Flood Variability Determines the Location of Lobe‐Scale Avulsions on Deltas: Madagascar
Flood Variability Determines the Location of Lobe‐Scale Avulsions on Deltas: Madagascar
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DOI：
10.1029/2020gl088797
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发表时间：
2020-09
影响因子：
5.2
通讯作者：
Sam A. S. Brooke;V. Ganti;A. Chadwick;M. Lamb
中科院分区：
文献类型：
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作者：
Sam A. S. Brooke;V. Ganti;A. Chadwick;M. Lamb
河流三角洲通过反复的叶规模撕脱而生长，这通常发生在与回水长度尺度相关的位置。相互竞争的假说将撕脱节点的起源归因于自然洪水流量变化（回水假说）或三角洲叶几何形状（几何假说）引起的形态动力学反馈。在这里，使用理论，历史洪水记录和遥感高程数据，我们分析了五个瓣规模的三角洲撕脱在马达加斯加，由陆地卫星图像捕获。撕裂长度为5-55公里，距离远大于回水长度尺度，与几何假设不一致。我们发现，陡峭的，淤泥层河流的马达加斯加洪水引起的河床冲刷，回水流体动力学驱动，传播上游比回水长度尺度。撕脱长度是预测洪水冲刷长度的3.1 ± 1.5倍，与低梯度三角洲相似，并与回水假设一致。结果表明，回水区不均匀流引起的侵蚀是三角洲撕脱位置的主要控制因素。
River deltas grow through repeated lobe‐scale avulsions, which often occur at a location that correlates with the backwater lengthscale. Competing hypotheses attribute the avulsion node origin to either the morphodynamic feedbacks caused by natural flood discharge variability (backwater hypothesis) or to the prograding delta lobe geometry (geometric hypothesis). Here, using theory, historical flood records, and remotely sensed elevation data, we analyzed five lobe‐scale delta avulsions in Madagascar, captured by Landsat imagery. Avulsion lengths were 5–55 km, distances significantly longer than the backwater lengthscale and inconsistent with the geometric hypothesis. We show that the steep, silt‐bedded rivers of Madagascar have flood‐induced bed scour, driven by backwater hydrodynamics, that propagates farther upstream than the backwater lengthscale. The avulsion lengths are 3.1 ± 1.5 times the predicted flood scour lengths, similar to low‐gradient deltas, and consistent with backwater hypothesis. Results demonstrate that erosion initiated by nonuniform flows in the backwater zone is a primary control on delta avulsion locations.