replacement of the existing bridge using the same alignment. Most of the proposed alternatives are for pedestrian use only.

OBEC recommends designing a bridge that will withstand the worst of the earthquakes and tsunamis that have occurred in the last 2,500 years.

Such an earthquake would involve a nearby Cascadia subduction earthquake, caused by a shift of tectonic plates 50 to 70 miles offshore. The proposed designs anticipate a magnitude 9.3 earthquake, the “largest believed possible off of the coast of Cannon Beach,” the study says.

Such an earthquake would be similar to or greater than the magnitude 9.0 earthquake that hit Japan March 11.

The Cannon Beach City Council will review the report at a City Council work session on Sept. 13, said City Manager Rich Mays. The report also will be a discussion topic during a community open house in September.

 Built in 1964 after an Alaska-generated tsunami destroyed the former bridge, the existing Ecola Creek Bridge is “vulnerable to failure in earthquakes,” the report notes. Most of the timber pilings also will be gone, so even a rudimentary pedestrian bridge could not be constructed after the earthquake.

Construction costs

Although it has been deemed too expensive to replace the current bridge, city officials have considered building a pedestrian bridge for nearly two years. The bridge could be used year-round and be available for emergency evacuations.

Design alternatives for a new pedestrian bridge were suggested to OBEC by the city staff and members of the city’s emergency preparedness committee.

 The new bridge would provide the shortest evacuation route to escape an oncoming tsunami. Estimates from a previous study indicate it would take five minutes for those at the Cannon Beach Elementary School, the northern portion of the downtown core and the Christian Conference Center to escape across the bridge.

Without the bridge, the only other escape route is along Spruce Street, which eventually ends at higher ground to the southeast. However, this route takes about 22 minutes in ideal conditions. Scientists say a nearby tsunami could reach land in 15 to 18 minutes, possibly sooner.

Poor soil

Most of the alternatives call for the pedestrian bridge to be built 30 feet west of the current bridge. If an earthquake occurred, the report says, the existing bridge would fall to the east, away from the new bridge.

The new alignment would also enable the bridge to be built on public, nonpark lands on both ends, which would minimize right-of-way issues.

Although the soil on Ecola Creek’s banks hasn’t yet been analyzed, the ground where the City Hall stands about a mile to the south recently underwent a geological examination. It was determined that the soil could turn to liquid the first 75 feet down, and the ground would spread between one and four feet. The soil was identified as a “Class E” – the poorest of possible soil types.

OBEC engineers say in their report that there was no reason to believe the soil near Ecola Creek would be different.

Because of the poor soil conditions, all of the designs must have foundations that are durable, deep and earthquake-resistant, they add.

Evacuation routes

A previous computer study involving probable evacuation routes that people would take to escape a tsunami estimated that 1,080 people would head across the bridge to the north side.

With at least 1,000 people crowding onto the bridge, a potential bottleneck also presents a concern, according to the OBEC report. About 72 persons per minute could cross a 6-foot-wide pedestrian bridge with four feet of walking space.

If 15 minutes is available before the tsunami arrives, 1,080 people could cross in time, according to the study. However, if delays occur, such as downed power lines, broken roads or obstacles in the way, people may arrive all at once with a short time to cross the bridge.

“In that scenario…the number of people crossing the bridge could be greatly reduced,” the report says.

However, a two-way multi-use bridge 12-feet wide could handle 216 evacuees per minute, it notes.

The bridge’s width may determine whether the city can obtain federal or state funding for its construction, according to the report. A narrower, 6-foot-wide bridge may be allowed, but funding may only be available for a bridge that is at least 12 feet wide.

Mays said the city would have to depend on outside funding to build the bridge. “There’s no way we can pay for it,” he said.

He added that it was too soon to begin considering whether a bond measure might be submitted to voters for funding.

Alternatives

The alternatives – and their advantages and disadvantages — presented in the report are:

• Floating pontoon bridge: $796,000

The 6-foot-wide pontoon bridge would be of timber construction, with the bridge ends connected to driven steel pile foundations able to withstand an earthquake and soil liquefaction. It wouldn’t resist a tsunami wave, however.

Pros: Lower cost than any of the alternatives; may resist a mild earthquake (magnitude 5 to magnitude 7).

Cons: Environmental agencies would object to having treated timber construction over rivers and creeks unless it was additionally sealed, and they may also object to pontoons in the water; probably won’t resist a strong earthquake or even a small tsunami; inadequate to move a large number of evacuees in a few minutes; surface level of the floating deck may be too steep at times to meet accessibility standards; and wooden deck could get slippery.

• Fixed timber bridge with 6-foot-wide deck: $1.05 million

Like the pontoon bridge, this bridge also would be built with 60-foot-long timber spans. It also would have steel pile foundations.

Pros: Could possibly survive both mild and severe earthquakes.

Cons: Timber construction may not be allowed in the creek; tsunami response is “particularly poor”; too narrow to adequately move a large number of evacuees in a short period; will have a shorter lifespan and require more maintenance than a concrete bridge.

• Fixed pre-stressed concrete pedestrian bridge with a 12-foot-wide deck: $1.7 million

This bridge would be built of concrete with a 12-foot-wide deck supported on 60-foot spans. It would have driven steel pile foundations.

Pros: Steel armored and pointing deck edges would resist tsunami-driven debris; it would resist both mild and extreme earthquakes; the environmental process for siting this bridge in creeks and rivers is well known and predictable; the deck surface is hard and nonslip and adequate to move a large number of evacuees in a short period; the concrete surface will have a long, useful life.

Cons: The report did not list any negatives.

• Replace western half of the existing road bridge with a new concrete bridge on the same alignment with a 10-foot-wide downstream sidewalk for tsunami evacuation: $2.9 million

The concrete portion of this replacement would be 27 feet wide, including a 1-foot-wide railing, 10-foot sidewalk, 4-foot-wide shoulder and 12-foot travel land. “To the extent possible,” the concrete addition would be designed to withstand a large earthquake and tsunami. The east half of the bridge would be replaced in the future.

Pros: Could receive environmental permits; earthquake resistant; hard, nonslip surface and 27-foot-width would allow easier pedestrian evacuation; little maintenance required.

Cons: No state or federal funding appears to be available; it may not effectively resist a tsunami or the debris generated by a tsunami.

• Replace all of existing bridge with new concrete bridge and 10-foot-wide sidewalk for evacuation: $4.8 million.

A precast concrete bridge would replace the existing 49-foot-wide bridge on the same alignment as it is now. It would be supported with 60-foot spans with steel pilings designed to withstand a large earthquake, soil liquefaction and a tsunami “to some extent,” the report says.

Pros: Environmentally acceptable; could withstand earthquakes; would allow easier evacuation of pedestrians in a short time because of its width; durable, with little maintenance required.

Cons: The bridge is the most expensive alternative and no state or federal funding appears to be available for this bridge; resistance to a tsunami may not be as effective as a 12-foot-wide deck because its greater width may be more susceptible to uplift force.

• Fixed timber suspension bridge with minimum 6-foot-wide deck: $1.6 million

This alternative would be constructed with timber and have a 6-foot-wide deck. Its 300-foot-long timber main span would be supported on steel towers on steel pilings. The main cables would be anchored 75 feet back on land at each end of the bridge and would be connected to mass concrete structures also on steel pilings.

 Pros: Would withstand mild and extreme earthquakes; may be environmentally acceptable because it has no piers in the water.

Cons: Debris from a tsunami would get caught on the main cables at the lower anchorages; narrow deck would restrict the number of evacuees crossing in a limited time; and the timber construction would require more maintenance.

• Use the pilings from the existing bridge in the loss of the deck for temporary crossing after an earthquake and tsunami: Cost not estimated

The report notes that the pilings of the existing bridge are still well within the zone where the soil will liquefy in an earthquake. In addition, the pins connecting the pilings to the deck have low resistance to the lateral movement caused by earthquakes.

“It is likely the existing timber substructure would not be usable to rapidly establish a transportation link after a bridge collapse,” the report says.

Although some of the bridge pilings remained after the 1964 tsunami and a temporary pedestrian “bridge” was created, that is unlikely to happen again. Because the existing bridge is heavier than the bridge it replaced, it is more likely to flatten the existing pilings, the report says.

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