Jeffrey C. Harris

Numerical Simulation of the 2011 Tohoku Tsunami Based on a New Transient FEM Co-seismic Source: Comparison to Far- and Near-Field Observations

In this work, we simulate the 2011 M9 Tohoku-Oki tsunami using new coseismic tsunami sources based on inverting onshore and offshore geodetic data, using 3D Finite Element Models (FEM). Such FEMs simulate elastic dislocations along the plate boundary interface separating the stiff subducting Pacific Plate from the relatively weak forearc and volcanic arc of the overriding Eurasian plate. Due in part to the simulated weak forearc materials, such sources produce significant shallow slip (several tens of meters) along the updip portion of the rupture near the trench. To assess the accuracy of the new approach, we compare observations and numerical simulations of the tsunamis far- and near-field coastal impact for: (i) one of the standard seismic inversion sources (UCSB; Shaoet al.2011); and (ii) the new FEM sources. Specifically, results of numerical simulations for both sources, performed using the fully nonlinear and dispersive Boussinesq wave model FUNWAVE-TVD, are compared to DART buoy, GPS tide gauge, and inundation/runup measurements. We use a series of nested model grids with varying resolution (down to 250 m nearshore) and size, and assess effects on model results of the latter and of model physics (such as when including dispersion or not). We also assess the effects of triggering the tsunami sources in the propagation model: (i) either at once as a hot start, or with the spatiotemporal sequence derived from seismic inversion; and (ii) as a specified surface elevation or as a more realistic time and space-varying bottom boundary condition (in the latter case, we compute the initial tsunami generation up to 300 s using the non-hydrostatic model NHWAVE). Although additional refinements are expected in the near future, results based on the current FEM sources better explain long wave near-field observations at DART and GPS buoys near Japan, and measured tsunami inundation, while they simulate observations at distant DART buoys as well or better than the UCSB source. None of the sources, however, are able to explain the largest runup and inundation measured between 39.5° and 40.25°N, which could be due to insufficient model resolution in this region (Sanriku/Ria) of complex bathymetry/topography, and/or to additional tsunami generation mechanisms not represented in the coseismic sources (e.g., splay faults, submarine mass failure). This will be the object of future work.

Modeling of SMF tsunami hazard along the upper US East Coast: detailed impact around Ocean City, MD

With support from the US National Tsunami Hazard Mitigation Program (NTHMP), the authors have been developing tsunami inundation maps for the upper US East Coast (USEC), using high-resolution numerical modeling. These maps are envelopes of maximum elevations, velocity, or momentum flux, caused by the probable maximum tsunamis identified in the Atlantic oceanic basin, including from far-field coseismic or volcanic sources, and near-field Submarine mass failures (SMFs); the latter are the object of this work. Despite clear field evidence of past large-scale SMFs within our area of interest, such as the Currituck slide complex, their magnitude, pre-failed geometry, volume, and mode of rupture are poorly known. A screening analysis based on the Monte Carlo simulations (MCS) identified areas for possible tsunamigenic SMF sources along the USEC, indicating an increased level of tsunami hazard north of Virginia, potentially surpassing the inundation generated by a typical 100-year hurricane storm surge in the region, as well as that from the most extreme far-field coseismic sources in the Atlantic; to the south, the MCS indicated that SMF tsunami hazard significantly decreased. Subsequent geotechnical and geological analyses delimited four high-risk areas along the upper USEC where the potential for large tsunamigenic SMFs, identified in the MCS, was realistic on the basis of field data (i.e., sediment nature and volume/availability). In the absence of accurate site-specific field data, following NTHMP’s recommendation, for the purpose of simulating tsunami hazard from SMF PMTs, we parameterized an extreme SMF source in each of the four areas as a so-called Currituck proxy, i.e., a SMF having the same volume, dimensions, and geometry as the historical SMF. In this paper, after briefly describing our state-of-the-art SMF tsunami modeling methodology, in a second part, we parameterize and model the historical Currituck event, including: (1) a new reconstruction of the SMF geometry and kinematics; (2) the simulation of the resulting tsunami source generation; and (3) the propagation of the tsunami source over the shelf to the coastline, in a series of nested grids. A sensitivity analysis to model and grid parameters is performed on this case, to ensure convergence and accuracy of tsunami simulation results. Then, we model in greater detail and discuss the impact of the historical Currituck tsunami event along the nearest coastline where its energy was focused, off of Virginia Beach and Norfolk, as well as near the mouth of the Chesapeake Bay; our results are in qualitative agreement with an earlier modeling study. In a third part, following the same methodology, we model tsunami generation and propagation for SMF Currituck proxy sources sited in the four identified areas of the USEC. Finally, as an illustration of our SMF tsunami hazard assessment work, we present detailed tsunami inundation maps, as well as some other products, for one of the most impacted and vulnerable areas, near and around Ocean City, MD. We find that coastal inundation from near-field SMF tsunamis may be comparable to that caused by the largest far-field sources. Because of their short propagation time and, hence, warning times, SMF tsunamis may pose one of the highest coastal hazards for many highly populated and vulnerable communities along the upper USEC, certainly comparable to that from extreme hurricanes.

MODELING OF WAVE-INDUCED SEDIMENT TRANSPORT AROUND OBSTACLES 1

We report on further developments of a hybrid numerical model to simulate wave-induced sediment transport. A 2D numerical wavetank (NWT) based on fully nonlinear potential flow (FNPF) equations is used to simulate fully nonlinear wav e generation and propagation. A 3D Navier-Stokes model with large eddy simulation (LES) is coupled to the NWT to simulate complex turbulent flows near the ocean bottom or a round obstacles. Wave kinematics in the 2D-NWT thus forces flow simulations in the 3D-NS -LES model, and resulting sediment transport over the seabed and around a partially buried obstacle. The latter is calculated in a non-cohesive suspended load transport model simulating the (scalar) sediment concentration, using a constant settling velocity. The 2D NWT is based on a higher-order boundary element method (BEM), with explicit 2nd-order time stepping. The computational grid, thus, is the 2D-NWT boundary and the 3D-LES near-field domain. In the present new formulation, the total velocity and pressure fie lds are expressed as the sum of irrotational (incident/far-field) and near-field viscou s perturbations. The LES equations are formulated and solved for the perturbation fields only, w hich are forced by the incident fields computed in the NWT. The feasibility of coupling the mo dels in an efficient manner is demonstrated.

Tsunami Generation and Modelling

Numerical simulations are performed to develop tsunami inundation maps for the US east coast (USEC), as envelopes of surface elevations caused by the probable maximum tsunamis (PMTs) in the Atlantic Ocean basin. These PMTs are triggered by various sources, identified from historical records or hypothetical, including : (i) near-field submarine mass failures (SMF) on or near the continental shelf break; (ii) an extreme hypothetical M9 seismic event occurring in the Puerto Rico Trench; (iii) a repeat of the historical 1755 M9 (Lisbon) earthquake occurring in the Madeira Tore Rise; and (iv) large scale volcanic flank collapses (80 and 450 km 3 ) of the Cumbre Vieja Volcano (CVV) on La Palma, in the Canary Archipelago. Tsunamis caused by: (1) earthquakes, are obtained from the estimated coseismic seafloor deformation; (2) SMF sources, modeled as rigid slumps, are generated using the 3D non-hydrostatic model NHWAVE; and (iii) the CVV sources are modeled as subaerial flows of a heavy fluid, using a 3D Navier-Stokes model. For each source, tsunami propagation to the USEC is then modeled in a series of nested grids of increasingly fine resolution, by one-way coupling, using FUNWAVE-TVD, a nonlinear and dispersive (2D) Boussinesq model. High-resolution inundation maps have been developed based on these results, so far for about a third of the USEC. A comparison of coastal inundation from each tsunami source shows similar alongshore patterns of higher and lower inundation, whatever the initial source direction; this is due to wave focusing and defocusing effects induced by the shelf bathymetry. Once developed for the entire USEC, inundation maps will fully quantify coastal hazard from the selected PMTs and allow developing site-specific mitigation measures and evacuation plans. Besides maximuminundation, other “products” available at high-resolution are maximum momentum flux, currents, and vorticity, although these are not systematically developed as maps in this phase of work.

Comparison of Fully Nonlinear and Weakly Nonlinear Potential Flow Solvers for the Study of Wave Energy Converters Undergoing Large Amplitude Motions

We present a comparison between two distinct numerical codes dedicated to the study of wave energy converters. Both are developed by the authors, using a boundary element method with linear triangular elements. One model applies fully nonlin-ear boundary conditions in a numerical wavetank environnment (and thus referred later as NWT), whereas the second relies on a weak-scatterer approach in open-domain and can be considered a weakly nonlinear potential code (referred later as WSC). For the purposes of comparison, we limit our study to the forces on a heaving submerged sphere. Additional results for more realistic problem geometries will be presented at the conference. INTRODUCTION Among the marine renewable energy sources, wave energy is a promising option. Despite the great number of technologies that have been proposed, currently no wave energy converter (WEC) has proven its superiority over others and become a technological solution. Usual numerical tools for modeling and designing WECs are based on boundary elements methods in linear potential theory [1-4]. However WECs efficiency relies on large amplitude motions [5], with a design of their resonance frequencies in the wave excitation. Linear potential theory is thus inadequate to study the behavior of WEC in such configuration.