CSDMS 2016 annual meeting poster AustinChadwick
How does delta shoreline sinuosity respond to changes in river discharge variability?
Vamsi Ganti, Imperial College London, London , United Kingdom. v.ganti@imperial.ac.uk
Hima Hassenruck-Gudipati, University of Texas at Austin, Austin Texas, United States. hima.gudipati@gmail.com
Michael Lamb, California Institute of Technology, Pasadena California, United States. mpl@gps.caltech.edu
Abstract:
Climate-driven changes in storm-induced flood events are amplified on coastal river deltas, where standing water downstream creates a region of non-uniform flow that is sensitive to river discharge regime. The sinuosity of modern and ancient delta shorelines, i.e. shoreline rugosity, is a potential imprint of discharge variability, especially where marine 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 channel avulsion location, avulsion timing, and lateral migration, all of which can be strongly influenced by discharge variability within the backwater zone. Scaling arguments predict that shoreline rugosity should increase linearly with avulsion timescale, inversely with avulsion lengthscale, and inversely with lateral migration rate. We present results from two scaled flume experiments that confirm this hypothesis, and furthermore illustrate the importance of discharge variability in controlling the dominant rates and scales in a growing delta. Under conditions of variable floods that maintain a dynamic backwater zone, river 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 channels 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 evaluating the impact of discharge variability on the shoreline rugosity of river-dominated deltas. Ongoing work focuses upon expanding this framework to deltas experiencing changes in base level.
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