Observed and modeled tsunami current velocities on California’s north coast

This thesis investigates currents produced by recent tsunamis in Humboldt Bay and Crescent City, California. The region is susceptible to both near- and far-field tsunamis and has a historic record of damaging events. Crescent City Harbor, 100 km north of Humboldt Bay, suffered US $28 million in damages from strong currents produced by the 2006 Kuril Islands tsunami and an additional US $26 million from the 2011 Japan tsunami. To better evaluate these currents in northern California, a Nortek Aquadopp 600 kHz 2D ADCP with a one-minute sampling interval was deployed in Humboldt Bay, near the existing NOAA NOS tide gauge station. The instrument recorded the tsunamis produced by the 2010 Mw 8.8 Chile earthquake and the 2011 Mw 9.0 Japan earthquake. Currents from the 2010 tsunami persisted in Humboldt Bay for at least 20 hrs with a peak amplitude of 0.35 m/s and dominant wave periods ranging between 0.5 and 1 hr. The 2010 tsunami signal represents a lower limit of the capability of an ADCP to measure small events. The 2011 tsunami signal lasted for approximately 40 hrs with peak amplitude of 0.84 m/s. Dominant wave periods ranged between 0.125 and 1 hr during the first 24 hrs after the tsunami arrival, and then narrowed to 0.25-1 hr over the duration of the signal. The strongest currents corresponded to the maximum change in water level, approximately 67 min after the initial wave arrival. In Crescent City, the first three and one-half hours of the 2011 tsunami was analyzed using video footage taken from the entrance to the small boat basin. The largest amplitude tide gauge water-level oscillations and most of the damage occurred within this time window. Currents reached a maximum speed of approximately 4.5 m/s with peak values exceeding 3 m/s for six cycles (tsunami periods). Tsunami current velocities in Humboldt Bay and Crescent City were compared to calculated velocities from the Method of Splitting Tsunamis (MOST) numerical model. The frequency and pattern of current amplification and decay at both locations are replicated by the MOST model for the first several hours after the tsunami onset. MOST underestimates peak current velocities by about 10 – 20% on average, and by as much as 50% for some peaks. At Humboldt Bay, MOST predicted attenuation of the signal after four hours but the actual signal persisted at a nearly constant level for at least twice as long. The results from this thesis demonstrate that ADCPs can effectively record tsunami currents for small to moderate events and can be used to calibrate and validate models (i.e. MOST) in order to better predict hazardous tsunami conditions and improve planned responses to protect lives and property, especially within harbors.