A Three Dimensional Numerical Model System of Coastal Circulation and Water Quality

A three dimensional numerical model system has been developed for coastal circulation and water quality applications. The model system is composed of a hydrodynamic model component and a pollutant transport model component. The hydrodynamic model solves the conservation of momentum, water mass, salt, and heat on a space staggered grid system. The solution technique employs a split mode semi-implicit algorithm for both the exterior vertically averaged flow and the interior vertical structure. This technique eases the time step restrictions typical of explicit algorithms yet is more efficient than fully implicit algorithms. The model has been tested against analytic solutions for a standing wave in a closed basin, tidal and wind flow in a basin with variable depth, wind flow in a closed basin, and density forcing in a channel. Simulations show good agreement with the analytic solutions. A companion pollutant transport model has also been developed. This model, in a vertically averaged mode, was applied to upper Narragansett Bay to predict levels of fecal coliform under various pollutant loadings. The required hydrodynamic data was model generated from the M2 and M4 tides plus mean river flows emptying into the area. Comparison of model tidal response with observations was good. A parametric study of dispersion rate, and source strength was undertaken to calibrate the model to known levels of fecal coliform in upper Narragansett Bay. Application included simulation of two storms of varying intensity under conditions of no combined sewer overflow (CSO) control and full CSO control. A third storm of larger intensity was used to examine a range of CSO controls to establish their relative benefit in terms of reduced fecal coliform concentration levels. An extension of the basic model approach was developed using boundary fitted coordinates as opposed to a square finite difference grid. This technique uses a transformation such that all boundaries are coincident with coordinate lines. The solution methodology is the same as the square grid model. The model has been tested in a two dimensional mode against analytic solutions for a standing wave in a closed basin, tidal flow in a annular basin with constant depth and wind driven flow in an elliptic cylindrical basin. Three dimensional tests of the code include wind and density forcing in a rectangular basin. All simulations show excellent agreement with the analytic solutions