Littoral Sediment From Rivers: Patterns, Rates and Processes of River Mouth Morphodynamics

Abstract

Rivers provide important sediment inputs to many littoral cells, thereby replenishing sand and gravel of beaches around the world. However, there is limited information about the patterns and processes of littoral-grade sediment transfer from rivers into coastal systems. Here I address these information gaps by examining topographic and bathymetric data of river mouths and constructing sediment budgets to characterize time-dependent patterns of onshore, offshore, and alongshore transport. Two river deltas, which differ in their morphology, were used in this study: the Elwha River, Washington, which builds a mixed sediment Gilbert-style delta, and the Santa Clara River, California, which builds a cross-shore dispersed sand delta from hyperpycnal flows. During and after sediment discharge events, both systems exhibited a similar evolution composed of three phases: (i) submarine delta growth during offshore transport of river sediment, (ii) onshore-dominated transport from the submarine delta to a subaerial river mouth berm, and (iii) longshore-dominated transport away from the river mouth following subaerial berm development. Although stage (ii) occurred within days to weeks for the systems studied and was associated with the greatest rates of net erosion and deposition, onshore transport of sediment from submarine deposit to the beach persisted for years following the river discharge event. These morphodynamics were similar to simple equilibrium profile concepts that were modified with an onshore-dominated cross-shore transport rule. Additionally, both study sites revealed that littoral-grade sediment was initially exported to depths beyond the active littoral cell (i.e., below the depth of closure) during the stage (i). Following several years of reworking by coastal processes, bathymetric surveys suggested that 14 and 46% of the original volume of littoral-grade sediment discharged by the Santa Clara and Elwha Rivers, respectively, continued to be below the depth of closure. Combined, this suggests that integration of river sediment into a littoral cell can be a multi-year process and that the full volume of littoral-grade sediment discharged by small rivers may not be integrated into littoral cells because of sand and gravel "losses" to the continental shelf.

On the overlooked impact of river dams on beach erosion worldwide

“…At low latitudes (as well as in enclosed seas), the weak wave regime induces a shallow depth of closure [30], which greatly limits the size of the nearshore zone. Consequently, sediment is more likely to bypass or drift directly from river outlets into the open ocean [31]. In our model, this results in low values of R cross at the equator, which lead to low sand budgets despite high riverine inputs.…”

Morphology Recovery and Convergence of Topographic Evolution in the Natori River Mouth after the 2011 Tohoku Tsunami

“…However, from y = 350 m, the linear decay from 2011 to 2014 and the random fluctuation from 2014 to 2018 are two notable trends. Similar to the cross-shore sediment transport which results in the onshore sediment movement in the early stage after a flood event [46,47], the transport after the tsunami is induced by skewed and shallow asymmetric waves to produce the onshore and offshore transports. Normally, the offshore sediment transport is low to negligible, and the sediment is transported dominantly from the submarine to the intertidal.…”

A method for estimating stored sediment volumes by check dam systems at the watershed level: example of an application in a Mediterranean environment

“…Watershed management aims to regulate cascades and fluxes of sediments moving from some distributed sources to downstream areas (Montgomery and Buffington 1997;Dunne et al 2003;Fryirs 2013;Dumitriu 2020). Consequently, addressing management efforts to preserve shorelines equilibrium in the proximity of river deltas (Komar 1977;Williams et al 2018;Warrick 2020) is sensible particularly where urban and tourist settlements, as well as infrastructure, exist or are being planned. Control works of watershed drainage networks, and especially check dams, affect sediment fluxes and budgets (Conesa García 2004;Boix-Fayos et al 2008;Díaz-Gutiérrez et al 2019;Hu et al 2019;Arabkhedri et al 2021).…”