Adam Spencer, The Triplicate

As Crescent City Harbor nears the completion of the first tsunami-resistant harbor in the Western Hemisphere, a report released Wednesday shows why such a burly structure is needed for California's northernmost commercial fishing port.

If a monster 9.1 earthquake struck off Alaska's coast, tsunami waves would rush toward the entire California coast within four to six hours, according to a report done by a team from the U.S. Geological Survey to help emergency responders prepare.

Crescent City is keenly aware of the devastation that can trail an earthquake off the Alaskan coast - that's the site of the 9.2 earthquake that generated the 1964 tsunami, which killed 11 people and destroyed 30 blocks here.

The USGS report details how tsunamis pose a threat to the entire California coast.

Scientists said a closer offshore quake would create more havoc. The twin ports of Los Angeles and Long Beach could be shuttered for at least two days because of strong currents, potentially losing $1.2 billion in business. The Oakland Airport would be flooded. Coastal communities would face mass evacuations, the report said.

This "helps them understand what a bad tsunami can be," said USGS seismologist Lucy Jones.

The team began work on the scenario before the magnitude-9.0 earthquake that struck Japan in March 2011 and triggered a tsunami. It went back to the drawing board after seeing the toll on Crescent City and other coastal cities. The group focused only on California, even though a powerful offshore Alaska quake would affect the entire West Coast.

Coastal planners held meetings this week around the state to digest the information and review their evacuation plans.

Under the scenario, it would take about four hours for tsunami waves to crash into places like Crescent City, near the Oregon state line, and about six hours to reach San Diego - theoretically allowing time for people to flee to higher ground.

The force of the waves would bring the same destruction felt in Crescent City to other ports, sinking boats docked in marinas and damaging harbors, the report said. The 2011 tsunami caused more than $20 million in damage to Crescent City Harbor, adding to some estimated $20 million in damage already inflicted from a 2006 tsunami spawned near the Kuril Islands.

Instead of replacing what was destroyed, local harbor leaders and California agencies pushed to build something better. The result is what several engineering firms behind the project call the first "tsunami-resistant" port in the western hemisphere. The project, estimated to cost more than $50 million, is designed to withstand the kind of tsunami expected once every 50 years.

Is it tsunami-resistant?

The new protection comes from the port's 244 wider steel pilings at 30 inches each, which are being driven at least 30 feet into the bedrock. The interior docks are extra strong, built with massive concrete modules, and they attach to the pilings with 600-pound pile guides made of galvanized steel. Bellingham Marine, the manufacturer of the dock and hoops, has called it the most technically complex project it has ever built.

The docks, pilings and pile guides are designed to withstand the forces the come with quickly rising and falling water during tsunami surges.

The other crucial component making the marina tsunami-resistant is a "wave attenuator" designed to decrease the force of a tsunami surge.

The 400-foot-long H Dock is the first dock a surge would hit at the entrance of the harbor. It's 7 feet tall, like an iceberg with the bulk hanging below the surface. Each 39-foot by 16-foot section of H Dock weighs 160,000 pounds, creating a massive wall of concrete that should slow down most tsunami surges. The pilings are closer together on H Dock, sometimes only an arm-span, adding to its strength.

Construction progress

More than 150 of the steel pilings have been installed so far, and all docks and pilings are expected to be structurally complete by mid-November, although other details like connecting water and electrical utilities could take longer, harbor officials said.

Harbor engineers and Dutra Construction, the lead contractor of the project, have discovered that some pilings are not perfectly plum, more than the 0.3 percent from vertical variance allowed in the design.

If the variance is too great, than docks could get jammed on the pilings during the quick rise and drop experienced during a tsunami surge, said Richard Young, Crescent City Harbormaster and CEO.

It may not be necessary to be within 0.3 percent of vertical, Young said, but there is a point where it could cause problems and "we expect them to correct the most egregious ones."

Dutra Construction has been pile driving seven days a week since Labor Day in order to finish by deadline, Young said.

Two Dutra excavators have been making steady progress on rock slope protection walls in the inner boat basin, but sometimes work is hampered by a shortage of 4-ton rock, needed for a point near the entrance of the harbor where tsunami surges are strongest. At times, Dutra has had to use a barge to ship 4-ton rock from its quarry in San Rafael, Young said.

Dutra has started to pour the concrete sidewalks on top of some of the rock walls, coming close to finishing some sections of rock wall work.

Dredging in the outer boat basin of the port is expected to be complete in a week or less after at least 168,000 cubic yards of silt and mud have been tugged out to sea, one 2,000-cubic-yard scow at a time.

The dredging became slow-moving when operators of the clam-shell scoop discovered the bedrock was shallower in some spots than the minus-15 depth called for in the design.

Before the season

More than 40 percent of all of Dungeness crab caught in California is landed in Crescent City. Last season's catch of 9,036,932 pounds was valued at $24,164,204.

The new inner boat basin will offer at least 230 slips to commercial fishing vessels, mostly crab fishermen.

The commercial Dungeness season is scheduled to begin Dec. 1, but could be delayed if sampled crabs show signs of late-molting. State regulators will only allow in-water construction work, like dock installation, through Nov. 15 to prevent impacts to salmon returning to spawn in nearby creeks.

The season start and dock finish leaves a narrow window before contractors need to have the docks installed and fishermen need to use them.

The Associated Press contributed to this report.

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