Search

As interest in the impact of sea-level rise on atoll islands increases, this study contributes to the growing database of observations of shoreline changes on South Pacific Islands, where few observations are currently available. Historical aerial photographs and recent satellite images were used to evaluate multidecadal surface area and shoreline changes on two atolls in French Polynesia: Manihi and Manuae. During the 40- to 50-year study period, atoll island surface area primarily increased or remained stable on Manihi and decreased on Manuae. Distinct ocean and lagoon shoreline changes were observed in different geographical regions of each atoll. On Manihi, ocean shoreline accretion rates were larger on the NW rim than the SE rim. On Manuae, atoll islands on the NE rim were eroding on the lagoon side and accreting on the ocean side, whereas islands on the SE rim showed the opposite trend. Sea-level rise is often thought to cause atoll erosion, but in this study, lagoon and ocean shorelines both eroded and accreted over a period when sea-level rise rates were greater than the global mean. Surface area changes related directly to anthropogenic activities were identified on only two of the 47 atoll islands. After completing a classification of the incident wave field, it was hypothesized that waves have an important role in controlling the shoreline change variability. Additional field surveys and in situ observations are needed to validate this hypothesis and to understand better island response to changing forcing factors. [ABSTRACT FROM AUTHOR]

Airborne light detecting and ranging (lidar) systems can survey hundreds of kilometers of shoreline with high spatial resolution (several elevation estimates per square meter). Sequential surveys yield spatial change maps of beach and dune sand levels. However, lidar data include elevations of the exposed, subaerial beach and, seaward of the waterline, the ocean surface. Here, a simple method is developed to find the waterline and eliminate returns from the ocean surface. A vertical elevation cutoff is used, with the waterline elevation (W) above the known tide level because of the superelevation from wave setup and runup. During each lidar pass, the elevation cutoff (W) is assumed proportional (C) to the offshore significant wave height Hs. Comparison of in situ and lidar surveys on a moderately sloped, dissipative California beach yields C ≈ 0.4, which is qualitatively consistent with existing observations of runup and setup. The calibrated method rejects ocean surface data, while retaining subaerial beach points more than 70 m seaward of the mean high waterline, which is often used as a conservative default waterline. [ABSTRACT FROM AUTHOR]

Two methods to treat wave breaking in the framework of the Hamiltonian formulation of free-surface potential flow are presented, tested, and validated. The first is an extension of Kennedy et al. (2000)'s eddy-viscosity approach originally developed for Boussinesq-type wave models. In this approach, an extra term, constructed to conserve the horizontal momentum for waves propagating over a flat bottom, is added in the dynamic free-surface condition. In the second method, a pressure distribution is introduced at the free surface that dissipates wave energy by analogy to a hydraulic jump (Guignard and Grilli, 2001). The modified Hamiltonian systems are implemented using the Hamiltonian Coupled-Mode Theory, in which the velocity potential is represented by a rapidly convergent vertical series expansion. Wave energy dissipation and conservation of horizontal momentum are verified numerically. Comparisons with experimental measurements are presented for the propagation of a breaking dispersive shock wave following a dam break, and then incident regular waves breaking on a mildly sloping beach and over a submerged bar. • Two wave breaking techniques are tested in a fully nonlinear model. • Breaking is implemented by extending the Hamiltonian Coupled-Mode Theory. • Energy dissipation and horizontal momentum conservation are verified numerically. • Good performance is shown in the presence of nonlinearity, dispersion and breaking. • Both methods can be implemented in other free-surface potential flow models. [ABSTRACT FROM AUTHOR]

Abstract With the objective of simulating wave propagation in the nearshore zone for engineering-scale applications, a two dimensional (2DV) model based on the Euler–Zakharov equations [73,54] is extended to three dimensions (3D). To maintain the flexibility of the approach with the goal of applying the model to irregularly shaped domains, the horizontal plane is discretized with scattered nodes. The horizontal derivatives are then estimated using the Radial Basis Function-Finite Difference (RBF-FD) method, while a spectral approach is used in the vertical dimension. A sensitivity analysis examined the robustness of the RBF-FD approach as a function of RBF parameters when estimating the derivatives of a representative function. For a targeted stencil size between 20 and 30 nodes, Piecewise-Smooth (PS) polyharmonic spline (PHS) functions are recommended, avoiding the use of Infinitely-Smooth (IS) RBFs, which are less appropriate for the desired applications because of their dependence on a shape parameter. Comparisons of simulation results to observations from two wave basin experiments show that nonlinear effects induced by complex bottom bathymetries are reproduced well by the model with the recommended RBF approach, validating the use of this method for 3D simulations of wave propagation. Highlights • A 3D combined spectral/RBF-FD method is developed for modeling highly nonlinear waves. • Sensitivity tests evaluate RBF parameters for a representative wave function. • Recommendations of the choice of RBF and its practical implementation are given. • Accurate results are shown for two wave basin experiments with complex bathymetries. [ABSTRACT FROM AUTHOR]

A new method is proposed for simulating the energy dissipation resulting from depth-limited wave breaking, in combination with a universal breaking onset criterion, in two-dimensional (2D) fully nonlinear potential flow (FNPF) models, based on a non-dimensional breaking strength parameter. Two different 2D-FNPF models are used, which solve the Laplace equation based on Chebyshev polynomial expansions or a boundary element method. In these models, impending breaking waves are detected in real time using a universal breaking onset criterion proposed in earlier work, based on the ratio of the horizontal particle velocity at the crest u , relative to the crest velocity c , B = u / c > 0. 85. For these waves, wave energy is dissipated locally with an absorbing surface pressure that is calibrated using an inverted hydraulic jump analogy. This approach is first validated for periodic spilling breakers over plane beaches and bars, for which results are shown to be in good agreement with experimental data. Recasting this breaking dissipation model in terms of a non-dimensional breaking strength, the hydraulic jump analog is shown to provide results similar to those of a constant breaking strength model, and to yield good agreement for periodic plunging breakers as well. The same approach is then applied to irregular waves shoaling over a submerged bar, and is shown to agree well with experimental data for the wave height, asymmetry, skewness, and kurtosis. Future work will extend this 2D breaker model to cases of three-dimensional (3D) breaking waves, simulated in existing 3D-FNPF models, in shallow or deep water conditions. • Two fully non-linear potential flow models are used for depth-limited breaking waves. • A new method with a constant non-dimensional breaking strength is proposed. • A unified onset criterion based on crest velocity is applied to detect wave breaking. • Two-dimensional results are validated against multiple experimental cases. • Good agreement is found for spilling and plunging, regular and irregular waves. [ABSTRACT FROM AUTHOR]

With the objective of modeling coastal wave dynamics taking into account nonlinear and dispersive effects, a highly accurate nonlinear potential flow model was developed. The model is based on the time evolution of two surface quantities: the free surface position and the free surface velocity potential. A spectral approach is used to resolve vertically the velocity potential in the domain, by decomposing the potential using an orthogonal basis of Chebyshev polynomials. With this approach, a wide range of relative water depths can be simulated, as demonstrated here with the propagation of nonlinear regular waves over a flat bottom with kh = 2 π and 4 π (where k is the wave number and h the water depth). The model is then validated by comparing the simulation results to experimental data for four non-breaking wave test cases: (1) nonlinear dynamics of a wave train generated by a piston-type wavemaker in constant water depth, (2) shoaling of a regular wave train on beach with constant slope up to the breaking point, (3) propagation of regular waves over a submerged bar, and (4) propagation of nonlinear irregular waves over a barred beach. The test cases show the ability of the model to reproduce well nonlinear wave interactions and the dynamics of higher-order bound and free harmonics. The simulation results agree well with the experimental data, confirming the model's ability to simulate accurately nonlinear and dispersive effects for non-breaking waves. [ABSTRACT FROM AUTHOR]