Including Tidal Effects in Tsunami Forecasting
Tuesday, 16 December 2014
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 interaction of tsunamis with a longer period water level variation due to tidal forcing, by comparing simulations of the 2011 Japan event at different at different locations with and without tidal effects. Our results demonstrate that while non-linear effects resulting from this interaction are minimal in water surface elevation, they can have a significant effect on inundation areas. Based on these findings we propose a simple, first-order correction to the standard MHW forecast, that can be performed on-the-fly by the SIFT system without the need for complex tidal models.