Since 1946 Crescent City has been hit with 10 tsunamis that have sunk boats, damaged marinas, and disrupted the fishing industry. The new harbor currently under construction is intended to be the first tsunami-resistant port and is designed to withstand a tsunami that might hit once every 50 years. Courtesy of the Crescent City Harbor District
In the absence of specific building codes, a team of engineers has designed a marina in California to resist a once-in-50-years tsunami, hoping the structure will serve as a model for others.
January 15, 2013—In March 2011, while engineers were at work designing a wave-resistant marina to replace a port in Crescent City, California, that had been damaged by a tsunami five years earlier, an earthquake struck off the shores of Japan. The natural disaster triggered a second tsunami, which also hit the Crescent City Harbor District’s marina, sinking 16 boats, damaging dozens of others, and threatening the local fishing economy.
The two tsunamis in five years caused $50 million in damage to the marina and nearby waterways and offered further proof that the harbor district’s next marina should be strong enough to withstand tsunamis, 10 of which have hit Crescent City since 1946, when the natural disasters were first recorded there. The 10 tsunamis brought waves ranging in height from several inches to 16 ft; Crescent City’s port is especially vulnerable to tsunamis due to geological features and because the port opens to the south, according to Ted Trenkwalder, P.E., S.E., M.ASCE, a structural engineer with Ben C. Gerwick, Inc., in Oakland, who is serving as the structural engineer for a new, more robust marina.
Trenkwalder says the engineers wanted to build a marina that would last 25 years and calculated that to do that they needed to prepare for a tsunami surge that might hit only once every 50 years. But figuring out how strong the new marina should be to withstand that type of tsunami—which could bring a 15 ft high wave—was a challenging task, Trenkwalder says. “Nothing like this had been done before,” he says. “Currently there are no building codes for tsunamis—nothing that says, ‘here’s how you do it.’ [So] we’re trying to establish as clearly as we can what the expected loads might be for a tsunami event,” he says.
The new port will be supported by 244 piles specially designed to
withstand the increased force from the strong currents and rising
waters a tsunami would bring. Engineers used mathematical and
3-D models to determine how strong the piles should be and then
had them built even stronger to ensure the docks could be rebuilt
quickly. Courtesy of Stover Engineering
Designing the marina was a two-year effort involving extensive mathematical modeling to predict a tsunami’s potential force, and also required cooperation between local, state, and federal agencies. The effort appears to have paid off. “We think we’ve done it,” Trenkwalder says.
Meanwhile the ASCE 7 Subcommittee on Tsunami Loads and Effects—chaired by structural engineer Gary Chock, P.E., S.E., M.ASCE, of Honolulu-based Martin & Chock and tasked with developing proposed revisions pertaining to tsunami loads and effects—is creating standards for designing tsunami-resistant buildings and waterfront structures. Although the standards will not address marinas like Crescent City’s specifically, Trenkwalder is a member of the subcommittee and is offering feedback on his team’s findings. He hopes the new marina will serve as a model for future tsunami-resistant ports along the California coast and other places.
In their calculations to protect against a 15 ft high wave similar to one that inundated Crescent City in 1964, the marina’s designers estimated the velocity, volume, and weight of the water, including all the sand and silt that the surge would carry with it. Engineers also considered current patterns that would form in the harbor when a tsunami wave strikes the rocks and creates eddies in the harbor. “The water comes in through the entrance channel, it hits the rock, and it shoots [up],” he says. “[It] forms an eddy within the harbor itself.”
A 50-year tsunami would move at a velocity of 22 ft per second, 10 times as fast as a normal wave, Trenkwalder says, and exert 20 to 30 times more force on the marina and its boats. As the water rises against the exposed surface area of the marina and its 250 vessels, the tsunami’s horizontal force will increase, so engineers also considered the exposed surface area of the harbor and the boats themselves.
The 30 in. diameter steel pipe piles are sunk 30 to 35 ft into rock
and are designed to withstand 75 percent of their 70 ksi yield
strength. The Dutra Group, of San Rafael, California, began
constructing the tsunami-resistant marina in late 2012 and is
expected to finish by the end of this year. Courtesy of the
Crescent City Harbor District
“We’re trying to resist the water that comes in through the strength of the piles and the floats,” Trenkwalder explains. “Just like it’s harder to stand up the higher the water is,” he says, it is harder to resist a higher wave unless the “legs” of the marina are strong enough.
For this reason, some 244 steel piles—each 30 in. in outer diameter and encircled by a 2 in. high-density polyethylene sleeve to prevent corrosion—were specified. Each pile has a minimum wall thickness of 5/8 in., Trenkwalder says, and they are being driven to 2 ft beneath the mud line.
Each pile has a yield strength of 70 ksi, and together they can withstand approximately 70 percent of their yield strength, Trenkwalder says. He says this is about 25 percent stronger than their calculations strictly called for, but the engineers wanted to be sure that in the unlikely event that an even stronger tsunami hits, the piles would survive and the port could be rebuilt more quickly than in years past, reducing financial losses for the fishing industry. “Even if the marina is decimated,” he says, the piles will remain.
The marina’s design was a collaborative effort between Stover Engineering, of Crescent City, which worked on such aspects as the mechanical floats, and San Francisco-based Treadwell & Rollo, which calculated such geotechnical aspects as the pile-soil strength, and the contractor, the Dutra Group, of San Rafael, California. Kennedy/Jenks Consultants was responsible for the electrical design. Dutra began constructing the tsunami-resistant marina in late 2012 and is expected to finish by the end of this year.