Abstract
At river mouths, fluvial jets and longshore currents (LSCs) generated by waves interact hydrodynamically. This idealized numerical modeling study simulates a large number of hydro-morphodynamic conditions (650) to explore the emergent hydrodynamics determined by different mouth bar volumes and geometries, river discharge, wave heights, and directions and their potential stress on river-mouth development. We find that in the absence of a river-mouth bar (RMB), interactions are driven by momentum balances, expressed either as the balance of wave momentum flux (Mw) and jet momentum flux (Mj), or the balance of river jet discharge (QJet) and longshore current discharge (QLSC). When a RMB is present, the topography modifies the structure of the jet by spreading it, and we quantify this mechanism through the lateral jet transfer rate (LJT). Secondly, topography generates complex longshore wave-driven circulation as a result of the protruding shoreface which serves as a platform on which counter LSCs develop. The balance in QJet/QLSC may be used as an indication of the type of circulation. High and oblique waves favor longshore circulation and RMB bypass, whereas low waves and normal-to-coast angles generate diverging LSCs on the mouth bar crest which interrupts the longshore circulation. A quantification of the dynamic diversion is proposed in the form of the non-dimensional Dynamic diversion index (DyD), which scales with the product of Mj and Mw, and can account for the absolute strength of hydrodynamic interactions occurring at river mouths. RMB morphology can affect DyD in multiple ways by strengthening or by weakening the interactions. The DyD effect seems to increase with increasing RMB size, indicating that the RMB scale regulates the interplay of the wave-driven circulation and the river jet which further controls the adjacent topography changes.
Highlights
River mouths are a focal point of interaction for jet currents originating from fluvial discharge, longshore currents (LSCs) generated by waves as well as circulation driven by wind, tides and density effects
We investigate jet- and wave-driven longshore circulation and their interaction at river mouths by using a numerical simulation which represents a full range of conditions expected in waveinfluenced river-mouth environments
From the results presented above, this metric (Mj/Mw) appears to be appropriate for describing other effects occurring at river mouths such as jet deflection and the relative dominance of either alongshore wave-driven circulation or crossshore jet circulation
Summary
River mouths are a focal point of interaction for jet currents originating from fluvial discharge, longshore currents (LSCs) generated by waves as well as circulation driven by wind, tides and density effects. River effluents may be seen as bounded jets (Dracos et al, 1992; Canestrelli et al, 2014), and jet spreading and deceleration for these river-mouth jets occurs more rapidly than for unbounded jets, without any constriction surfaces (Özsoy and Ünlüata, 1982; Fagherazzi et al, 2015; Kundu et al, 2016) Processes such as bed friction, lateral mixing, and expansion at the outlet, jet and plume detachment, act to diverge jets from their idealized shape (Wright, 1977; Edmonds and Slingerland, 2007; Fagherazzi et al, 2015). Wind waves cause jet deflection and spreading (Nardin and Fagherazzi, 2012), as do tides (Leonardi et al, 2013) and salinity differences It seems that apart from potential vorticity which acts to conserve the jet structure, all other element act against this conservation, resulting in the multitude of coastal jet and plume structures that occur in nature
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