Weak wave/tide interaction in suspended sediment-stratified flow: a case study

The formulation of weak wind-wave/low-frequency current interaction is extended to the case of suspended sediment-stratified flow. The influence of suspended sediment stratification on flow dynamics is described in terms of a sediment stratification parameter defined as von Karman's constant times a depth-independent function of the relative friction velocity and the relative settling velocity of suspended particles that is specified by a solution for the problem on the vertical structure of the suspended sediment-stratified near-bottom logarithmic layer. This ‘extended’ formulation is inserted in a two-dimensional non-linear, finite-difference, high-resolution hydrodynamic model and the modified model is applied to clarify the roles of wind-wave/tide interaction and suspended sediment stratification—individually and in combination—in the formation of the M₄ and M₆ overtides in Cádiz Bay. It is shown that the predictions for the M₄ and M₆ overtides have much in common and much in contrast with the M₂ tide. For the M₂ tide the influence of suspended sediment stratification shows up most vividly in the spatial variability of the tidal characteristics, but is not evident in changes in the M₄ and M₆ overtides. On the other hand, the influence of wave-induced changes on the M₂ tidal amplitude and phase is only of minor importance, but for the M₄ and M₆ overtides these changes are quite significant. When taken together, the effects of the two factors under investigation are very nearly balanced. This, however, does not mean that the conventional assumption of ignoring these factors is valid in shallow-water dynamics. Simply, that their resulting effect vanishes.