Sediment flux driven channel geometry adjustment of bedrock and mixed gravel‐bedrock rivers,Earth Surface Processes and Landforms

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Sediment flux driven channel geometry adjustment of bedrock and mixed gravel‐bedrock rivers
Earth Surface Processes and Landforms ( IF 2.8 ) Pub Date : 2020-09-18 , DOI: 10.1002/esp.4996
Edwin R.C. Baynes _{1,

2,

3} , Dimitri Lague ₁ , Philippe Steer ₁ , Stéphane Bonnet ₄ , Luc Illien ₁

Affiliation

Sediment supply (Qs) is often overlooked in modelling studies of landscape evolution, despite sediment playing a key role in the physical processes that drive erosion and sedimentation in river channels. Here, we show the direct impact of the supply of coarse‐grained, hard, sediment on the geometry of bedrock channels from the Rangitikei river, New Zealand. Channels receiving a coarse bedload sediment supply are systematically (up to an order of magnitude) wider than channels with no bedload sediment input for a given discharge. We also present physical model experiments of a bedrock river channel with a fixed water discharge (1.5 l/min) under different Qs (between 0 and 20 g/l) that allow the quantification of the role of sediment in setting the width and slope of channels and the distribution of shear stress within channels. The addition of bedload sediment increases the width, slope, and width‐to‐depth ratio of the channels, and increasing sediment loads promote emerging complexity in channel morphology and shear stress distributions. Channels with low Qs are characterised by simple in‐channel morphologies with a uniform distribution of shear stress within the channel while channels with high Qs are characterised by dynamic channels with multiple active threads and a non‐uniform distribution of shear stress. We compare bedrock channel geometries from the Rangitikei and the experiments to alluvial channels and demonstrate that the behaviour is similar, with a transition from single thread and uniform channels to multiple threads occurring when bedload sediment is present. In the experimental bedrock channels, this threshold Qs is when the input sediment supply exceeds the transport capacity of the channel. Caution is required when using the channel geometry to reconstruct past environmental conditions or to invert for tectonic uplift rates, because multiple configurations of channel geometry can exist for a given discharge, solely due to input Qs.

中文翻译：

沉积物通量驱动的基岩和砾石-基岩混合河流的河道几何调整

尽管沉积物在驱动河道侵蚀和沉积的物理过程中起着关键作用，但在景观演变的建模研究中，沉积物供应 (Qs) 经常被忽视。在这里，我们展示了粗粒、硬质沉积物的供应对新西兰朗吉泰克河基岩通道几何形状的直接影响。对于给定的排放量，接收粗粒沉积物供应的渠道系统地（最多一个数量级）比没有床负荷沉积物输入的渠道更宽。我们还展示了在不同 Qs（0 到 20 g/l）下固定排水量（1.5 l/min）的基岩河道的物理模型实验，允许量化沉积物在设置宽度和坡度方面的作用。通道和通道内剪切应力的分布。河床沉积物的加入增加了河道的宽度、坡度和宽深比，增加的沉积物负载促进了河道形态和剪应力分布的复杂性。低 Qs 的通道的特征是简单的通道内形态，通道内的剪应力分布均匀，而高 Qs 的通道的特征是具有多个活动线程的动态通道和剪应力的非均匀分布。我们比较了 Rangitikei 的基岩通道几何形状和冲积通道的实验，并证明行为是相似的，当存在床载沉积物时，会发生从单线和均匀通道到多线的转变。在实验基岩通道中，这个阈值 Qs 是当输入的沉积物供应超过通道的运输能力时。在使用通道几何结构重建过去的环境条件或反转构造抬升速率时需要小心，因为对于给定的放电，通道几何结构的多种配置可能存在，仅由于输入 Qs。

更新日期：2020-09-18

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