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Fiedler, JW, Smit PB, Brodie KL, McNinch J, Guza RT.  2019.  The offshore boundary condition in surf zone modeling. Coastal Engineering. 143:12-20.   10.1016/j.coastaleng.2018.10.014   AbstractWebsite

Numerical models predicting surfzone waves and shoreline runup in field situations are often initialized with shoreward propagating (sea-swell, and infragravity) waves at an offshore boundary in 10-30 m water depth. We develop an offshore boundary condition, based on Fourier analysis of observations with co-located current and pressure sensors, that accounts for reflection and includes nonlinear phase-coupling. The performance of additional boundary conditions derived with limited or no infragravity observations are explored with the wave resolving, nonlinear model SWASH 1D. In some cases errors in the reduced boundary conditions (applied in 11 m depth) propagate shoreward, whereas in other cases errors are localized near the offshore boundary. Boundary conditions that can be implemented without infragravity observations (e.g. bound waves) do not accurately simulate infragravity waves across the surfzone, and could corrupt predictions of morphologic change. However, the bulk properties of infragravity waves in the inner surfzone and runup are predicted to be largely independent of ig offshore boundary conditions, and dominated by ig generation and dissipation.

Chen, YZ, Guza RT, Elgar S.  1997.  Modeling spectra of breaking surface waves in shallow water. Journal of Geophysical Research-Oceans. 102:25035-25046.   10.1029/97jc01565   AbstractWebsite

Predictions from Boussinesq-equation-based models for the evolution of breaking surface gravity waves in shallow water are compared with field and laboratory observations. In the majority of the 10 cases investigated, the observed spectral evolution across the surf zone is modeled more accurately by a dissipation that increases at high frequency than by a frequency-independent dissipation. However, in each case the predicted spectra are qualitatively accurate for a wide range of frequency-dependent dissipations, apparently because preferential reduction of high-frequency energy (by dissipation that increases with increasing frequency) is largely compensated by increased nonlinear energy transfers to high frequencies. In contrast to the insensitivity of predicted spectral levels, model predictions of skewness and asymmetry (statistical measures of the wave shapes) are sensitive to the frequency dependence of the dissipation. The observed spatial evolution of skewness and asymmetry is predicted qualitatively well by the model with frequency-dependent dissipation, but ij predicted poorly with frequency-independent dissipation. Although the extension of the Boussinesq equations to breaking waves is ad hoc, a dissipation depending on the frequency squared (as previously suggested) reproduces well the observed evolution of wave frequency spectra, skewness, and asymmetry.

Raubenheimer, B, Guza RT, Elgar S.  1996.  Wave transformation across the inner surf zone. Journal of Geophysical Research-Oceans. 101:25589-25597.   10.1029/96jc02433   AbstractWebsite

Sea and swell wave heights observed on transects crossing the mid and inner surf zone on three beaches (a steep concave-up beach, a gently sloped approximately planar beach, and a beach with an approximately flat terrace adjacent to a steep foreshore) were depth limited (i.e., approximately independent of the offshore wave height), consistent with previous observations. The wave evolution is well predicted by a numerical model based on the one-dimensional nonlinear shallow water equations with bore dissipation. The model is initialized with the time series of sea surface elevation and cross-shore current observed at the most offshore sensors (located about 50 to 120 m from the mean shoreline in mean water depths 0.80 to 2.10 m). The model accurately predicts the cross-shore variation of energy at both infragravity (nominally 0.004 < f less than or equal to 0.05 Hz),nd sea swell (here 0.05 < f less than or equal to 0.18 Hz) frequencies. In models of surf zone hydrodynamics, wave-energy dissipation is frequently parameterized in terms of gamma(s), the ratio of the sea swell significant wave height to the local mean water depth. The observed and predicted values of gamma(s) increase with increasing beach slope beta and decreasing normalized (by a characteristic wavenumber k) water depth kh and are well correlated with beta/kh, a measure of the fractional change in water depth over a wavelength. Errors in the predicted individual values of gamma(s) are typically less than 20%. It has been suggested that infragravity motions affect waves in the sea swell band and hence gamma(s), but this speculation is difficult to test with field observations. Numerical simulations suggest that for the range of conditions considered here, gamma(s) is insensitive to infragravity energy levels.

Raubenheimer, B, Guza RT, Elgar S, Kobayashi N.  1995.  Swash on a Gently Sloping Beach. Journal of Geophysical Research-Oceans. 100:8751-8760.   10.1029/95jc00232   AbstractWebsite

Waves observed in the inner surf and swash zones of a fine grained, gently sloping beach are modeled accurately with the nonlinear shallow water equations. The model is initialized with observations from pressure and current sensors collocated about 50 m from the mean shoreline in about 1 m depth, and model predictions are compared to pressure fluctuations measured at five shoreward locations and to run-up. Run-up was measured with a vertical stack of five wires supported parallel to and above the beach face at elevations of 5, 10, 15, 20, and 25 cm. Each 60-m-long run-up wire yields time series of the most shoreward location where the water depth exceeds the wire elevation. As noted previously, run-up measurements are sensitive to the wire elevation owing to thin run-up tongues not measured by the more elevated wires. As the wire elevation increases, the measured mean run-up location moves seaward, low-frequency (infragravity) energy decreases, and higher-frequency sea swell energy increases. These trends, as well as the variation of wave spectra and shapes (e.g., wave skewness) across the inner surf zone, are well predicted by the numerical model.