A two-dimensional hydrodynamic model for the St Lucia Estuary mouth.

Abstract

The reduced fresh water input into the St Lucia Estuary combined with the increase of sediment in the St Lucia Lake System has necessitated the implementation of a dredging programme. To ensure the effectiveness of the dredging programme, the behaviour of the sediment under various flow and tidal conditions needs to be determined. To establish how sediment will move, it is necessary to understand the hydrodynamics of the estuary. To achieve this, a hydrodynamic model which can be linked to a sediment transport model needs to be developed. Various existing types of hydrodynamic and sediment transport models are reviewed, to determine their suitability for the above purpose. Results of the analysis indicate that a two-dimensional hydrodynamic model is required. The two-dimensional hydrodynamic model developed is based on the momentum and continuity equations for an unsteady, non-uniform, free-surface flow for an incompressible fluid. The two dimensions are in the horizontal plane and flow is averaged over the depth. The equations are non-linear and are not decoupled, thus a numerical technique was needed to solve them. An Alternating Direction Implicit technique has been used. Boundary conditions in the modelled region were specified as flow velocity at the upstream boundary, and water levels, relative to the Mean Lake Level, at the downstream boundary. Two short simulations using hypothetical data were run on a 80826 IBM compatible. Results of the simulation indicate two areas where irregularities in the model output are a consequence of the use of hypothetical data in defining the boundary conditions. Recommendations for the collection of data in order to improve and calibrate the model are discussed.