The Influence of Sediment Load on Tidal Dynamics, a Case Study: Cádiz Bay

A two-dimensional, non-linear, finite-difference, hydrodynamic model was applied to Cadiz Bay to study the influence of sediment load on tidal dynamics. The sediment load effect is represented parametrically as a dependence of the drag coefficient on the relative settling velocity (the ratio of the settling velocity of suspended particles to the bottom friction velocity) and the relative friction velocity (the ratio of the bottom friction velocity to its critical value at which sediment particles begin to go into suspension). This dependence is derived from a solution of the equations describing the vertical structure of the sediment-stratified bottom logarithmic layer. A comparison of the model predictions with and without allowance for the sediment load effect shows that the latter is responsible for small local changes of the amplitude and phase of tidal elevation and the maximum depth-averaged tidal velocity, the result counting in favour of the conventional approach whereby the influence of sediment load on tidal dynamics is considered to be negligible. However, it is apparent after close inspection of the model predictions that the sediment load effect tends to enhance the time-space variability of the tidal characteristics. In particular, it results in an increase in the maximum depth-averaged velocity and a decrease in the drag coefficient for the periods of flood and ebb currents, thus reducing the shear bottom stress and the tidal energy dissipation by about half.