ABSTRACT

Incising gravel channels respond with a robust critical transition to the lateral alluvial supply. Previous experiments have studied how supply affects bedload transport, but I studied a setup with converging banks, where the supply–transport relation implies feedback. While increasing flows, a short range of transitional flows showed sharp increase of bedload, accompanied by channel response via longitudinal homogenization (connectivity) and maximum sediment storage. To slightly vary initial bed mixing (channel history), I repeated the experiments three times, which validated the robustness of the transition. This transition synchronized hydraulics and transport along the channel, leading to a wetted width consistent with downstream hydraulic geometry that allowed a critical sediment evacuation with minimum energy and bed alteration. For flows higher than the transitional, longitudinal connectivity persisted, as most of the landslide material was redistributed by fluvial action. Finally, the largest flows dissipated energy excess via coarsening and channel migration, akin to non-incising rivers with floodplain. A conceptual model of the geomorphic cycle of gravel river reaches might contain the proposed critical transition, but only if this transition proves to be robust to bank cohesion and to spatial and temporal size of the experimental setup.

摘要

切割砾石通道以向横向冲积供应的稳健关键过渡作出响应。之前的实验研究了供应如何影响床载运输，但我研究了一个具有汇聚银行的设置，其中供应 - 运输关系意味着反馈。在流量增加的同时，短范围的过渡流量显示出床载量急剧增加，并伴随着通过纵向均化（连通性）和最大沉积物储存的渠道响应。为了稍微改变初始床混合（通道历史），我重复了三次实验，验证了过渡的稳健性。这种转变同步了水力和沿河道的运输，导致湿润宽度与下游水力几何形状一致，从而允许以最小的能量和河床改变进行关键的沉积物疏散。对于高于过渡点的流量，纵向连通性持续存在，因为大部分滑坡物质通过河流作用重新分布。最后，最大的流量通过粗化和渠道迁移消散了能量过剩，类似于带有漫滩的非切割河流。砾石河段地貌循环的概念模型可能包含所提出的关键转变，但前提是这种转变被证明对河岸凝聚力以及实验装置的空间和时间规模具有鲁棒性。