Budging boulders: controls on coarse sediment evacuation in bedrock rivers

Overview

The pace at which bedrock river channels incise into solid Earth determines how relief is generated across mountain ranges, controlling the long-term evolution of landscapes (e.g., Schildgen et al., 2007; Whipple, 2009). Understanding the mechanisms through which bedrock channels incise is an ongoing challenge in Earth surface processes research. Existing mechanistic models of bedrock incision by particle impacts capture the competing effects of sediment supply in terms of either providing erosional ‘tools’ that actively incise exposed bedrock (e.g., Beer et al., 2014), or the development of an alluvial cover that protects the underlying bedrock from particle impacts (e.g., Sklar and Dietrich, 2001, 2004, 2006). Steep bedrock channels also episodically receive large inputs of coarse hillslope material (e.g., boulders) through processes such as earthquake or storm-induced mass wasting, that may cover and protect the underlying bedrock from incision for hundreds to thousands of years (e.g., DeLisle et al., 2021). This is comparable to the frequency at which these types of extreme sediment generating events occur. This complicates our understanding of how incision into bedrock channels keeps pace with rock uplift. If these types of channels are rarely free of boulder cover, it is unclear where and how the underlying bedrock is eroded, and the influence this has on longer-term channel and landscape evolution.
Shobe et al. (2018) have shown that boulder delivery to channels can significantly slow-down rates of bedrock channel incision. But, their work has assumed that the flow has to be capable in sliding the boulders downstream before the underlying bedrock can be eroded, and we still do not fully understand how interactions between flow and boulder movement affect bedrock incision (Shobe et al., 2021). We hypothesise that two additional processes might also affect the interaction between boulders and bedrock incision. The first is the toppling/rotation of boulders, which could expose the underlying bed, and is likely to be linked to the erosion/deposition of sediment around the boulders. The second is the role of bedload and suspended load in abrading boulders in-situ, and increasing their mobility. The aim of this project is to understand controls on boulder mobility in bedrock rivers through observations of boulder movement and in-situ abrasion in response to variable flow conditions. This will be established through one, or both, of the follow objectives:
1. Develop new physical experiments to document and quantify spatial and temporal aspects of boulder movement and thresholds of motion in a fully controlled environment.
2. Undertake investigations and measurements of boulder mobility in bedrock channels using methodologies developed in the flume.

Methodology

The project will have a strong practical focus, and we expect that the project aims will be addressed using a combination of novel flume experiments and complimentary fieldwork. But, the student will play a pivotal role in designing the project, and the supervisors will support the student in tailoring the project and balance between the approaches to meet the student’s particular interests. Possible locations to undertake fieldwork include the European Alps (with project collaborator Dr Alex Beer) or Himalayas/western Canada/Taiwan. This project will combine large-scale landscape processes and observations with granular phenomenon but could similarly be developed to focus on either aspect in more detail.
Flume experiments will be developed in collaboration with Dr Ed Baynes at the River Science Laboratory at Loughborough University. Experiments could include using repeat topographic data to measure how bed topography and boulder position changes in response to imposed flow and sediment supply conditions; and/or testing the use of tri-axial accelerometers to record boulder motion. The findings from the latter flume experiments could be used to guide fieldwork where instrumentation will be installed into real boulders and left over a period of high flow to make comparable observations on boulder movement. Boulder motion data will be analysed using statistical techniques, as well as high-resolution topographic data from laser scanning to show how boulder and bed morphology adjusts through time. Field measurements could also be designed to quantify rates and spatial patterns of in-situ boulder abrasion.

Project Timeline

Year 1

Literature review
Design/develop/trial a series of flume experiments

Year 2

Run and analyse flume experiments and data
and/or
Use observations from trial flume experiments to design fieldwork methodology
Undertake fieldwork

Year 3

Retrieve and analyse field data (if applicable)
Write thesis and papers

Year 3.5

Finish writing thesis and papers

Training
& Skills

We anticipate that this project will be completed as a series of publications, with support and training in scientific writing. Technical training will depend on the direction that the student wishes to take the project, but could include laser scanner operation and data collection, flume/experimental design and operation, and statistical analysis of acquired data. IAPETUS2 provides a wide range of training opportunities to its students. The student will gain extensive experience of working with and developing laboratory experiments, as well as geomorphological field skills. With respect to this project, two of the most relevant are the ‘Introduction to modelling in Python’ and ‘Advanced statistics in R’ modules, but we will discuss the student’s needs and interests at the outset of the project. In addition, training will be provided in quantitative analysis of DEMs, as well as Python programming for topographic analysis and running command line tools. The student will gain experience in efficiently analysing large and complex datasets.

References & further reading

Beer, A.R., Turowski, J.M., Fritschi, B. and Rieke‐Zapp, D.H., 2015. Field instrumentation for high‐resolution parallel monitoring of bedrock erosion and bedload transport. Earth Surface Processes and Landforms, 40(4), pp.530-541.
DeLisle, C., Yanites, B. J., Chen, C.-Y., Shyu, J. B. H., & Rittenour, T. M. (2021). Extreme event-driven sediment aggradation and erosional buffering along a tectonic gradient in southern Taiwan. Geology. https://doi.org/10.1130/G49304.1
Schildgen, T.F., Hodges, K.V., Whipple, K.X., Reiners, P.W. and Pringle, M.S., 2007. Uplift of the western margin of the Andean plateau revealed from canyon incision history, southern Peru. Geology, 35(6), pp.523-526.
Shobe, C. M., Tucker, G. E., & Rossi, M. W. (2018). Variable-threshold behavior in rivers arising from hillslope-derived blocks. Journal of Geophysical Research: Earth Surface, 123(ja), 1931-1957. https://doi.org/10.1029/2017JF004575
Shobe, C. M., Turowski, J. M., Nativ, R., Glade, R. C., Bennett, G. L., & Dini, B. (2021). The role of infrequently mobile boulders in modulating landscape evolution and geomorphic hazards. Earth-Science Reviews, 103717. https://doi.org/10.1016/j.earscirev.2021.103717
Sklar, L. S., & Dietrich, W. E. (2001). Sediment and rock strength controls on river incision into bedrock. Geology, 29(12), 1087–1090.
Sklar, L. S., & Dietrich, W. E. (2004). A mechanistic model for river incision into bedrock by saltating bed load. Water Resources Research, 40, W06301. https://doi.org/10.1029/2003WR002496
Sklar, L. S., & Dietrich, W. E. (2006). The role of sediment in controlling steady-state bedrock channel slope: Implications of the saltation–abrasion incision model. Geomorphology, 82(1–2), 58–83. https://doi.org/10.1016/j.geomorph.2005.08.019
Whipple, K.X., 2009. The influence of climate on the tectonic evolution of mountain belts. Nature Geoscience, 2(2), pp.97-104.

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