Boulder 3D: sediment mobility in bedrock landscapes

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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.