The Role of Backwater Hydraulics in Mediating Avulsion Location, Channel Migration Rate, and Delta Shoreline Rugosity

Abstract:

River deltas earn their name from a characteristic planform-triangular shape, but in reality demonstrate a wide range of morphologies. The sinuosity of delta shorelines, i.e. shoreline rugosity, is particularly variable worldwide even among deltas where waves and tides are not dominant processes. We hypothesize that river-dominated deltas built through construction of depositional lobes develop a characteristic shoreline rugosity that is determined by long-term patterns in avulsion location, avulsion timing, and channel migration, all of which can be strongly influenced by backwater hydrodynamics. Scaling arguments predict that shoreline rugosity should increase linearly with avulsion timescale, inversely with avulsion lengthscale, and inversely with channel lateral migration rate. We present results from two scaled flume experiments that confirm this hypothesis, and furthermore illustrate the importance of backwater hydrodynamics in controlling the dominant rates and scales in a growing delta. Under the case of variable discharge floods that maintain a dynamic backwater zone, avulsions occur at a fixed distance from the shoreline, resulting in the construction of lobes of constant size even during shoreline progradation. In addition, erosion caused by drawdown hydrodynamics during floods eliminates alternating bars, which slows lateral migration of the channel and allows for more elongate delta lobes. Based on these results, and a compilation of modern river-dominated deltas, we propose a new dimensionless phase space for the occurrence backwater-mediated deltas with rugose shorelines.