Date of Award
Master of Science in Ocean Engineering
Stéphan T. Grilli
This body of work uses state of the art numerical models to assess and reduce tsunami hazard. The first manuscript describes the use of these models to explore nonlinear interaction between tide and tsunami in the context of hazard assessment. Inundation due to several probable maximum tsunamis (PMTs) is considered in the Hudson River Estuary (HRE). Of the sources considered, a submarine mass failure (SMF) poses the most significant tsunami threat in this region and across the entire US East Coast. The next manuscript focuses on the how SMF mechanics effect tsunami generation. In addition to inundation, SMF tsunamis are dangerous because of their short or nonexistent warning times. The final manuscript discusses developments to an algorithm which extend the range of tsunami detection by shore-based HF radar, thereby increasing warning times.
Tsunami hazard assessment in the Hudson River Estuary based on dynamic tsunami-tide simulation
The first manuscript is part of a tsunami inundation mapping activity carried out along the US East Coast (USEC) since 2010, under the auspice of the National Tsunami Hazard Mitigation program (NTHMP). Two densely built low-lying regions are situated along this coast: Chesapeake Bay and HRE. HRE is the object of this work, with specific focus on assessing tsunami hazard in Manhattan, the Hudson River and East River areas. In the NTHMP work, inundation maps are computed as envelopes of maximum surface elevation along the coast and inland, by simulating the impact of selected PMTs in the Atlantic Ocean margin and basin. At present, such simulations assume a static reference level near shore equal to the local mean high water (MHW) level. Instead we simulate maximum inundation resulting from dynamic interactions between the incident PMTs and a tide, which is calibrated to achieve MHW at its maximum level. To identify conditions leading to maximum tsunami inundation, each PMT is simulated at four different phases of the tide and results are compared to those obtained for a static reference level in the HRE. We conclude that changes in inundation resulting from the inclusion of a dynamic tide in the specific case of the HRE, although of scientific interest, are not significant for tsunami hazard assessment and that the standard approach of specifying a static reference level equal to MHW is conservative. However, in other estuaries with similarly complex bathymetry/topography and stronger tidal currents, a simplified static approach might not be appropriate.
Modeling coastal tsunami hazard from submarine mass failures: effect of slide rheology, experimental validation, and case studies of the US East coast
We first validate two models simulating tsunami generation by deforming submarine mass failures (SMFs) against laboratory experiments for SMF made of glass beads moving down a steep slope. These are two-layer models, in which the upper layer is water, simulated with the non-hydrostatic 3D (-layer) non-hydrostatic model NHWAVE, and the SMF bottom layer is simulated with depth-integrated equations and represented either as a dense Newtonian fluid or a granular medium.
Using the dense fluid model, we assess model convergence with grid resolution, and sensitivity of slide motion and generated surface elevations to slide parameters. A more limited validation is conducted for the granular slide model. Both models can accurately simulate time series of surface elevations measured at 4 gages, while providing a good simulation of both the geometry and kinematics of the moving slide material.
The viscous slide model, which at present is the only one that can be applied to an arbitrary bottom bathymetry, is then used to simulate the historic Currituck SMF motion, in order to determine relevant viscous slide parameters to simulate SMF tsunamis on the east coast. The same parameters are then applied to simulate tsunami generation from a possible SMF sited near the Hudson River Canyon.
Simulations are performed for 3 deforming slides with different dissipation parameters and the rigid slump, and results compared; all SMFs have the same initial volume, location, and geometry. Simulations of tsunami propagation are then done for the tsunamis in two levels of nested grids, using the Boussinesq model FUNWAVE-TVD, and maximum surface elevations computed along a 5 m depth contour off of the coast of New Jersey and New York.
At most nearshore locations surface elevations caused by the rigid slump are significantly larger (up to a factor of 2) than those caused by the 3 deforming slides. Hence, the rigid slump provides a conservative estimate of SMF tsunami impact in terms of maximum inundation/runup at the coast, while using a more realistic rheology with some level of SMF deformation, in general, leads to a reduced tsunami impact at the coast. This validates as conservative the tsunami hazard assessment and inundation mapping performed to date as part of NTHMP, on the basis of Currituck SMF proxies simulated as rigid slump.
Algorithms for tsunami detection by High Frequency Radar : development and case studies for tsunami impact in British Columbia, Canada
To mitigate the tsunami hazard along the shores of Vancouver Island in British Columbia (Canada), Ocean Networks Canada (ONC) has been developing a Tsunami EarlyWarning System (TEWS), combining instruments (seismometers, pressure sensors) deployed on the sea floor as part of their Neptune Observatory, and a shore-based High-Frequency (HF) radar. This HF radar can remotely sense ocean currents up to a 80 km range, based on the Doppler shift they cause in ocean waves at the radar Bragg frequency. Using this method, however, tsunami detection is limited to shallow water areas where they are sufficiently large due to shoaling and, hence, to the continental shelf.
To extend detection range into deep water, thereby increasing warning time, the authors have proposed a new detection algorithm based on spatial correlations of the raw radar signal at two distant locations along the same wave ray. In a previous work, they validated this algorithm for idealized tsunami wave trains propagating over a simple sea floor geometry in a direction normally incident to shore. In the final manuscript, this algorithm is extended and validated for realistic tsunami case studies conducted for seismic sources and using the bathymetry off of Vancouver Island, BC. Tsunami currents computed with a state-of-the-art long wave model are spatially averaged over the HF radar cells aligned along individual wave rays, obtained by solving geometric optic equations. A model simulating the radar backscattered signal in space and time as a function of the simulated tsunami currents is applied for the characteristics of the WERA HF radar deployed by ONC near Tofino, BC. Finally, numerical experiments show that the proposed algorithm works on realistic tsunami data. This is used to develop relevant correlation thresholds for tsunami detection.
Shelby, Michael R., "Modeling Tsunamis for Improved Hazard Assessment and Detection" (2016). Open Access Master's Theses. Paper 891.