Growth of river mouth bars in sheltered bays in the presence of frontal waves

One of the key processes for the formation of deltas and their fluvial networks is the deposition of mouth bars in front of prograding distributaries. Waves influence mouth bar growth, but it is not clear how and to what extent. Toward this end, we conduct a modeling study on deltas forming in sheltered bays, where waves are locally generated and both longshore currents and surf zone are absent. We focus on the simplified case of a homopycnal river plume subject to frontal wave attack, and we begin by isolating the effects of waves on jet spreading. 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. To investigate how waves influence mouth bar morphodynamics, a set of numerical simulations is run with Delft3D-SWAN utilizing a geometry and wave characteristics typical of sheltered bays. 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. The distance from the river mouth to the stagnated mouth bar decreases with increasing wave height and wave period. The timescale of bar formation is inversely proportional to wave height and directly proportional to wave period. Our modeling study suggests that wave influence on mouth bar growth is complex. Small waves, like the ones modeled here, promote mouth bar formation via increased jet spreading and faster formation time, which in turn should create deltas with more distributary channels. On the other hand, large waves suppress mouth bar formation, as seen in other studies, leading to fewer distributary channels.