Modelling how sediment suspension controls the morphology and evolution of sand-bed rivers

Sand-bed rivers dominate the drainage of the Earth's surface. For example, the world's 10 largest rivers, that drain almost 20 per cent of global continental land and deliver 33 per cent of the terrestrial sediment supplied to the oceans, are all sand-bed channels. Many river catchments, in which sand-bed channels are present, are subject to anthropogenic activities such as dam construction, water abstraction, river engineering, or deforestation. As a result, the rivers in these catchments can experience sudden and catastrophic environmental problems such as major bank retreat that promotes building collapse, river bed aggradation and flooding, and channel shifting that leads to habitat degradation.

Despite over five decades of research, recent state-of-the-science reviews illustrate that we still lack the rigorous, quantitative, physically-based process understanding and modelling capability required to understand how rivers respond to environmental change. Our failure to develop such models to date has major environmental, social and economic ramifications, because most rivers on Earth are affected by natural or human-induced disturbances, the consequences of which we cannot yet predict robustly.

Dam construction, water abstraction, river engineering, deforestation and changing flood regimes are recognised as major catalysts of dramatic channel pattern change and associated bank erosion, loss of land and infrastructure, river bed scour or aggradation, habitat degradation and enhanced flood risk. These issues are a particular concern in the case of sand-bed rivers, because they are present at a range of scales from relatively small channels (widths <100 m) up to the largest rivers on Earth (widths 5-10 km). For example, the world’s 10 largest rivers (draining 17% of global continental land and delivering 33% of the terrestrial sediment supplied to the oceans) are all sand-bed channels.

Given the global significance of these issues, why have we struggled to make rapid progress in our ability to model and understand sand-bed river morphology and evolution?

This NERC-funded three-year project that started on 1 January 2015, led by Professor Phil Ashworth and Professor Andrew Nicholas, quantified the dominant hydro-dynamic processes operating in morphology the sandy-bed South Saskatchewan River, six hours east of Calgary, in Western Canada. Field data was used to set-up and then validate a numerical model of sandy braided river dynamics that will run over years to decadal time scales.

Strick, R.J.P., Ashworth, P.J., Sambrook Smith, G.H., Nicholas, A.P., Best, J.L., Lane, S.N., Parsons, D.R., Simpson, C.J., Unsworth, C.A. and Dale, J. (2018) Quantification of bedform dynamics and bedload sediment flux in sandy braided rivers from airborne and satellite imagery. Earth Surface Processes and Landforms. 44, 4, p. 953-972

Unsworth, C.A., Nicholas, A.P., Ashworth, P.J., Best, J.L., Lane, S.N., Parsons, D.R., Sambrook Smith, G.H., Simpson, C.J. and Strick, R.J.P. (2020) Influence of dunes on channel-scale flow and sedimenty transport in a sand bed braided river. Journal of Geophysical Research: Earth Surface. 125, 11, p. 1-26