Talk abstract:
Water quality issues have been a major concern in the San Diego-Tijuana region stemming in part from an inoperative wastewater treatment plant near Punta Bandera, Mexico that drains untreated sewage directly into the surf. From there, the wastewater can easily travel up the mostly straight, sandy San Diego-Tijuana coastline, carried by wave-driven currents. The goal of this project is to build a model that can predict the concentrations of wastewater along the shoreline through a tracer transport model that uses currents estimated from wave properties at a wave buoy. A simple, fast model built on publicly available datasets could inform beach closures and prevent illness in beachgoers. The model is a one-dimensional finite difference shoreline model that uses an upwind-advection scheme for tracer transport and an exponential decay term to parameterize offshore diffusion and pathogen decay. To test the performance of the one-dimensional model, a three-dimensional ocean-wave coupled model of the San Diego-Tijuana coastal ocean was used. The one-dimensional shoreline model reproduces the distribution of tracer concentration along the coastline from the three-dimensional ocean-wave model, with an average correlation coefficient > 0.5 at all coastline locations. The one-dimensional model correctly identifies the presence of high dye concentrations at 89% of time steps. In addition to being a potential tool for public health, the one-dimensional shoreline model provides insight into alongshore transport of tracers. For example, the Tijuana River estuary mouth induces a small but significant drop in the correlation between the one-dimensional model results and the ocean-wave model. This demonstrates the need for further research into the effect of coastline features such as low-flow estuaries and inlets on alongshore transport of pollutants and other tracers.