Joint acoustic research by HSU and Humboldt County shows solid promise for measuring the velocity of tsunami currents like those triggered along the California coast last March by the Japan tsunami.
A device called an acoustic profiler, deployed under a pilot project from a dock at Humboldt Bay’s Fairhaven Terminal on the North Spit, captured the velocity signals of both the Mar. 11 Japan and the Feb. 27, 2010 Chile tsunamis.
“That was really exciting and confirmed to us that this pilot project succeeded in measuring tsunami currents,” says Humboldt State graduate researcher Amanda Admire. “We also now have the velocity data posted online in near real-time.”
The link is http://nrs-isa.humboldt.edu/tsunami/content/data_adcp_realtime.php.
The research supports tsunami science and local mitigation efforts along the earthquake- and tsunami-prone North Coast. One of the objectives is to determine the response of Humboldt Bay to tsunami-produced currents and to compare the measured currents to those predicted by numerical models.
“If we can gain a deeper understanding of the currents that are generated, then over the long-term we can engineer the infrastructure of harbors and marinas to withstand the impact better,” says Professor Lori Dengler of Humboldt State’s Department of Geology. She is an internationally recognized expert in earthquake and tsunami hazard mitigation.
Dengler conducted a 10-day reconnaissance of Japan disaster sites in May. Water heights topped five stories in some cities, she found. Sea walls were overrun and obliterated. In other locations, water heights exceeded 30 feet and leveled whole blocks.
“There is a lot of discussion in Japan about rebuilding sea walls and other abatement structures,” says Dengler, “but the question is whether you rebuild for a more common 100-year event or a 1,000-year catastrophe like the one on March 11th. I don’t think 60-foot sea walls are economically or structurally viable, and a lot of other factors will have to be considered, not just in Japan but here in Humboldt and all up and down our Pacific coast. You have to take into account coastal configurations, population distribution and land use, as well as tsunami hazard zones and evacuation routes. Gauging the velocity of tsunami currents in Humboldt Bay is the kind of site information we need to underpin tsunami mitigation and infrastructure planning.”
The prototype measuring device is a Nortek Aquadopp 600 kHz acoustic two-dimensional current profiler. The Fairhaven dock in Humboldt Bay was chosen for the pilot project because of its proximity to NOAA’s (National Oceanic and Atmospheric Administration) tide gauge site, which continuously records water levels. The dock also affords easy access to Humboldt State scientists who fine-tune the instrument.
The profiler has demonstrated the feasibility of the pilot approach with the recording of two major tsunamis. “For the first time, we have actual quantitative data that show both the strength and the time history of tsunamis in Humboldt Bay,” says Dengler.
The Japan tsunami reached speeds of about two knots and persisted over two days. That was not damaging, but “this record allows us to calibrate numerical models and more accurately project the likely forces exerted by much larger tsunamis,” Dengler emphasizes.
Ironically, a second prototype was about to be deployed in Crescent City in March, but minor installation details kept it sitting on shore. That turned out to be fortunate, as the appointed dock was destroyed by the Japan tsunami.
On the upside, says Admire, “We lucked out and saved our expensive equipment. It was a bummer not to get the data, but we would have lost the instrument. We are still pursuing deployment in Crescent City, but have not set a date yet.”
The device measures the velocity of particles in the water. It is deployed in Humboldt Bay pointing roughly east because the channel near Fairhaven Terminal runs north-south. Accordingly, velocity measurements are collected across the channel. The instrument is set up to record one-minute average velocities, running continuously and collecting data.
Pacific Gas & Electric Company provided $30,000 to purchase and install the instrument in Humboldt Bay. The Humboldt Bay Harbor Recreation and Conservation District has provided logistical support. Additional operating expenses have been financed with donations from private individuals.
Dengler is the lead scientist and former HSU Oceanography Professor Greg Crawford, who helped her kick off the operation three years ago, continues to consult on the project.
Troy Nicolini of the National Weather Service in Eureka assisted with site scouting, and Katrina Sigler and Mike Willcutt from Security National Properties aided in establishing the site. David Hull, Adam Wagschal, Alan Bobillot and John Powell of Humboldt Bay Harbor District assisted in the design, development and deployment. Humboldt State’s Richard Alvarez and Steve Monk provided diving support and HSU’s Jose Montoya is the instrument technician.
The Humboldt State team collaborates with numerical modeler Burak Uslu of NOAA’s Pacific Marine Environmental Laboratory.