Experimental Investigation of the Morphodynamic Controls on Delta-Lobe Formation and Shoreline Rugosity

River deltas grow primarily through repeated avulsions, where abrupt redistribution of sediment and water delivered to the ocean creates new delta lobes and rugose shorelines. Previous work isolates sediment caliber and cohesion as a primary control on the planform morphology of deltas; however, the effect of varying water discharges and backwater hydrodynamics on the creation of new delta lobes and development of shoreline rugosity is yet to be explored. Here we report on two physical experiments conducted with non-cohesive sediment in the river-ocean facility at Caltech, where a 7 m long, 7 cm wide alluvial river drains into a 6 m by 3 m "ocean" basin, building its own delta under subcritical flow and constant sea level conditions. The first experiment was conducted under constant sediment and water discharge, while in the second experiment we alternated between a low flow and a high flow such that the backwater hydrodynamics are persistent through time. We find that in the first experiment, channel dynamics are dominated by pronounced channel migration and avulsions at the location of imposed change in flow confinement, which result in smooth shorelines that are radially symmetric. However, in the second experiment, channel migration is arrested due to the creation (during low flow) and destruction (during high flow) of bars, which results in progradation of the delta front in a localized area, producing significant shoreline rugosity. The avulsion length in this experiment is determined by the backwater length, and delta progradation results in downstream translation of the avulsion node, thus determining the delta-lobe size and consequently the degree of shoreline rugosity. Our results indicate that deltas can exhibit rugose shorelines in the absence of cohesive sediment, and the time-iterative process of downstream-translating avulsion locations that are controlled by backwater hydrodynamics offer an alternative mechanism for development of shoreline rugosity.