HR: 1340h
AN: OS23D-1341    [Abstracts]
TI: Effects of the Tides on the Probability of Tsunami Inundation at Seaside, Oregon
AU: * Mofjeld, H O
EM: Harold.Mofjeld@noaa.gov
AF: NOAA/Pacific Marine Environmental Laboratory, 7600 Sand Point Way NE, OE2, Seattle, WA 98115 United States
AU: Venturato, A J
EM: Angie.J.Venturato@noaa.gov
AF: Joint Institute for the Study of the Atmosphere and Ocean, Box 354235 University of Washington, Seattle, WA 98195 United States
AU: Gonzalez, F I
EM: Frank.I.Gonzalez@noaa.gov
AF: NOAA/Pacific Marine Environmental Laboratory, 7600 Sand Point Way NE, OE2, Seattle, WA 98115 United States
AU: Titov, V V
EM: Vasily.Titov@noaa.gov
AF: Joint Institute for the Study of the Atmosphere and Ocean, Box 354235 University of Washington, Seattle, WA 98195 United States
AB: The tides along the U.S. West Coast are large enough to have a major impact on tsunami inundation. An analysis of the first-order linear effects of tides on the total tsunami wave heights has been carried out for Seaside, Oregon, as part of the FEMA FIRM Pilot Study (Gonzalez et al., this session). Since the tsunami may arrive at a random time relative to the phase of the tide, a probabilistic approach is required to characterize the tidal effects. Both near- and distant-source generation creates tsunamis with long-duration wavetrains that last over several tidal cycles. As a result, the combination of tsunami and next higher-high water tends to produce the highest total wave height unless the tsunami wave amplitudes are substantially greater than the local tidal range (MHHW-MLLW=2.7 m at Seaside, Oregon). Computing the net height exceedance probability requires combining the height probabilities for the tsunamis (as described by Geist and Parsons, this session) with those of the tides and other background water levels, including the various tidal variations over the 18.6-year nodal cycle. Tsunamis generated regionally by great earthquakes in the Cascadia Subduction Zone will generate the highest tsunamis at Seaside. Model tsunamis of relatively short duration are used in the study; therefore, it is necessary to extend them in time via statistical measures in order to model the full tsunami wavetrains. This procedure follows the observation that the amplitudes of Pacific tsunamis decay exponentially with an e-folding time of 2 days. Since the tsunami wavetrains are irregular in time, the later waves are simulated using random band-limited fluctuations generated by wavelet methods. Complicating the analysis is the lack of a long-term tide gage at Seaside. For this study, tidal predictions are used based on harmonic constants from the NOAA/CSDL ENPAC 2003 tide model; the estimates of non-tidal sea level fluctuations are based on observations at South Beach, Oregon, located 150 km to the south. Open-coast tidal datums at Seaside, computed from the tidal harmonic constants, are significantly greater than those observed inside the very shallow mouth of the Necanicum River, for example, where MHHW-MLLW=1.8m. The procedures described here are therefore for the open coast at Seaside.
DE: 9355 Pacific Ocean
DE: 4560 Surface waves and tides (1255)
DE: 4564 Tsunamis and storm surges
DE: 1255 Tides--ocean (4560)
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting