Fengyan Shi

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.

A fully nonlinear Boussinesq model in generalized curvilinear coordinates

Abstract Based on the fully nonlinear Boussinesq equations in Cartesian coordinates, the equations in generalized coordinates are derived to adapt computations to irregularly shaped shorelines, such as harbors, bays and tidal inlets, and to make computations more efficient in large near-shore regions. Contravariant components of velocity vectors are employed in the derivation instead of the normal components in curvilinear coordinates or original components in Cartesian coordinates, which greatly simplifies the equations in generalized curvilinear coordinates. A high-order finite difference scheme with staggered grids in the image domain is adopted in the numerical model. The model is applied to five examples involving curvilinear coordinate systems. The results of these cases are in good agreement with analytical results, experimental data, and the results from the uniform grid model, which shows that the model has good accuracy and efficiency in dealing with the computations of nonlinear surface gravity waves in domains with complicated geometries.

A Curvilinear Version of a Quasi-3D Nearshore Circulation Model

A curvilinear version of the nearshore circulation model SHORECIRC is developed based on the quasi-3D nearshore circulation equations derived by Putrevu and Svendsen [Eur. J. Mech. 18 (1999) 409-427]. We use a generalized coordinate transformation and re-derive the equations in tensor-invariant forms. The contravariant component technique is used to simplify both the transformed equations and boundary conditions. A high-order finite-difference scheme with a staggered grid in the image domain is adopted for the numerical model. Very good convergence rates with both grid refinement and time refinement are obtained in a simple convergence test. The model is then applied to four cases involving either a non-orthogonal quadrilateral grid or a generalized curvilinear grid. The versatility of the curvilinear model in dealing with curved shorelines, nearshore breakwaters and other complicated geometries is demonstrated in the test cases. The accuracy of the model is shown in the paper through model/data comparisons in two of the case studies.

Tide-surge Interaction Intensified by the Taiwan Strait

Science Foundation of Fujian Province [2009J01223]; National High-tech RD Program [2006AA09A302-6]; National Oceanographic Partnership Program [N00014-06-1-0945]

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.

Hydrodynamic and sediment transport modeling of New River Inlet (NC) under the interaction of tides and waves

The interactions between waves, tidal currents, and bathymetry near New River Inlet, NC, USA are investigated to understand the effects on the resulting hydrodynamics and sediment transport. A quasi-3-D nearshore community model, NearCoM-TVD, is used in this integrated observational and modeling study. The model is validated with observations of waves and currents at 30 locations, including in a recently dredged navigation channel and a shallower channel, and on the ebb tidal delta, for a range of flow and offshore wave conditions during May 2012. In the channels, model skills for flow velocity and wave height are high. Near the ebb tidal delta, the model reproduces the observed rapid onshore (offshore) decay of wave heights (current velocities). Model results reveal that this sharp transition coincides with the location of the breaker zone over the ebb tidal delta, which is modulated by semidiurnal tides and by wave intensity. The modulation of wave heights is primarily owing to depth changes rather than direct wave-current interaction. The modeled tidally averaged residual flow patterns show that waves play an important role in generating vortices and landward-directed currents near the inlet entrance. Numerical experiments suggest that these flow patterns are associated with the channel-shoal bathymetry near the inlet, similar to the generation of rip currents. Consistent with other inlet studies, model results suggest that tidal currents drive sediment fluxes in the channels, but that sediment fluxes on the ebb tidal delta are driven primarily by waves.

Modeling coastal tsunami hazard from submarine mass failures: effect of slide rheology, experimental validation, and case studies off the US East Coast

We perform numerical simulations to assess how coastal tsunami hazard from submarine mass failures (SMFs) is affected by slide kinematics and rheology. Two types of two-layer SMF tsunami generation models are used, in which the bottom (slide) layer is depth-integrated and represented by either a dense Newtonian fluid or a granular flow, in which inter-granular stresses are governed by Coulomb friction (Savage and Hutter model). In both cases, the upper (water) layer flow is simulated with the non-hydrostatic 3D σ-layer model NHWAVE. Both models are validated by simulating laboratory experiments for SMFs made of glass beads moving down a steep plane slope. In those, we assess the convergence of results (i.e., SMF motion and surface wave generation) with model parameters and their sensitivity to slide parameters (i.e., viscosity, bottom friction, and initial submergence). The historical Currituck SMF is simulated with the viscous slide model, to estimate relevant parameters for simulating tsunami generation from a possible SMF sited near the Hudson River Canyon. Compared to a rigid slump, we find that deforming SMFs of various rheology, despite having a slightly larger initial acceleration, generate a smaller tsunami due to their spreading and thinning out during motion, which gradually makes them less tsunamigenic; the latter behavior is controlled by slide rheology. Coastal tsunami hazard is finally assessed by performing tsunami simulations with the Boussinesq long wave model FUNWAVE-TVD, initialized by SMF tsunami sources, in nested grids of increasing resolution. While initial tsunami elevations are very large (up to 25 m for the rigid slump), nearshore tsunami elevations are significantly reduced in all cases (to a maximum of 6.5 m). However, at most nearshore locations, surface elevations obtained assuming a rigid slump are up to a factor of 2 larger than those obtained for deforming slides. We conclude that modeling SMFs as rigid slumps provides a conservative estimate of coastal tsunami hazard while using a more realistic rheology, in general, reduces coastal tsunami impact.

Numerical Simulation of Nearshore Hydrodynamics and Sediment Transport Downdrift of a Tidal Inlet

AbstractNearshore hydrodynamics and sediment transport patterns induced by waves and tide adjacent to a structured tidal inlet with complex bathymetry are investigated to determine the potential causes of downdrift beach erosion. A coupled wave and hydrodynamic model is used to simulate the nearshore hydrodynamics and morphodynamics. Near the inlet, the tidal-induced pressure gradient dominates the wave radiation stress gradient only in the first half of the flood duration. The nearshore hydrodynamic pattern for the rest of the tidal cycle is driven mainly by the wave-driven pressure gradient. The wave-driven pressure gradient results from alongshore variation of water surface elevation induced by nearshore wave focal points caused by wave refraction over irregular bathymetry (with ebb tidal shoals and nonparallel shoreline depth contours). The resulting alongshore sediment transport patterns suggest that the direction of the time-averaged alongshore sediment transport rate near the inlet and at the downd...

MODELING OF SURFZONE BUBBLES USING A MULTIPHASE VOF MODEL

The ability to make optically-based observations in the surf-zone is strongly influenced by the presence of suspended sediment particles and of air bubbles, both of which are present due to the action of breaking waves. Wave breaking is instrumental in injecting large volumes of air into the water column. This air volume subsequently evolves into a distribution of bubble sizes that interact with the fluid turbulence and are advected by the organized flow. The bubble population in the surf-zone is intensified due to the greater intensity of breaking processes, leading to increase in turbulence intensity and associated energy dissipation. The bubble sizes are also affected by the densely sedimented flows that could alter the relationship between turbulent perturbing forces and surface tension-based restoring forces leading to the determination of critical bubble diameters.

Quasi-3D Nearshore Circulation Equations: a CL-Vortex Force Formulation

INTRODUCTION Recently much attention has been paid to different formulations of surface wave force in wave-driven ocean and coastal circulations (e.g., McWilliams et al., 2004, Mellor, 2003, Smith, 2006 and others). Basically, the analytical expressions for surface wave force and wave-current interaction can be classified into two types. One is the classical wave ‘radiation stress’ concept presented by Longuet-Higgins and Stewart (1962, 1964) and many others in depth-integrated and short wave-averaged equations. Mellor (2003) recently used the same concept to derive short wave-averaged 3-D equations with a depth-dependent wave-induced force. A direct application of this kind of depth-dependent wave-induced force was conducted by Xia et al. (2004) who related the vertical variation of current to the vertical structure of radiation stresses. Another type of wave force is the surface wave force initially derived by Garrett (1976) in the study of Langmuir circulation generation. The wave driving forces include the wave dissipation term and the wave-averaged vortex forcing term which has been identified later by Leibovich (1980) and Smith (1980) as the vertically integrated form of the ’CL vortex-force’ derived by Craik and Leibovich (1976). Dingemans et al. (1987) also presented a similar formulation of this type of wave driving force, though the current refraction, that may result in the vortex-force term, was recognized to give insignificant contributions under the conditions of slowly varying wave fields. Smith (2006) extended the formulation of Garrett (1976) to include finite-depth effects and provided some insight into physical interpretation of each forcing term in depth-integrated equations. A