IAP2-22-361

Sediment cover and erosion dynamics in bedrock river channels

The pace at which bedrock river channels incise into Earth’s surface determines how relief is generated across mountain ranges, controlling the long-term evolution of landscapes (e.g., Schildgen et al., 2007; Whipple, 2009). Existing 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, 2004, 2006). To understand how future changes in flow regime and sediment supply (e.g., climate change or anthropogenic pressures in mountain communities) will modify rates of bedrock incision, a better understanding of factors controlling the spatial and temporal evolution of sediment cover in bedrock channels is necessary. Understanding sediment cover is difficult because we do not have good methods for measuring the amount of cover under the high-flow conditions when there is the potential for most erosion. Field observations and theory have suggested that sediment cover can adjust quickly and in non-linear ways through a flood event in response to changes in flow, sediment supply and channel morphology (e.g. Cook et al., 2020), but these adjustments are not fully understood. This project therefore will use a combination of physical and numerical modelling techniques to measure and predict changes in sediment cover in ways that are not possible in the field.

The aim of this project is to explore feedbacks between sediment cover on the bed (i.e., grain size, areal extent, thickness), sediment supply, hydraulic conditions, and bedrock topography. For example, how does sediment cover vary through a flood hydrograph for different grain size distributions? How does bedrock micro or macro topography relate to the area, thickness, or the grain size of overlying sediment cover? This will be established through both of the following objectives:

1. Develop new physical flume experiments to document and quantify feedbacks between sediment cover, flow dynamics and bedrock surface morphology.
2. Develop a numerical model of the flume experiment to simulate bedrock incision over accelerated time and larger river-reach scales.

Click on an image to expand

Image Captions

(Left) Exposed sediment cover on the bed of the Tamur River during low flow conditions in east Nepal and (Right) the series of steps going from the field location, to a topographic model, to 3D printed tiles installed in a laboratory flume.

Methodology

The project will have a strong practical focus, and supervisors will support the student in tailoring the project and approaches to meet the student’s particular interests. We expect that the project aims will be addressed using novel flume experiments which the student will play a pivotal role in designing in collaboration with project partner Dr Ed Baynes at the River Science Laboratory at Loughborough University. The flume experiments will build on work developed by Hodge and Hoey (2016a and 2016b) which used 3D printing to replicate a 1:10 scale model of a bedrock river bed in a flume (see images). This PhD will use a range of 3D milled channel topographies that are being developed for a new project on flow resistance in rough-bed channels, and the student will benefit from collaboration with the team working on that project. Methods used in the flume could include adjusting the channel topography by adding roughness elements and local width constrictions, varying grain size distributions to include boulders, measuring high-resolution bed topography, image processing to map sediment cover and grain size, and measuring hydraulics using flow velocity profiles and PIV. The second focus of the project is to extend the existing numerical model of Creed et al. (2017) to incorporate bedrock incision using existing relations based on particle impact rate, substrate erodibility and grain size (e.g., Turowski et al., 2007). By simulating the flume experiments numerically, the model will be used to test and potentially adapt these existing empirical relationships for bedrock incision.

Project Timeline

Year 1

Literature review
Design flume experiments

Year 2

Build and run flume experiments, and draft paper based on them
Develop numerical model

Year 3

Analyse flume and modelling data
Start writing thesis and draft paper on modelling results

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 experimental design and numerical modelling. The student will also be encouraged to attend the British Society for Geomorphology Windsor Workshop for new PhD students. 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 working with large data sets and numerical models. 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 programming for the development of the numerical model. 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.

Cook K.L., Turowski J.M., Hovius N. 2020. Width control on event scale deposition and evacuation of sediment in bedrock‐confined channels. Earth Surface Processes and Landforms : esp.4993. DOI: 10.1002/esp.4993

Creed, M.J., Apostolidou, I.G., Taylor, P.H., Borthwick, A.G.L. (2017) A finite volume shock-capturing solver of the fully coupled shallow water-sediment equations. Int. J. Numer. Meth. Fluids. 84, 509–542

Hodge R.A., Hoey T.B. 2016a. A Froude-scaled model of a bedrock-alluvial channel reach: 1. Hydraulics. Journal of Geophysical Research: Earth Surface 121 : 2015JF003706. DOI: 10.1002/2015JF003706

Hodge R.A., Hoey T.B. 2016b. A Froude-scaled model of a bedrock-alluvial channel reach: 2. Sediment cover. Journal of Geophysical Research: Earth Surface 121 : 2015JF003709. DOI: 10.1002/2015JF003709

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.

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

Turowski, J.M., Lague, D. and Hovius, N., (2007). Cover effect in bedrock abrasion: A new derivation and its implications for the modeling of bedrock channel morphology. Journal of Geophysical Research: Earth Surface, 112(F4).

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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