Tidal hydrodynamics and their implications for the dispersion of effluents in Mazatlan harbor: An urbanized shallow coastal lagoon

Abstract

In order to investigate the Urias Coastal Lagoon (UCL) hydrodynamics, a vertically integrated semi-implicit, non-linear, finite difference model, has been applied. The flow dynamics in this model has been described by the depth integrated shallow water equations and has been forced by prescribed free surface elevations at the open boundary in the inlet of the lagoon. The predicted instantaneous tidal elevation and the vector field of tidal velocities, reflect reasonably well the flood and ebb conditions in the coastal lagoon. Maximum tidal velocities of 0.6 m/s at the navigation channel of the lagoon and tidal ranges of 1.2 m were predicted for spring tides. Residual current of 0.01-0.06 m/s have also been predicted. The advection-diffusion process of a hypothetical pollutant released at two discrete points in the UCL depended on the intensity of water circulation

sites with slow instantaneous tidal velocities and residual currents of small magnitude presented slow advection and diffusion of the pollutant and may be considered vulnerable to the contamination, specifically the head of the lagoon where the pollutant was difficult to be removed by the tidal currents. The main channel, where the tidal currents exceed 0.6 m/s and the residual currents reached 0.06 m/s, behaved as a natural conduct for the pollutant motion. The forces involved in water circulation within the channel would be the best driving mechanism to flush contaminants from the UCL into the Ocean.