NUMERICAL SIMULATION OF THE THREES
To reliably or convincingly model this complicated hydrodynamic system, it was
necessary to determine what conditions would cause Threes to break. Therefore, one of
the authors (Buonaiuto) conducted water-borne field observations at Shinnecock Inlet,
serendipitously capturing the famous Threes being enjoyed by more than a dozen surfers
and one dedicated researcher on July 23 and 24, 2003. The conditions that promoted the
breaking of Threes were obtained from a moored buoy located 30 nautical miles off of
Fire Island Inlet (National Data Buoy Center Station 44025). The station indicated
heights ranged from 1.9 to 2.3 m, periods were between 7 and 9 sec, and that the waves
approached from 170 to 192 deg clockwise from the north. The wind direction at the
buoy during this period was from the south-southwest and ranged from 190 to 210 deg.
Simulation Model CGWAVE
Waves propagating through this jettied inlet are influenced by wave reflection and
diffraction. Numerical representation of waves inside this inlet requires a model capable
of describing the variation in wave field in a confined inlet. A review of various types of
wave prediction models used in coastal engineering applications is provided by
Panchang, Xu, and Demirbilek (1999). It is generally agreed that models based on mild-
slope equation (MSE) and/or Boussinesq equations are best suited for modeling waves at
jettied inlets. These classes of wave models are based on the conservation of mass and
momentum equations and are most widely applied for predicting the transformation of
waves in shallow water under influence of complex coastal bathymetry and
configurations of protective structures such as jetties and breakwaters. The finite element
CGWAVE model (Demirbilek and Panchang 1998; Panchang and Demirbilek 2001,
2002; Panchang et al. 2000; Xu, Panachang, and Demirbilek 1996) is such a model, and it
was established at Shinnecock Inlet to examine the conditions necessary for the presence
of Threes.
CGWAVE can simulate regular or random waves by solving the combined MSE
elliptic refraction-diffraction equation. The model is applicable to both long and short
waves. The governing equations represent wave shoaling, refraction, diffraction,
reflection, wave breaking, and dissipation processes in all water depths. Being elliptic,
the model solves a boundary value problem that can accommodate internal non-
homogeneities and boundaries. It therefore forms a well-accepted basis for performing
wave simulations in coastal regions with arbitrarily shaped (engineered or natural)
boundaries and arbitrary depth variations without limitations on the angle of incidence or
the degree and direction of wave reflection and scattering that can be modeled. Irregular
wave conditions are represented by superposition of regular (monochromatic) wave
simulations (e.g., Demirbilek and Panchang 1998; Demirbilek, Xu, and Panchang 1996;
Chawla et al. 1998; Zhao et al. 2001). CGWAVE calculates for a triangular finite-
element formulation with grid sizes varying throughout the modeling domain based on
the local wavelength. The model allows one to specify the desired reflection properties
along the coastlines and other internal boundaries. The model is implemented in the U.S.
Army Corps of Engineers' Surface-water Modeling System (SMS) with automated pre-
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