Alongshore sediment bypassing as a control on river mouth morphodynamics

Abstract

River mouths, shoreline locations where fluvial and coastal sediments are partitioned via erosion, trapping, and redistribution, are responsible for the ultimate sedimentary architecture of deltas and, because of their dynamic nature, also pose great management and engineering challenges. To investigate the interaction between fluvial and littoral processes at wave-dominated river mouths, we modeled their morphologic evolution using the coupled hydrodynamic and morphodynamic model Delft3D-SWAN. Model experiments replicate alongshore migration of river mouths, river mouth spit development, and eventual spit breaching, suggesting that these are emergent phenomena that can develop even under constant fluvial and wave conditions. Furthermore, we find that sediment bypassing of a river mouth develops though feedbacks between waves and river mouth morphology, resulting in either continuous bypassing pathways or episodic bar bypassing pathways. Model results demonstrate that waves refracting into the river mouth bar create a zone of low alongshore sediment transport updrift of the river mouth, which reduces sediment bypassing. Sediment bypassing, in turn, controls the river mouth migration rate and the size of the river mouth spit. As a result, an intermediate amount of river discharge maximizes river mouth migration. The fraction of alongshore sediment bypassing can be predicted from the balance between the jet and the wave momentum flux. Quantitative comparisons show a match between our modeled predictions of river mouth bypassing and migration rates observed in natural settings.

Shoreline Changes on the Wave-Influenced Senegal River Delta, West Africa: The Roles of Natural Processes and Human Interventions

“…The dominant natural mode of behaviour of the Senegal delta shoreline is thus one imprinted by strong longshore transport of sand generated by Atlantic waves from NW to N. The Senegal River mouth is thus a fine example of a wave-influenced delta illustrating the relationship between river-mouth migration, spit elongation and spit breaching by the river mouth [31], although a simple relationship between these processes cannot be expected because of the influence of fluctuations in river discharge and river-mouth bar dynamics [11]. Whereas high river discharge and the formation of river-mouth bars can lead to reduced sediment bypassing, which affects in turn the river-mouth migration rate and the size of the river-mouth spit [31], reduced discharge at the river mouth, tantamount to a decrease in hydraulic efficiency, can lead to bypassing of sediment around the mouth, thus reducing migration [31,35]. Natural breaches of spits barring river mouths and tidal inlets are a commonly cyclic process determined by a combination of spit lengthening, river discharge and river hydraulic efficiency, and also in many cases, storm wave action [31,36].…”

“…Whereas high river discharge and the formation of river-mouth bars can lead to reduced sediment bypassing, which affects in turn the river-mouth migration rate and the size of the river-mouth spit [31], reduced discharge at the river mouth, tantamount to a decrease in hydraulic efficiency, can lead to bypassing of sediment around the mouth, thus reducing migration [31,35]. Natural breaches of spits barring river mouths and tidal inlets are a commonly cyclic process determined by a combination of spit lengthening, river discharge and river hydraulic efficiency, and also in many cases, storm wave action [31,36].…”

“…The Langue de Barbarie lengthened by 11 km between 1850 and 1900 (about 220 m a year), with a distal tip located 15 km south of St. Louis at the turn of the 20th century, and the spit was affected over this 50-year period by seven breaches [27]. Between 1900 and 1973, 13 other breaches occurred across the Langue de Barbarie [27], thus suggesting a breaching timescale (see Nienhuis et al [31]) of~6 years. There were no breaches between 1973 and 2003.…”

Littoral steering of deltaic channels

“…These small channels, however, are often 'deflected' 504 (Bhattacharya and Giosan, 2003) and show downdrift migration along an otherwise straight 505 coastline. River mouth processes likely dictate at this scale, such that the dynamics that set 506 channel orientation are not determined by fluvial sediment supply, but rather by alongshore 507 sediment bypassing (Kirk, 1991;Nienhuis et al, 2016). When downdrift migration occurs 508 without the net progradation typical of river deltas, channel orientation is generally variable, as 509 spit breaching will reset the channel orientation on decadal timescales (Zenkovich, 1967).…”

“…1). The presence of the channel itself affects coastal 37 processes, as river mouths can limit bypassing of littoral sediment (Nienhuis et al, 2016). As 38 such, a mechanistic understanding of the basic controls on channel orientation has been 39 previously lacking.…”