A new method of generating and analyzing directional spectral wave a recently has been implemented in the U.S. Army Engineer Waterways Experiment Station's Coastal Engineering Research Center's (CERC) directional spectral wave basin. The generation program simulates a stroke time series for each of the 61 independent paddles of the directional spectral wave generator using frequency domain methods and a "235" FFT algorithm. Either deterministic amplitude and random phase or random amplitude and phase options with a "TMA" or Ochi-Hubble spectral form and wrapped normal directional spreading function can be used. The analysis package allows processing of predicted or measured gage data using an eigen function procedure, Fourier series expansion of the directional spectrum, or Maximum Likelihood Methods (MLM). A description of the directional spectral wave generator and basin, directional spectral simulation and generation procedures, and subsequent analysis are presented. Finally, comparisons of predicted and measured results show excellent agreement.
Because of current economic conditions, the trend seems to be returning to shallower offshore waters in some areas for exploration of new domestic reserves. The offshore community will be interested in the transformation of wave energy in shallow and intermediate water depths due to shoaling, refraction, and diffraction on coastal structures such as pipeline crossings, ship moorings, artificial islands and breakwaters. CERC'S directional spectral wave basin provides a means to simulate these depth-limited ocean environments in a laboratory physical model. Directional spreading also can be included to more realistically simulate prototype wave conditons. Structures respond differently to directional waves than they do to unidirectional or monochromatic waves as movements and forces transverse to the main wave direction tend to reduce design forces and loads.
In this paper, a brief description of the directional spectral wave generator (DSWG) and basin is given. The modular design of the DSWG allows concurrent operation of modules from different locations and future expansion. A description of the directional spectral wave simulation and generation software is presented next. It incorporates a TMA depth-limited or Ochi-Hubble frequency spectrum and a wrapped normal spreading function. Unimodal or multimodal spectra in frequency or direction can be generated. Frequency domain methods are used to simulate realizations of stroke time series for each of the 61 DSWG paddles corresponding to the desired sea state. Next, the analysis package which allows processing of predicted or measured wave gage data using eigen function procedure, Fourier series expansion of the directional spectrum, or MLM is described. Finally, comparison of theoretical and measured wave properties for different TMA spectra and directional spreading combinations are presented.
The directional spectral wave basin is approximately 96 ft long by 121 ft wide (Figure 1). The maximum water depth is 2 ft. Wave absorbers (Briggs and Barnes, 1987) are installed along the basin perimeter and consist of 2-in. layers of rubberized horsehair installed between two layers of expanded sheet metal. The wave absorber slope is 37 deg along the basin front and side walls. The absorber slope is adjustable along the wall in the rear of the wave generator.