Hydrodynamic Simulations in Support of Beach Water Quality Assessments at a Tropical Beach with Fringing Reefs

High bacteria levels are a recurrent concern at Dorado Public Beach, a sandy beach protected by fringing reefs, located approximately 20 miles West of San Juan, Puerto Rico. Currently, assessing fecal indicator bacteria levels at local beaches in Puerto Rico requires tedious in-situ measurements and lengthy processing times. This process delays the response of coastal managers, potentially putting beachgoers' health and safety at risk. We performed numerical simulations of the nearshore hydrodynamics and transport of Enterococci bacteria to provide a more timely assessment of water quality at the beach. Three sets of oceanographic and meteorological forcing conditions were selected to drive the model in order to represent the most typical conditions at the site. The numerical simulations were performed with the Delft3D 4 modeling suite. The hydrodynamic and wave modules were coupled to simulate the flow conditions and evaluate how wave generation would transport the bacteria within the nearshore and onto the beach. Depth-averaged simulations were set up for this investigation with boundary-fitted curvilinear grids having a finer resolution in the area of interest. Bacteria input were modeled as a point source, with subsequent transport as a passive tracer by the resolved hydrodynamic flows and waves. The hydrodynamic component was validated with in-situ measurements from two acoustic Doppler current profilers, one located near the point source of the bacteria, and the other further offshore. Additionally, the numerical simulations were compared with results from SWAN being run operationally by IOOS-CARICOOS.