Examining the Effects of Directional Wave Spectra on a Nearshore Wave Model

Author(s): Dillon, C.; Linhoss, A.; Jensen, R.; Smith, J.; Skarke, A.

Wave models are an integral part of coastal engineering due to their ability to quantitate information that is either unobtainable or unavailable. However, these models rely heavily on their inputs for accuracy. One critical input for nearshore models is the directional wave spectrum. The directional wave spectrum is the product of a frequency spectrum and a directional spreading function. There are many ways to compute the directional wave spectra depending on how either the frequency spectra or the spreading function is computed.

In this study, five methods for computing the directional wave spectrum were investigated. Using an offshore buoy, a Fast Fourier Transformation (FFT) of the time series of the buoy's heave generated a frequency spectra. Directional coefficients generated by the FFT were used to calculate three directional spreading functions: the maximum entropy method, the maximum likelihood method, and the Longuet-Higgins method. Using only the observed mean wave direction, the significant wave height, and the peak period from the offshore buoy, a frequency spectra was generated using the JONSWAP method, which applies a parametric shape based on the observed parameters. Since no FFT coefficients were created a cosine squared and a cosine 2s spreading function were used with the JONSWAP frequency spectra.

This study investigated how these five directional wave spectra perform within the nearshore spectral wave model, STWAVE. To accomplish this task, STWAVE was run five times in a half plane mode, meaning only wave propagation towards shore is retained. Each experimental run contained a different directional wave spectral computation, a bathymetry grid of 100 by 100 m resolution, a constant JONSWAP bottom friction value of 0.004, and spatially constant winds taken from the offshore buoy. No currents or changes in water level were included.

The results of the five experimental runs show that direction was the most affected parameter by the directional wave spectra input. Many differences observed between the five directional wave spectra occurred due to the differing placement of energy in the higher frequencies between the two frequency spectra methods, thus affecting where wave-bottom interaction begins. Another conclusion of this study, is that for the study’s environment, which was shallow and low energy, the wave-bottom interactions dictate the spectra in the nearshore. Thus, no matter the complexity of the directional wave spectra used as the model input, the wave-bottom interactions will tend to converge all spectra according to the limits of the bathymetry.

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