The slippery slope to resolving enigmatic river system dynamics

Background

Fluvial systems, from mountain streams, rivers to estuaries determine the transport of sediment, nutrients and pollutants across the Earth’s surface. As the principal vectors connecting land to ocean, they are uniquely critical to life on land and below water. The evolution of fluvial channels by sediment transport, erosion and deposition, i.e. morphodynamics, control these transport vectors. Prediction of morphodynamics up to river system scale is central to our understanding, management and hazard mitigation of the natural environment. However, a fundamental challenge remains as key processes scale between the microscale of sediments (𝜇m) to the macroscale river systems (100’s km).

Critically, existing models do not accurately predict natural dynamics at the mesoscale (the bridge between microscale and macroscale) of channels and bars (i.e. sandbanks). This is important as such scales are the limiting resolution of macroscale system models. Mesoscale models are flawed as they are based on either microscale physics or calibration parameters. They lack accurate physical descriptions of morphodynamics where the bed topography is not flat^[1,4].  For example, channels and bars, which are the fundamental building blocks that form any fluvial or estuarine environment, are built by the interaction of along-channel currents and downslope sediment transport on the side-slopes of the channels. By balancing along-channel erosion, the amount of downslope sediment transport critically determines local channel morphology and thus mesoscale dynamics. Thus, sediment transport on slopes depend on the interaction of micro to mesoscale processes that are not understood and are therefore over-simplified in current models. As a result, all macroscale models need severe calibration to reach a stable equilibrium or resemble measured data^[1,2,3]. Critically macroscale models are more sensitive to calibration parameters than to forcing due to environment change.

Aims & objectives

The aim of this PhD is to advance a new understanding of the micro to mesoscale physics that control sediment transport and link this to macroscale transport models. This will be achieved through the following four objectives:

• Experimentally test controls on suspended sediment over cross-sloped channels
• Develop and execute high-fidelity numerical models to resolve complex flow-particle interactions
• Derive parametrisations describing suspended sediment transport using the experimental and numerical results
• Implement and test the new physical suspended sediment model in an idealized fluvial system

Methodology

This project will adopt physical experiments and Computational Fluid Dynamics (CFD) to study the suspension, transport and deposition of fine sediments on sloped beds. The experiments will be conducted in a new stratified recirculating flume, in which a range of environmental conditions can be varied, such as water density stratification, current velocity and sediment characteristics. This flume set-up uses state-of-the-art optical flow metrology which allows for the tracking of suspended particles and will therefore afford a new standard of data quality to resolve the microscale physics of sediment suspension in an idealized channel.

CFD will be adopted to numerically resolve mixing of sediment in sloped channels. In particular, high-fidelity techniques such as Large-Eddy and Direct Numerical Simulations. These simulations will enable a wide range of parameter studies to be carried out, validated against laboratory experiments. Finally, results will be used to develop parametrisations of sediment transport processes and tested in a system-scale model.

Impact

This project will lead to a step-change in our understanding of, and ability to fully and correctly parameterize, the microscale processes of fine sediment transport. Understanding the dynamics of fine sediments is crucial to understand the behaviour of fluvial systems under changing environmental conditions. Without this, it is impossible to make reliable predictions of e.g. increased flood risk, the effectiveness of management strategies and engineering interventions, shipping fairway maintenance and port access, the dispersal of pollutants such as microplastics, and ecological restoration measures.

Find out more via our free webinar

The University of Hull is running a webinar at 6pm on 21 November to provide more information about this project and the eight other projects hosted by Hull. The webinar will close with a Q&A giving you the opportunity to delve deeper into research opportunities, training provision and potential career paths. Book your place.

References: [1] Baar et al. (2019), Nature Communications 10(1), 1-12. [2] Schuurman et al. (2013), Journal of Geophysical Research: Earth Surface 118(4), 2509-2527. [3] Van der Wegen & Roelvink (2012), Geomorphology 179, 152-167. [4] Hepkema et al. (2019), Journal of Geophysical Research: Earth Surface 124(10), 2417-2436.