Velocity skew controls the flushing of a tracer in a system of shallow bays with multiple inlets

The exchange of dissolved constituents between a shallow bay and the ocean is governed by regular tidal fluxes as well as by wind generated storm surges and currents. These mechanisms regulate the rate at which pollutants and nutrients are removed from the lagoons. Here we determine the main hydrodynamic drivers controlling the removal of a conservative tracer from a bay with multiple inlets. The transport of the tracer is simulated using the numerical model Delft3D in a system of shallow lagoons along the coast of the Delmarva Peninsula, Virginia, USA. The tracer flushing time is evaluated using the Eulerian approach and the decay of the tracer concentration in time is approximated with an exponential curve. We assess the influence of tidal amplitude, local winds, and time of release of the tracer with respect to the tidal cycle on flushing time. Results show that wind-driven fluxes are a prevailing factor controlling the tracer transport and, therefore, the tracer concentration within the lagoons. Variations in tidal phase and amplitude along the inner shelf also promote the flushing of the tracer out of the bays, while the time of tracer release with respect to the tidal phase has been found to play a relatively negligible role. The tracer flushing time is proportional to a velocity skew index, accounting for the asymmetry of the ebb-flood velocities at the inlets, while the tidal prism has minimal effect on flushing time. Our simulations revealed that the average flushing time of these bays is around 24-27 days, decreasing to 21 days if favorable wind conditions exist. Finally, a simplified approach is presented to compute the decay of tracer concentration in time as a function of hourly variable wind characteristics as well as seasonal changes in meteorological conditions, without the need of large scale simulations.