Isolating the key scales of river bank roughness as drivers of erosion through high-resolution monitoring and modelling

Overview

River bank erosion is a major global hazard, causing millions of people to be displaced annually. The primary driver of river bank erosion is the stresses imparted on the river bank by the flow of water. Topographic undulations in the form of the river bank increase form drag, which alters the hydraulic forces and the flow close to the bank. As hydrological regimes around the world are changed as a result of climate change, the magnitude and frequency of high and lows flows will change, and along with them the stress placed on the river bank. The erosion and deposition of sediment by fluvial bank erosion plays a pivotal role in maintaining the ecological and geomorphological diversity of fluvial channels. Previous research has shown how the rate at which sediment is exhumed from floodplains by the processes of bank failure, sediment entrainment, and transportation has far reaching implications for geomorphology, ecology, infrastructure management, and nutrient and contaminant tracking (Florsheim et al., 2008; Camporeale et al., 2013). As such, there is a need to better understand the relationship between flow and form as a driver of bank erosion, at a range of river scales and in a changing climate.

Previous research has suggested that the roughness of the river bank caused by the topography, geology and delivery of large blocks of failed material (slump blocks) to the bank toe, reduces hydraulic action and thus can act to slow down rates of bank erosion (Hackney et al., 2015; Konsoer et al., 2017). Bank roughness may deflect high velocity flow away from the bank (Wood et al., 2001). However, recent field observations collected from high resolution (sub-centimeter) datasets, have shown that when fully submerged, bank roughness, and slump blocks in particular, may deflect flow up, over and toward the bank, enhancing rates of bank erosion (Hackney et al., 2015). As high flows become more frequent, the role that bank roughness play in driving erosion becomes increasingly important. Important questions remain as to whether large and small rivers behave the same way, and whether roughness at the bank toe is more important than roughness at the bank top. Advances in high-resolution laser and sonar technologies allow us to capture the processes occurring at a river bank during a range of flows, at a range of river channel scale, at previously unattainable resolutions (Leyland et al., 2017). These new datasets hold the key to better understanding, and managing, the role that river bank form plays in driving erosion rates.

Methodology

Our ability to capture and model the flow near to, and form of, river banks has increased dramatically over recent years. This project will utilise state-of-the-art field and numerical modelling techniques (see figures) to isolate the impact that varying scales of river bank roughness play in driving bank erosion at a range of flow magnitudes. As such the project is split into two components:
1) Fieldwork: The student will collect high resolution (sub centimetre) topography of river banks using terrestrial laser scanners to capture bank roughness. Data will be collected on a range of river scales, from the world’s largest rivers (e.g. the Mekong) to actively eroding banks at the River Coquet, UK. Scans will be repeated through the course of year to capture bank erosion rates and changes in bank topography. This will be coupled with high-resolution flow data collection using Acoustic Doppler Current Profilers that can resolve three-dimensional flow close to the river bank. Surveys using an autonomous vehicle will be used capture flow at a range of flow stages to allow the relationship between flow, topography and erosion to be fully understood.
2) Numerical modelling: Develop an existing three dimensional numerical model that predicts flow-form interactions. The field data collected in the fieldwork will be used as model boundary conditions and validation data. The model will predict the drag created by the riverbank morphology and riverbank zones that will experience either erosion or deposition.

The two approaches will allow a re-assessment of flow bank form interaction and our ability to explore bank erosion under different inherited flow conditions.

Project Timeline

Year 1

Literature review and identification of fieldwork sites. Training in field equipment, data analysis and processing. Initial field campaigns starting from winter of year 1 to capture high flows, progressing through end of year 1 and into year 2.

Year 2

Continuation of fieldwork and data analysis and processing, identification of different roughness scales and links to near bank flow dynamics and erosion rates. Training on numerical model set up and initial model set up and parameterisation. Attendance at national conference to present findings.

Year 3

Numerical modelling simulations to isolate the role of different roughness scales on flow, stress and bank erosion. Attendance at international conference to present findings.

Year 3.5

Completion of thesis write up and preparation of manuscripts

Training
& Skills

During the PhD the student will gain necessary training and skills in both field data collection and processing (Newcastle) and numerical model set-up parameterisation and use (Durham). The student will become proficient in high-resolution laser-scan data collection and processing as well as the collection and processing of three-dimensional flow characteristics. Training in Computational Fluid Dynamics (CFD) will be provided at Durham and the student will be be trained to use field data to parameterise and run CFD simulations.

References & further reading

Camporeale, C., E. Perucca, L. Ridol, A. M. Gurnell (2013), Modeling the interactions between river morphodynamics and riparian vegetation, Reviews of Geophsyics, 51, 379 – 414

Florsheim, J. L., J. F. Mount, A. Chin (2008), Bank erosion as a desirable attribute of rivers, BioScience, 58(6), 519-529

Hackney, C.R.. Best, J., Leyland, J., Darby, S.E., Parsons, D., Aalto, R., Nicholas, A. (2015) Modulation of outer bank erosion by slump blocks: Disentangling the protective and destructive roles of failed material on the three-dimensional flow structure, Geophysical Research Letters, 42, 10,6333 – 10,670.

Konsoer, K., Rhoads, B., Best, J., Langendoen, E., Ursic, M., Abad, J., Garcia, M. (2017) Length scales and statistical characteristics of outer bank roughness for large elongate meander bends: The influence of bank material properties, floodplain vegetation and flow inundation, Earth Surface Processes and Landforms, 42, 13, 2024 – 2037.

Leyland, J., Hackney, C.R., Darby, S.E., Parsons, D.R., Best, J.L., Nicholas, A.P., Aalto, R., Lague, D. (2017), Extreme flood-driven fluvial bank erosion and sediment loads: direct process measurements using integrated Mobile Laser Scanning (MLS) and hydro-acoustic techniques, Earth Surface Processes and Landforms, 42, 334 – 346.

Konsoer, J. Wood, A. L., A. Simon, P. W. Downs, C. R. Thorne (2001), Bank-toe processes in incised channels: The role of apparent cohesion in the
entrainment of failed bank materials

Wood, A.L., Simon, A., Downs, P.W., Thorne, C.R. (2001) Bank toe processes in incised channels: The role of apparent cohesion in the entrainment of failed bank materials, Hydrological processes, 15(1), 39 – 61.

Further Information

Dr Chris Hackney, Christopher.hackney@ncl.ac.uk, 0191 2087920

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