A nearshore breaker prediction system for Puerto Rico and the United States Virgin Islands in support of beach safety and drowning prevention

More than half of Puerto Rico´s population does not know how to swim and, usually, there are no lifeguards at all on Puerto Ricos beaches. In addition, Puerto Rico has a strongly seasonal cycle in wave heights, and marine conditions can change very quickly as intense swell events arrive. Intense surfzone currents caused by wave-induced pressure gradients pose a threat to beachgoers at hundreds of beaches throughout the region. These are the main facts that account for an average of 25 beach drownings per year. The present study describes the development and implementation of a nearshore breaker prediction system for the PR/USVI region. A high-resolution operational wave model, the CariCOOS Nearshore Wave Model, has been developed in order to adequately resolve wave transformation across the PR/USVI archipelago. The performance of the model for offshore and coastal locations was validated using wave observations from four CariCOOS buoys, with very good results. After ensuring that the model performed satisfactorily at the CariCOOS buoy locations, a field validation experiment was conducted to evaluate model performance in the nearshore region and within the surfzone. A field experiment involving bottom-mounted wave sensors was carried out at the Tres Palmas Marine Reserve in Rincn, Puerto Rico in order to evaluate the performance of one of the high-resolution nested grids. The model was shown to be able to resolve wave transformation with very good results up to just seaward of the surf zone. This shows that the model is able to resolve wave transformation including shoaling, diffraction and refraction at this location. After model validation, a simple method was developed to estimate nearshore breaker heights based on the model output at strategically placed virtual buoys. Each virtual buoy was located just outside the surfzone for each beach, so that the available wave energy flux just seaward of the surfzone could be estimated from the SWAN output. Based on this available wave energy flux, an estimate of the maximum expected breaker heights was obtained using a semi-empirical formulation for breaker heights. While a rigorous validation of breaker heights is very difficult due to the complexity of measuring breaker heights in the surfzone, the breaker height predictions were compared to visually estimated breaker heights at a beach in Rincόn, Puerto Rico. While the results of this comparison should be taken with caution, given the subjective nature of visually estimated breaker heights, the results suggest that in general the breaker model can estimate the arrival of high breaker events at this beach with reasonable accuracy. Following this methodology for estimating the range of expected breaker heights, a custom webpage was created in which the range of expected breaker heights as a function of time are provided via a nearshore virtual buoy system at 84 beaches throughout the PR/USVI archipelago.