Journal of Waterway, Port, Coastal, and Ocean Engineering

This paper reports a numerical simulation of tsunami propagation for the 1993 Southwest Hokkaido earthquake tsunami. The model is based on the Boussinesq equation, which includes the effects of frequency dispersion. Energy dissipation due to breaking at the tsunami wave front is modeled. The validity of the model was tested with the existing laboratory data of dispersive wave trains breaking on a slope. The model was then applied to the simulation of the 1993 Southwest Hokkaido earthquake tsunami around the southern part of Okushiri Island. Comparison with the physical model demonstrates that it is the dispersion of the wave front which caused focusing of the wave energy at the narrow region on the lee side of the island, consequently increasing the tsunami height.

Estimates of significant wave heights made from finite record lengths have a small but important statistical sampling variability. This variability leads to positive biases in the maximum significant wave height in a storm and in design wave heights estimated from extreme value distributions fitted to the data. Since wave hindcasts are often verified by comparing the maximum hindcast and measured significant wave heights in a storm, the bias due to sampling variability can make a correct hindcast appear to be too low. In this paper we show how the magnitude of the biases can be calculated from the sample length and the shape of the wave spectrum. These theoretical calculations agree very well with results from several sets of measurements in the North Sea. We recommend that new wave measurements be recorded continuously whenever possible, and that hindcast verifications and extreme wave calculations be made with smoothed versions of the measured time series of significant wave height.

Prediction of tide levels is one of the important problems in determining the schedule making of activities or constructions in coastal and oceanic area. Accurate predictions of tide levels could not be obtained without a long length (about one month or more) of tide measurements by conventional methods. A method of using a short length (a few days) of tide measurements to predict tide levels over a longer time is proposed in the present paper. In the method, a harmonic tide level model is used to predict tide levels. The parameters of the tide level model, that is, the amplitudes of the harmonic components, are estimated by Kalman filtering technique using a few-day tide record with the assumption of known angular frequencies. The proposed model is then applied to the short-term tide level prediction of the Kaohsiung Harbor, Taiwan. As a result, the tide levels predicted by the proposed method are in good agreement with the observed data.

Extensive field observations are used to characterize seiches (periods 0.5–30 min) in three small harbors with similar surface areas (∼1 km ² ), water depths (5–12 m), and swell wave climates. On the continental shelf just offshore of each harbor mouth, the energy levels of waves in the infragravity frequency band 0.002– 0.03 Hz (periods 0.5–10 min) vary by more than a factor of 200 in response to comparably large variations in swell energy levels. Energy levels in this swell-driven frequency band also vary (less dramatically) in response to changes in the swell frequency and with tidal stage. Motions at longer seiche periods (10–30 min) are primarily driven by meteorological and other processes (a tsunami-generated seiche is described). As has often been observed, the amplification of seiche energy within each harbor basin (relative to energy in the same frequency band outside the harbor) varies as a function of seiche frequency, and is largest at the frequency of the lowest resonant harbor mode (i.e., the Helmholtz or grave mode). At all three harbors, the average amplification of the grave mode decreases (by at least a factor of 2) with increasing seiche energy, a trend consistent with a nonlinear dissipation mechanism such as flow separation in the harbor mouth or sidewall and bottom friction.

This study describes a field experiment on wind waves in the Gulf Intracoastal Waterway. Wind-wave height and period were measured at the Aransas National Wildlife Refuge in Texas and compared to hindcast results generated by the wind-wave model for restricted fetches in the Automated Coastal Engineering System. The comparisons showed that the present technology did not predict the measurements well. The restricted fetch method showed a tendency to overpredict waves when the wind was blowing along the channel and underpredict waves when the wind was blowing across the channel. The poor performance of the restricted fetch model in long, narrow channels was attributed to the measured data being far outside the bounds of the data used to develop the model (fetch width-to-length ratio, as well as wave height and period). It was shown that wave damping could reduce wave heights for along-channel winds. Analysis of an elliptical fetch suggested that fetch width-to-length ratio is an important variable. Because this parameter is not included in the wave-prediction equations, the formulas should only be applied to water bodies similar in width-to-length ratio to those used to develop the model.

The numerical wave model HARBD has been used extensively within and outside the U.S. Army Corps of Engineers for estimating waves in harbors. The model was originally developed for long waves (harbor oscillations). It has been extended for use with short waves (wind waves and swell) and validated by several investigations, most notably Chen's (1986) fundamental investigation of long waves in a rectangular harbor. This study focuses on short-wave validation. The model is tested with several simple domains, in most cases with uniform depth. It is validated with laboratory data for irregular wave diffraction around a semi-infinite breakwater. Both broad and narrow directional spreads are considered. The model is also compared to widely accepted curves for regular and irregular wave diffraction through a breakwater gap. Field validation of the model for Kaumalapau Harbor, Lanai, Hawaii, is discussed. Tests demonstrate the effect of input parameters on model results and provide guidelines about the choice of parameter values in future modeling efforts.

Shoreline change models such as GENESIS use refracted wave data identified at nearshore water depths that are seaward of the breaking limit for the largest waves in the model's ensemble. Adjustment of the manner in which wave data are input to the model can potentially improve the physical reality of the model's predictions. This paper presents a method whereby incipient breaking wave conditions are computed for each refracted wave case and then transformed offshore to a given reference water depth for subsequent input to the shoreline change model. This allows for consideration of wave transformations that occur between the reference water depth and the point of incipient breaking. This adjustment is shown to be of particular importance in those cases where the nearshore bathymetry is irregular.