ERDC/CHL CHETN-I-64
September 2001
definition of the land-mass sheltering, and bathymetry. Generation models are based on a
statistical representation of waves using two-dimensional (frequency-direction) wave spectra.
Spectral wave models, also known as phase-averaged models, do not save information about the
relative phase of the spectral wave components because the phases are random (i.e., components
of the spectrum are not locked together as in cnoidal waves, which reinforce to give consistently
higher crest elevations because the wave crests are in phase). Generation-scale modeling has
evolved from empirical relationships (based on dimensional analysis) to solutions of the action
or energy balance equation. Jensen (1994) describes the evolution of spectral wave generation
models. The U.S. Army Engineer Research and Development Center (ERDC), Coastal and
Hydraulics Laboratory (CHL) presently uses the model WISWAVE (Hubertz 1992) for the WIS
hindcast because of the model's efficiency and accuracy. The WAM model (WAMDI
Group1988) is also used in some applications.
shoaling, breaking, and wind input dominate in intermediate water depths (depth less than
approximately 50 to 200 ft (15 to 60 m)), which is within a few miles to tens of miles from the
coast. Wave heights may increase or decrease in shallower depths due to wave refraction and
shoaling and wave directions refract to become more shore normal (wave crests parallel to
shore). In very shallow depths, waves break where the wave height is of the same order as the
water depth. To represent the bathymetry features that cause refraction, shoaling, and breaking,
transformation-scale grid resolution is of the order of 100 to 1,000 ft (30 to 300 m). Accurate
nearshore bathymetry is required. The input to calculate wave transformation is the output from
a wave generation model (e.g., WIS hindcast) or field wave measurements. CHL presently uses
the steady-state spectral wave model STWAVE (Smith, Sherlock, and Resio 2001) for nearshore
wave transformation applications.
Local Scale Modeling: In areas where wave properties change on a subwavelength scale, a
high-resolution local-scale model is required. These processes include reflection from
breakwaters and jetties, diffraction around coastal structures, and phase-dependent wave
nonlinearities (generation of harmonics and subharmonics), as well as refraction, shoaling, and
breaking. Grid domains on the local scale are generally small (on the order of a few miles or
less) because the models are computationally intensive and the processes are localized.
Numerical model grids must contain 8-10 grid cells per wavelength (resolution of tens of feet).
Input to calculate local-scale waves is typically output from a wave transformation model or field
wave measurements. Accurate bathymetry and structure configuration is also required. CHL is
presently developing the BOUSS-2D model (based on the Boussinesq equations) for local scale
modeling (Nwogu and Demirbilek 2001). The model CGWAVE (Demirbilek and Panchang
1998) is also applied at CHL for local-scale wave modeling and harbor resonance, but it does not
include wave nonlinearities.
DESCRIPTION OF STWAVE: The purpose of applying STWAVE is to quantify the change
in wave parameters (wave height, period, direction, and spectral shape) between the offshore,
where the wave field is fairly homogeneous on the scale of miles, and the nearshore, where
waves are strongly influenced by variations in bathymetry, water level, and current. Wave
parameters in the nearshore vary significantly on the scale of tens to hundreds of feet. Nearshore
wave information is required for the design of almost all coastal engineering projects. Waves
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