CSDMS 2013 annual meeting poster Jaap Nienhuis
Growth and abandonment: quantifying first-order controls on wave influenced deltas
Andrew D Ashton, WHOI Cambridge Massachusetts, United States.
Liviu Giosan, WHOI Cambridge Massachusetts, United States.
River delta evolution is characterized by cyclical progradation and transgression: the delta cycle. We investigate the growth and decay of the individual or main lobes of deltas with strong wave influence with the aim to quantitatively compare marine to terrestrial controls.
We apply a model of plan-view shoreline evolution to simulate the evolution of a deltaic environment. The fluvial domain is represented by deposition of sediment along the shoreline, developing along a predefined single or multi-channel fluvial network. We investigate the influence of wave climate, fluvial sediment input and network geometry.
For growing deltas, we present a sediment-flux-based approach to quantify the relative influence of fluvial versus marine (wave) controls on morphology. Wave domination requires that the magnitude of the fluvial bedload flux to the nearshore region be less than the alongshore sediment transport capacity of waves removing sediment from the mouth. Fluvial dominance occurs when fluvial sediment input exceeds the wave-sustained alongshore sediment transport for all potential shoreline orientations, both up- and downdrift of the river mouth. For a single delta (or delta lobe), this transition depends not only on the fluvial river sediment flux and wave energy, but also on the directional wave climate.
Channel bifurcation is critical; it splits the sediment discharge from the river, while the potential alongshore sediment flux per channel remains equal. Fluvial dominance persists until sufficient bifurcations have split the fluvial sediment flux among the channels or until the occurrence of a river avulsion. This simplified model allows us to quantify the transition from fluvial to wave dominance and enables comparisons with natural examples near this transition, such as the Tinajones lobe of the Sinu River Delta, Colombia, and the Po Delta, Italy.
During delta abandonment, model results suggest littoral sediment transport can result in four characteristic modes of wave reworking, ranging from diffusional smoothing of the delta (or delta lobe) to the development of downdrift-extending recurved spits. The directional characteristics of the wave climate, along with the pre-abandonment delta shape, determine the mode of reworking. Simple analysis of pre-abandonment delta shape and wave characteristics provides a framework for predicting the mode of delta reworking; model predictions agree with the observed morphology of historically abandoned delta lobes, including the Nile, Ebro, and Rhone. These results provide insight into the potential evolution of active delta environments facing near elimination of fluvial sediment input.
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