Downstream Control on Delta Network Dynamics
Abstract
Bifurcations are the gatekeepers of delta distributary networks, controlling how much water and sediment enters each of their downstream branches. Previous studies of bifurcations have focused on upstream controls on their dynamics. However, in the long term, distributary networks organize to deliver sediment to areas with accommodation space to fill, suggesting a downstream control. We investigate the downstream control on delta bifurcations through both a quasi-1D numerical model and laboratory experiments. From the numerical model, we find a qualitative change in dynamics due to downstream conditions. While a perfectly symmetric bifurcation is always a solution numerically, it may be either stable or unstable to a small perturbation. Under bypass conditions (sediment in equals sediment out), if perturbed, an unstable symmetric bifurcation evolves towards a stable asymmetric state, becoming "frozen". Alternatively, downstream deposition prevents the bifurcation from remaining frozen indefinitely; the development of a relative slope advantage towards the opposite branch causes ongoing oscillations (avulsions). Model results are compared to experiments in a fixed-wall bifurcation. Experiments consist of a depositional stage where the branches of the bifurcation are allowed to prograde, and a bypass stage where the sediment can exit the system. Continuous measurements of water and sediment discharge are used to quantify avulsion magnitude and frequency. The flow patterns associated with measured discharge asymmetry are assessed from overhead images, as well as from bed and water surface scans. Experiments generate dynamics that are more complex than in the model, but a change in the dynamics between the depositional and bypass stages is nevertheless observed. Overall, results suggest that the dynamics of delta distributary networks are largely controlled from downstream.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMEP53A0919S
- Keywords:
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- 1804 Catchment;
- HYDROLOGYDE: 1825 Geomorphology: fluvial;
- HYDROLOGYDE: 1850 Overland flow;
- HYDROLOGYDE: 1861 Sedimentation;
- HYDROLOGY