GROWTH AND EVOLUTION OF RIVER MOUTH BARS UNDER WAVE ATTACK

Abstract:

A key process for the formation of deltas and their fluvial networks is the deposition of mouth bars. Predicting mouth bar formation on marine coastlines is complex because of the interactions between waves, tides, vegetation, water and sediment discharge. In this work an analytical model for the hydrodynamic interaction between incoming waves and a turbulent expanding jet is developed and tested with the numerical model Delft3D coupled to the wave model SWAN. Both the analytical model and Delft3D predict that incoming surface gravity waves increase the spreading of the jet and the interaction between wave and current boundary layers causes an increase in bottom friction. Our numerical results show that in the presence of waves, mouth bars form up to 35% closer to the river mouth and 40% faster when compared to cases without waves. From a morphodynamic point of view, we show that wave characteristics (height, period and direction) play an important role in the formation of mouth bars. In our numerical experiments waves affect bar development in three ways: by modifying the direction of the river jet, by increasing bottom shear stresses at the river mouth, and by changing bottom friction and hence increasing jet spreading. In addition, we show that high waves with long period prevent the formation of mouth bars and therefore should be considered in the design of restoration schemes for deltaic land.