HOW DOES WAVE-CLIMATE AFFECT DELTA EVOLUTION?

The interplay between coastal and river processes influence delta morphologies. In particular, channel lengthening contributes to in-channel aggradation and avulsions, which alter the location sediment is delivered to the coast, affecting alongshore sediment transport patterns. Alongshore transport then redistributes that sediment and reshapes the coastline, tending to alter the rate of channel elongation. Ratliff and others (2018) used numerical modelling to examine how river and coastal processes affect each other and delta morphology. Their initial work used symmetric wave climates (wave climates that cause zero net alongshore sediment transport on average) as an initial simplification. We build upon these initial investigations by introducing asymmetric wave climates (i.e., there is net alongshore sediment transport on average). In a series of numerical model experiments we altered four major parameters: wave asymmetry (ratio of sediment transport going to the left vs right, looking offshore), wave height, coastline stability (ratio of “high angle” to “low angle” wave influences), and channel superelevation threshold for avulsions. We find that increasing wave asymmetry results in delta asymmetry, but raising wave heights and lowering the superelevation threshold both tend to inhibit this effect. Experiments also showed that for very asymmetric deltas, avulsions tend to occur on the updrift portion of the delta. In these cases, because of rotated coastline orientations, avulsions changed the length of the river very little (and consequently sediment flux out of the river mouth varied little). An asymmetric wave climate tends to make deltas migrate downdrift. Unexpectedly, when deltas migrate downdrift rapidly enough, avulsions in the downdrift direction are inhibited, and when the erosive updrift portion of the migrating delta impinges on the river mouth, the erosion slows river progradation and inhibits avulsions.