Wave-angle control of delta evolution
Animation S1: Side-by side animations of simulated delta growth for results shown in Figure 3: symmetrical delta with A = 0.5, U = 0.4; asymmetrical delta with A = 0.8, U = 0.3. (6.185Mb)
Animation S2: Side-by side animations of simulated delta growth for results shown in Figure 3: symmetrical delta with A = 0.5, U = 0.4; asymmetrical delta with A = 0.7, U = 0.4. (6.094Mb)
Ashton, Andrew D.
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KeywordDepositional asymmetry; Large-scale coastal evolution; Numerical modeling; Plan-view delta evolution
Wave-influenced deltas, with large-scale arcuate shapes and demarcated beach ridge complexes, often display an asymmetrical form about their river channel. Here, we use a numerical model to demonstrate that the angles from which waves approach a delta can have a first-order influence upon its plan-view morphologic evolution and sedimentary architecture. The directional spread of incoming waves plays a dominant role over fluvial sediment discharge in controlling the width of an active delta lobe, which in turn affects the characteristic rates of delta progradation. Oblique wave approach (and a consequent net alongshore sediment transport) can lead to the development of morphologic asymmetry about the river in a delta's plan-view form. This plan-form asymmetry can include the development of discrete breaks in shoreline orientation and the appearance of self-organized features arising from shoreline instability along the downdrift delta flank, such as spits and migrating shoreline sand waves—features observed on natural deltas. Somewhat surprisingly, waves approaching preferentially from one direction tend to increase sediment deposition updrift of the river. This ‘morphodynamic groin effect’ occurs when the delta's plan-form aspect ratio is sufficiently large such that the orientation of the shoreline on the downdrift flank is rotated past the angle of maximum alongshore sediment transport, resulting in preferential redirection of fluvial sediment updrift of the river mouth.
Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 38 (2011): L13405, doi:10.1029/2011GL047630.