Including Tidal Effects in Tsunami Forecasting

Abstract:

Recently a new tsunami forecast system SIFT (Short-term Inundation and Forecasting of Tsunamis) has been declared operational by the National Weather Service (NWS) Tsunami Warning Centers. The SIFT system assimilates real-time information from a network of observing systems deployed in the open ocean, to produce on-the-fly estimates of tsunami impact at specific coastal communities. These estimates are computed via the tsunami simulation code MOST (Method of Splitting Tsunami) and include forecast products such as tsunami arrival time, duration of the event, predicted tsunami currents, maximum sea surface elevation and expected inundation areas. These computations are performed under the assumption that the mean sea level remains constant at Mean High Water (MHW) during the entire tsunami event. This assumption produces conservative tsunami forecasts that tend to err on the side of caution with the possibility of substantial overestimates of the inundation areas. To avoid this problem and produce more accurate, operational tsunami forecasts, we investigate the effects of tsunami interaction with tides. The nonlinear dynamic interaction is simulated by first, simulating tidal elevations and currents with Oregon State University tidal model, to obtain boundary and initial conditions to force the MOST tsunami model. Tsunami boundary and initial conditions can be added to those for the tide to study the combined effect. Our results show that even at locations with strong tidal forcing, the tsunami/tide interaction effect has a weakly non-linear effect on the tsunami elevation waveform. This interaction, however, will have a significant effect on the extent of the inundation area. Based on these findings we propose a simple, linear correction to the standard MHW forecast for tsunami time series and inundation area, that can be performed on-the-fly by the SIFT system without the need for complex tidal models.