Waterfalls, Hillslopes & Sediment: Understanding critical controls of landscape evolution

长期以来，瀑布一直被认为是任何景观的标志性特征(Gilbert，1896年)，但在控制短期和长期景观演变的速度和模式方面也至关重要(Baynes等人，2015；2018)。景观通过瀑布的上游运移响应构造抬升的变化，不仅调整河流形态，而且还改变相邻山坡的形态，这对山坡的稳定性有重要影响。尽管它们很重要，但驱动瀑布侵蚀的物理过程仍然很少受到限制，许多研究倾向于简化方法，尽管最近的研究强调了物理现实中的许多复杂性(例如，Steer等人，2019年；Scheingross等人，2019年)。这一博士将利用实验室建模、野外工作和基于地理信息系统的地形分析(例如，利用SWT算法；Hillier，2008)，以一种综合的方法来处理瀑布过程的复杂性及其在控制更广泛地貌的响应时间方面的作用。具体的复杂性包括不同的推移质泥沙供应(即‘工具’和‘覆盖’效应；斯克拉尔和迪特里希2001)、河道内和山坡上的岩性(强度和结构)以及构造抬升速率对瀑布退缩速度和形态的作用。实地研究的具体景观将由学生在项目的早期阶段确定，但将包括可以分离和量化不同驱动因素的地点，例如新西兰Rangitikei河的泥沙供应(Baynes等人，2020年)。根据新冠肺炎的限制，替代的野外地点可能包括苏格兰的冰后景观。独特的模拟模型实验将在受控的实验室条件下(见Baynes等人，2018年)在法国的Rennes 1大学进行，允许测试一系列不同的气候和构造情景，并量化特定的瀑布过程，包括水流水力学和泥沙供应的作用。该项目的发现将导致对瀑布和连体山坡侵蚀过程的理解的阶段性变化，对更广泛的地貌演变建模和解释过去的地貌变化具有令人兴奋的影响。

Waterfalls have long been considered iconic features of any landscape (Gilbert, 1896), but are also critical in controlling the pace and pattern of landscape evolution over both short and long timescales (Baynes et al., 2015; 2018). Landscapes respond to changes in tectonic uplift via the upstream migration of waterfalls, which not only adjust river morphology, but also the processes that the morphology of adjoining hillslopes, which has significant influence on hillslope stability. Despite their importance, the physical processes that drive waterfall erosion remain poorly constrained with many studies favouring a simplified approach despite recent research highlighting numerous complexities in the physical reality (e.g., Steer et al., 2019; Scheingross et al., 2019).This PhD will harness laboratory modelling, fieldwork and GIS-based topographic analyses (e.g., with the SWT algorithm; Hillier, 2008) in a combined approach to tackle the complexities of waterfall processes and their role in controlling the response time of the wider landscape. Specific complexities include the roles of varying bedload sediment supply (i.e., the 'tools' and 'cover' effects; Sklar and Dietrich 2001), lithology (strength and structure) in-channel and on hillslopes, and tectonic uplift rate on waterfall retreat rate and morphology. Specific landscapes for field study will be determined by the student during the early stages of the project, but will include locations where different driving factors can be isolated and quantified, such as bedload sediment supply in the Rangitikei River, New Zealand (Baynes et al., 2020). Depending on Covid-19 restrictions, alternative field locations could include the postglacial landscapes of Scotland. Unique analogue model experiments will be performed under controlled laboratory conditions (see Baynes et al., 2018 for an example) at the Université de Rennes 1 (France), allowing a range of different climate and tectonic scenarios to be tested and specific waterfall processes quantified including the role of flow hydraulics and sediment supply.The findings from this project will lead to a step-change in the understanding the erosion processes of waterfalls and coupled hillslopes, with exciting implications for the modelling of wider landscape evolution and interpreting past landscape change.