Alongshore sediment bypassing as a control on river mouth morphodynamics

• Published in: Journal of geophysical research. Earth surface 2016-04, Vol.121 (4), p.664-683
• Main Authors: Nienhuis, Jaap H., Ashton, Andrew D., Nardin, William, Fagherazzi, Sergio, Giosan, Liviu
• Format: Article
• Language: English
• Subjects: alongshore sediment transport; Bypasses; bypassing; Coastal sediments; Coastal zone management; Delft3D; Deltas; Dynamics; Erosion; Evolution; Fluvial sediments; Freshwater; Geomorphology; Hydrodynamics; Mathematical models; Migration; Momentum; Momentum flux; Momentum transfer; Morphology; Mouth; Predictions; River basins; River channels; River discharge; River flow; River mouth; river mouth migration; river mouth spit; River mouths; Rivers; Salmon; Sediment; Sediment transport; Sediments; Shorelines; Water discharge
• ISSN: 2169-9003; 2169-9011
• DOI: 10.1002/2015JF003780

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. Key Points Model of waves and river mouth morphodynamics shows littoral bypassing and river mouth migration Bypassing pathways through river mouth bar migration, spit breaching, and channel dynamics Effects on spit width and bypassing make migration fastest for intermediate river discharge