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

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.