Spectral Wave Model Sensitivity Under Hurricane Wind Forcing Over a Narrow Shelf Steep-Reef Environment

Reefs are important coastal structures that serve as natural barriers that block and dissipate wave energy. Benthic maps estimate that coral reefs and colonized hard bottoms cover an area of 1056 km2  in Puerto Rico and the U.S. Virgin Islands (PRVI). This is equivalent to about 30% of the total land area of PRVI, indicating the significance of reef coverage in this region. Although reefs have been studied for their biological role in the coastal oceans in the region, there has been a lack of study regarding the physical role that reefs play as natural barriers and their importance for coastal protection during tropical cyclones. To address this, as part of the IOOS Coastal and Ocean Modeling Testbed (COMT) the impact of reef systems on the ADCIRC+SWAN computed waves, water surface elevations, coastal inundation, and currents is being considered. Two hurricanes were selected as test cases, Hurricane Georges (1998) which has been the most damaging hurricane in recent history, and Hurricane Irene (2011) for which an extensive dataset of wind, waves, and current observations exists obtained by the CARICOOS buoy and meteorological stations network. In this paper we present the results of a sensitivity test of numerical factors and physics parameterizations and their effects on model execution times and wave field transformation and dissipation over reefs.

Sensitivity to execution times was measured through varying the maximum number of SWAN solver iterations, percent of grid nodes required to achieve a given percent of numerical accuracy, and spectral frequency resolution. Due to the significant extension of reef coverage in the region, the physics parameterization sensitivity is based on the effect of wave-wave triad interactions over reef areas. Figures 1 and 2 show the differences in significant wave heights caused by wave-wave triad interactions at different time snaps during Hurricane Georges. Differences of more than 0.5 m can be seen at some areas of the reef crests as well as on the reef lagoons. Transects of 1D and 2D wave spectra across reef and shelf break areas are used to further determine the energy distribution caused by wave-wave triad interactions.

Results reiterate the importance of reefs and shallow sand banks as natural barriers that have a big role on the dissipation of wave energy under tropical cyclone forcing. Given the narrow spatial scales over which this energy dissipation occurs (less than 500 m across reef) detailed grids that capture both the complex geometry and provide enough grid points to abruptly transform and dissipate the wave field are necessary. The sensitivity test shows that processes such as wave-wave triad interactions need to be accounted for in narrow shelf steep-reef environments. In addition, these results could serve as an initial step towards quantifying the possible impact on coastal communities of climate change and sea level rise under additional stress and destruction of reef structures, such as an increase of coastal hazards and the economic cost of coastal infrastructure damage in Puerto Rico and the U.S. Virgin Islands due to loss of reef coverage.