The Impact of Earthquake-Driven Sediment Supply on Mountain River Dynamics

Overview

Mountain river systems play a critical role in the long-term evolution of high-relief landscapes. River incision into bedrock sets the local base level of the hillslopes, and rivers also transport sediment produced by erosion out of the mountain belt. These two roles can act against each other. The incision of bedrock rivers requires exposure of bedrock in the river bed, because otherwise the bedrock is protected from incision by the overlying sediment cover. In contrast, the high overall rates of erosion mean that the river systems have to transport significant amounts of sediment, inhibiting bedrock exposure in the channels.

Understanding and predicting the interplay of these two processes is complicated by the episodic and spatially variable nature of the sediment supply to these rivers. Sediment can be supplied by a variety of mechanisms, including hillslope processes such as landslides and debris flows, and erosion of sediment stores adjacent to the channel. Furthermore, the grain size of these sediment supplies will also vary in time and space. The aim of this project is to understand the response of mountain river systems to spatially and temporally variable sediment inputs, and to assess the impact of this response for short-term hazard assessment and long-term channel incision.

This project will address the following research questions:
• Where is sediment stored, and where is sediment transported?
• How does grain size affect sediment storage and transport, and how do those processes affect grain size?
• What are the dominant controls on the passage of sediment through the channel network?

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Image Captions

BK.jpg: Figure 1: Upper Bhote Kosi, Nepal; a possible field site for this project.

Methodology

This project will primarily use numerical modelling, but there is potential to support this with some field data collection in Nepal and/or the Philippines.

The project will develop and adapt the 1D numerical model SEDROUT (Hoey and Ferguson, 1994). SEDROUT is a morphodynamic model that predicts flow and sediment transport rates and the resulting changes in bed elevation along a river channel. SEDROUT was initially designed to investigate the impact of size-selective sediment transport on downstream patterns of grain size. Subsequent uses have included quantifying the evacuation of mining sediment from the Fraser River (Ferguson et al., 2015).

The first part of this project will use SEDROUT to analyse the routing of coarse sediment though a mountain river under different spatial and temporal patterns of sediment supply. The spatial pattern of sediment supply to the channel will be parameterised using data collected by the supervisors from the landslides that occurred following the 2008 Mw 7.9 Wenchuan earthquake in China and the 2015 Mw 7.8 Gorkha earthquake in Nepal. These experiments will address questions such as the timescales required for sediment evacuation, the different responses of different grain size fractions, and the spatial pattern of sediment storage along the channel.

Later parts of this project will further develop the SEDROUT model. One possible development is use the branching channel network formulation of SEDROUT (Verhaar et al, 2008), allowing the combined impact of sediment fluxes from different tributaries to be evaluated. Another development is to implement bedrock channels into the model, allowing the model to behave as a bedrock-alluvial channel under conditions of reduced sediment supply. This development will require consideration of the impact of exposed bedrock on sediment transport processes and flow resistance. Such a model will enable sediment cover extents to be tracked along the channel, thus enabling the long-term patterns of bedrock exposure to be analysed.

Parameterising and testing SEDROUT under these different scenarios will require information about the volume and grain size distribution of typical sediment inputs to mountain rivers. In addition to the data already archived at Durham University, there is potential for the student to collect their own field data in Nepal and/or the Philippines. The exact scope of the fieldwork can be developed by the student, but could, for example, focus on the grain size of landslides deposits along the Bhote Kosi river.

Project Timeline

Year 1

Training in key field and modelling skills
SEDROUT model development
Experimental design and pilot experiments
Fieldwork planning

Year 2

Fieldwork
Numerical Modelling experiments
Further model development
Data analysis
UK conference attendance to present preliminary data

Year 3

Finish modelling experiments
International conference to present key findings
Submission of paper based on key findings

Year 3.5

Analysis and write up

Training
& Skills

Training is fundamental to the development of postgraduate research students and, together with the DTP, University and Department we provide a substantial training programme. Priorities for training are determined from the ‘Training Needs Analysis’ carried out in the initial supervisory meeting with the student.

Research training is based around a number of themes: Recognition and validation of problems; Demonstration of the original, independent and critical thinking, and the ability to develop theoretical concepts; Knowledge of recent advances within research field and in related areas; Understanding relevant research methodologies and techniques and their appropriate application within research field; Ability to analyse and critically evaluate findings and those of others; and Summarising.

Training that is specific to this project will include some of the following:
– Development and use of SEDROUT morphodynamic model
– Quantitative analysis of spatially-distributed datasets, including landslide locations and areas as well as topography
– Field surveying and data analysis (differential GPS, use of Unmanned Aerial Vehicles, Terrestrial Laser Scanning, Structure from Motion techniques)
– Use of Matlab, R and/or Python for data processing and analysis. Advanced statistics.
– Fieldwork safety (First Aid)

References & further reading

Attal, M., & Lave, J. (2006). Changes of bedload characteristics along the Marsyandi River (central Nepal): Implications for understanding hillslope sediment supply, sediment load evolution along fluvial networks, and denudation in active orogenic belts. In S. D. Willett, N. Hovius, M. T. Brandon, & D. Fisher (Eds.), Tectonics, Climate, and Landscape Evolution (Vol. 398, pp. 143-171). Geological Society of America.

Croissant, T., Lague, D., Steer, P., & Davy, P. (2017). Rapid post-seismic landslide evacuation boosted by dynamic river width. Nature Geoscience, 10, 680-684.

Cui, Y., Parker, G., Pizzuto, J., & Lisle, T. E. (2003). Sediment pulses in mountain rivers: 2. Comparison between experiments and numerical predictions. Water Resources Research, 39(9), 1240.

Ferguson, R.I., Church, M., Rennie, C.D., & Venditti, J.G. (2015). Reconstructing a sediment pulse: Modeling the effect of placer mining on Fraser River, Canada. Journal of Geophysical Research: Earth Surface, 120(7), 1436-1454.
Hodge, R.A., Hoey, T.B., & Sklar, L.S. (2011), Bedload transport in bedrock rivers: the role of sediment cover in grain entrainment, translation and deposition, J. Geophys. Res..

Hoey, T.B., & Ferguson, R. I. (1994). Numerical-simulation of downstream fining by selective transport in gravel-bed rivers – model development and illustration. Water Resources Research, 30(7), 2251-2260.

Verhaar, P. M., Biron, P. M., Ferguson, R. I., & Hoey, T. B. (2008). A modified morphodynamic model for investigating the response of rivers to short-term climate change. Geomorphology, 101(4), 674-682.

Zhang, L., Stark, C., Schumer, R., Kwang, J., Li, T., Fu, X., et al. (2018). The advective-diffusive morphodynamics of mixed bedrock-alluvial rivers subjected to spatiotemporally varying sediment supply. Journal of Geophysical Research: Earth Surface, 123, 1731-1755.

Further Information

Please contact Dr Rebecca Hodge (rebecca.hodge@durham.ac.uk) or any of the other supervisors for further information on any aspect of this project. Note that there is scope to tailor the specific components of this project towards the individual interests of the student.

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