Abstract
Kaneohe Bay, located on the windward (northeastern) side of the island of Oahu, Hawaii, presently has a shallow water (30 m) wave energy test berth and is under consideration to develop up to two additional berths in deeper waters (60 m ‐ 70 m) potentially making it the location of the first full scale wave energy test site (WETS) in the United States (Figure 1). One objective of the WETS is to provide a location that contains all necessary in‐water and land‐side infrastructure to support simple connection of up to three wave energy conversion (WEC) devices for testing purposes. To support the site‐selection process, it is necessary to determine the anticipated incident wave climate on the study site, as well as the effects of the WEC on the propagation of waves into shore. As such, a numerical model was developed in order to better comprehend both the existing condition (i.e. no WEC device) wave conditions and those that may be present when a WEC device (or WEC array) is installed. Specific concerns include, but are not limited to, impacts of the WEC device(s) on the near‐shore recreational surf climate as well as resultant shoreline erosion.
As deepwater waves approach the coast, they are transformed by certain processes including refraction (as they pass over changing bottom contours), diffraction (as they propagate around objects such as headlands), shoaling (as the depth decreases), and ultimately, energy dissipation (due to bottom friction and by breaking). The propagation of deepwater waves into the study site was modeled using the open‐source program, SWAN (Simulating WAves Nearshore), developed by Delft Hydraulics Laboratory. SWAN has the capability of modeling all of the aforementioned processes in shallow coastal waters.
The SWAN model is a non‐stationary (non‐steady state) third generation wave model based on the discrete spectral action balance equation. SWAN is fully spectral over the total range of wave frequencies. Wave propagation is based on linear wave theory, including the effect of wave generated currents. The processes of wind generation, dissipation, and nonlinear wave‐wave interactions are represented explicitly with state‐of‐the‐science, third‐generation formulations. SWAN provides many output quantities including, but not limited to, two dimensional spectra, significant wave height (Hs), wave period (mean and peak, Tp), wave direction (peak and mean, MWD), and directional spreading. The SWAN model has been successfully validated and verified in laboratory and complex field cases. Sandia National Labs and Sea Engineering, Inc. (SEI) have validated the model at nearby Waimanalo Bay as well as several locations on the mainland United States (e.g. Santa Cruz Bight, Monterey Bay, and Humboldt Bay, California).