Poor H Dock.
When 4-foot tsunami waves crashed into the Crescent City Harbor in rural Northern California last week, the dock nearest the harbor’s entrance — which was designed to absorb the brunt of the surge energy — took a beating.
State Sen. President Pro Tem Mike McGuire, who represents the North Coast, called H Dock “a sacrificial lamb.” And the Crescent City Harbor District said in a statement that the dock “functioned as designed — sacrificing itself to protect other infrastructure.”
But despite state and local officials’ relief over how well the deck performed during the storm, it appears the damage to the harbor in the small city in Del Norte County — which calls itself “Comeback Town,” because of its history with tsunamis — was worse than initial assessments suggested.
Harbor officials now estimate the pounding waves on July 30 caused $1 million in damage to the harbor, which was rebuilt to be “tsunami resistant” after it was destroyed in a 2011 tsunami.
“At first glance, the damage appeared modest” on H Dock, the Harbor District said in a statement Tuesday. Further assessments, however, found severe impacts below the water line to infrastructure “that supports the safe and functional operation of the harbor.”
Added Harbormaster Mike Rademaker: “The visible damage only tells part of the story.”
The tsunami, which followed a massive 8.8-magnitude earthquake off the sparsely-populated eastern coast of Russia on July 29, triggered alerts in Japan, Canada and along the entire U.S. West Coast. Ultimately, it did little damage outside of Northeast Russia, where it nearly destroyed a floating pier at a submarine base.
Read more: Why one of the biggest earthquakes ever recorded caused so little damage
Crescent City, a low-lying and uniquely tsunami-prone town of about 6,200 people, appears to have felt the most impact in California.
At around 2:40 a.m. on July 30, surging waves caused H Dock’s floating concrete decking to lift along its pilings, according to the Harbor District.
As the water continued to rise, the structure could not support the decking. It was temporarily submerged, “resulting in segment separation and progressive structural failure,” the Harbor District said in a statement last week.
In a statement Tuesday, harbor officials said the incident illuminated “a new vulnerability in floating dock design: a previously underrecognized hydrodynamic failure mechanism.”
Video footage and data modeling from July 30 point to “a fluid dynamics effect on docks that has rarely been captured on video.”
The tsunami accelerated currents beneath the floating decking of H Dock, creating a zone of low pressure that “produced a downward force strong enough to overcome the dock’s buoyancy, pulling the structure downward into the water,” the statement read.
“This is similar to the way an airplane wing generates lift — but in reverse,” Harbormaster Rademaker said. “Instead of lifting the structure up, the water moving rapidly under the dock actually ducked it down. It’s a dramatic and under-appreciated mode of failure.”
Harbor officials believe that electrical conduits through the center of H Dock’s decking were badly damaged, as were potable water lines. Fire suppression plumbing lines were ripped apart.
The tsunami waves also deposited a large amount of sediment and debris throughout the harbor basin. Extensive dredging operations will be required in order to restore safe depths for boat navigation, harbor officials said, adding that repairs will be “more costly than comparable land-based work” because it of the specialized labor needed.
Harbor officials said Tuesday that they are working on a tight deadline to assess the damage. California law requires city and county governments to declare a local emergency within 10 days of a disaster as a prerequisite for requesting state or federal assistance.
The Del Norte County Board of Supervisors “is expected to issue such a proclamation later this week,” the Harbor District statement read.
No boats were damaged in last week’s storm, and no injuries were reported.
In 2011, a tsunami that followed a devastating earthquake in Japan caused $50 million in damage to the Crescent City Harbor. The storm damaged numerous boats, which were repeatedly slammed against docks and each other.
The inner boat basin was completely reconstructed to be more tsunami resilient.
Thirty-inch steel pilings replaced the 16-inch pilings used in the previous dock structure and were driven between 21 and 37 feet into bedrock, compared with just 10 feet in the previous design, according to the Harbor District.
H Dock was designed to bear the brunt of storms, with closely-spaced pilings designed to absorb and dissipate energy before it reached interior docks.
State and local officials said the design prevented impacts from last week’s tsunami from being much worse.
“The infrastructure built after 2011 saved vessels and lives,” Rademaker wrote. “Now, we have a chance to take the design to the next level. We’re not asking for a blank check to return to the status quo, we’re taking the opportunity to use what we’ve learned to build smarter.”
Crescent City is uniquely susceptible to tsunamis for many reasons, including exposure — it is low-lying and juts out into the Pacific — and the bowl shape of the continental shelf, which traps the energy of tsunamis and bounces waves back and forth.
Including last week’s surge, 42 tsunamis have been recorded in Crescent City since the first tide gauge was installed in 1933.
Crescent City earned its Comeback Town nickname in 1964, after a tsunami killed 11 people and destroyed 29 city blocks, forever transforming downtown.
This story originally appeared in Los Angeles Times.
