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 has built a mixed sediment Gilbert-style delta, and the Santa Clara River, California, which has built 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. Stage (ii) occurred rapidly (i.e., days to weeks) after the river discharge event for the systems studied and was associated with the greatest rates of net erosion and deposition. These morphodynamics were similar to simple equilibrium profile concepts that include an onshore-dominated cross-shore transport rule. Additionally, both study sites had littoral-grade sediment initially deposited at 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 most of the sediment deposited below the depth of closure remained in place. The ‘below closure depth’ sediment represents 14% and 46% of the original volume of littoral-grade sediment discharged by the Santa Clara and Elwha Rivers, respectively. This provides evidence 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.