River Mouth Hydrodynamics: The Role of the Outlet Geometry and Transient Tidal and River Discharge Conditions on the Jet Structure

The hydrodynamics of river mouths are the result of a complex interaction between river flow, tidal conditions, and outlet geometry. This complex interaction of factors shapes the jet that flows onto the continental shelf and influences the dynamics of these areas. This work analyses the influence of idealized but realistic outlet geometries under steady and unsteady hydrodynamic conditions on the jet structure. The analysis is carried out using a numerical model which is validated by comparison with the classical jet theory for highly simplified conditions where this theory applies. The results show that both the outlet geometry and the transient hydrodynamic conditions have a significant influence on the jet structure and evolution along the nearshore. For constant river discharge and water level conditions, the results indicate that the nearshore profile plays a key role in determining the expansion or contraction of the jet. The momentum balance shows that the jet behavior is related to the momentum transport and the barotropic terms. In cases where the river discharge and tidal conditions are transient, the jet alternates between a structure with two velocity maxima at the edges or a single peak in the center during the tidal cycle depending on the phase lag between the tidal conditions and the river hydrograph. These two different jet structures play an important role in the morphodynamic evolution of the river mouths and bar development, favoring in some cases the formation of lateral levees parallel to the channel walls.