Boulder 3D: sediment mobility in bedrock landscapes

河流通过切割暴露在河床上的基岩来切割山脉，促使邻近的山坡变陡。当这些山坡达到临界值陡峭时，它们就很容易发生山体滑坡，这对山区社区和基础设施是一个重大危险。这种基岩切割和山坡变陡的过程是气候和构造变化信号在地球表面被记录的方式，这些反馈驱动着地貌的长期演变。然而，我们对导致基岩切割的机制的了解仍然有限。河流切割从根本上受泥沙可获得性的控制；当颗粒撞击河床上裸露的基岩时，河床表面会磨损。但在陡峭的山区，我们也知道，大石头可以在河床上停留长达数百年，保护河床免受颗粒撞击。由于基岩河流往往很难进入，河床上的巨石往往被淹没，我们缺乏对巨石如何以及何时移动以及如何重新切开下面基岩的了解。为了了解山脉景观如何响应气候驱动的水和沉积物供应变化，我们必须了解基岩清除巨石覆盖的过程，动员的巨石会发生什么，以及这如何调节基岩切割的速度和模式。我们对基岩河流如何切割的了解很大程度上是通过缩小物理实验来实现的，但在实验室中复制自然过程是困难的。在自然界中，切割主要发生在非常大和罕见的水流中，因此不可能直接观察泥沙如何沿着河床移动。在深水池中，航道内的仪器设备通常会被破坏或丢失。在这个项目中，我将开发一种新的方法，结合现场和实验室实验，跟踪巨石如何通过天然基岩渠道，并了解导致它们从河床上清除的过程。最先进的微型传感器将被钻入巨石中，这些巨石将被放置在一个可接近的小基岩通道中，以记录它们流动的流动条件。到目前为止，这些传感器更常用于动物跟踪研究，但最近电池寿命的改善和更小的传感器尺寸现在使它们可以用来跟踪甚至非常微妙的巨石运动。部署巨石的现场符合一套严格的标准，即使巨石被水流运送到很远的距离，也可以回收，从而确保持续的数据收集。由于这是一项实地实验，我将能够选择一系列不同形状、大小和地质的巨石，在自然环境中测试几个假设。我还将对巨石进行重复的3D扫描，以探索巨石是如何被水流中携带的细物质的颗粒撞击磨损到更小的尺寸的。这些现场实验将得到实验室实验的补充，以观察在自然航道中无法观察到的过程。我将把这两组实验的新知识和数据整合到一个计算机模型中，以探索这些现实的门槛对更广泛的景观演变的影响。我将重点了解河流流量和悬浮泥沙的变化(由于气候变化和极端降水)将如何在十年-千年的时间尺度上调制巨石覆盖的模式，使我们对沉积物覆盖、基岩切割、山坡陡化和滑坡潜力之间的反馈的理解发生阶段性变化。该模型的具体应用包括喜马拉雅山脉等山区，在这些地区，在极端水流期间的巨石运输对社区和基础设施构成重大危险。

Rivers carve mountain ranges by incising into bedrock exposed on riverbeds, driving steepening of adjacent hillslopes. When these hillslopes reach a threshold steepness, they become susceptible to landsliding which is a significant hazard to mountain communities and infrastructure. This process of bedrock incision and hillslope steepening is how signals of climate and tectonic change are recorded in Earth's surface, and these feedbacks drive the long-term evolution of landscapes. However, our understanding of the mechanisms that result in bedrock incision are still limited. River incision is fundamentally controlled by sediment availability; when particles impact exposed bedrock on the riverbed, the surface is worn down. But in steep mountainous terrain, we also know that large boulders can sit on the riverbed for up to hundreds of years, protecting the bed from particle impacts. Because bedrock rivers are often difficult to access and boulders on the bed are often submerged, we lack an understanding of how and when boulders are moved and how incision of underlying bedrock recommences. To understand how mountain landscapes are responding to climate driven changes in water and sediment supply, it is vital that we understand the processes through which bedrock is cleared of boulder cover, what happens to the mobilised boulders and how this modulates rates and patterns of bedrock incision. Much of our understanding of how bedrock rivers incise is through scaled down physical experiments, but it is difficult to replicate natural processes in laboratories. In nature, incision predominantly occurs during very large and infrequent flows, so it is impossible to directly observe how sediment is moving along the channel bed. In-channel instrumentation is usually destroyed or lost in deep pools. In this project I will develop a novel methodology combining field and laboratory experiments to track how boulders move through a natural bedrock channel and understand the processes that lead to their clearance from riverbeds. State of the art miniature sensors will be drilled into boulders that will be put in a small and accessible bedrock channel to document the flow conditions at which they mobilise. Until now, these sensors are more commonly used in animal tracking research but recent improvements in battery life and smaller sensor sizes now make it feasible to use them to track even very subtle boulder motion. The field site that the boulders will be deployed in meets a strict set of criteria making it possible to retrieve boulders even if transported significant distances by the flow, ensuring continuous data collection. As this is a field experiment, I will be able to choose a range of different shape, size and geology of boulders to test several hypotheses in a natural environment. I will also perform repeat 3D scans of the boulders to explore how boulders are worn down to smaller sizes by particle impacts from fine material carried in the flow. These field experiments will be supplemented by laboratory experiments to observe processes that cannot be observed in the natural channel. I will incorporate new knowledge and data from both sets of experiments into a computer model to explore the effects these realistic thresholds have on wider landscape evolution. I will focus on understanding how changes in river flow and suspended sediment (due to climate change and precipitation extremes) will modulate patterns of boulder cover over decadal-millennial timescales, bringing about a step-change in our understanding of feedbacks between sediment cover, bedrock incision, hillslope steepening and landsliding potential. Specific applications of this model include mountainous regions such as the Himalayas, where boulder transport during extreme flows represents a significant hazard to communities and infrastructure.