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

Lead Research Organisation: University of Exeter
Department Name: Geography

Abstract

Sand-bed rivers dominate the drainage of the Earth's surface. For example, the world's 10 largest rivers, that drain almost 20% of global continental land & deliver 33% 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 the environmental, social and economic significance of these rivers, we have struggled to produce robust models of how sand-bed rivers work, how they transport their sediment, how rivers change over decades and centuries, how they produce the variety of channel patterns we see in the world, and how rivers respond to a change in environmental drivers such as climate, erosion rates and human interference.

Very recent research indicates that the morphology, functioning and pattern of sand-bed rivers is strongly dependent upon whether the sand that they carry is transported in suspension (i.e. carried in the water column) or as bedload (moving in contact with the bed). In addition, theory suggests that, over the range of sediment sizes and flow conditions that are typical of sand-bed rivers, there is a dramatic shift from bedload to suspension-dominated sand transport. However, the physical mechanisms that control the link between how sand is transported and the resulting river morphology remain largely unexplained. This project will develop new models and quantitative understanding of the role of sediment suspension as a control on the morphology of sand-bed rivers. We will do this by implementing a research strategy that involves three key elements: First, we will apply an innovative image acquisition technique to obtain datasets that quantify river bed morphology at very high spatial resolutions (cm) over large areas (km) and multiple timescales (days to years). Second, we will use state-of-the-art field instrumentation to obtain concurrent measurements of flow and sediment transport processes and their relationship to river morphology over a range of discharges. Third, we will develop and apply two- and three-dimensional numerical models to quantify the interactions between riverine processes and channel morphology at bedform, bar and whole river scales. We will use field datasets to test our models in sand-bed rivers of different sizes and with contrasting flow regimes and bed sediment texture. Once validated, our models will provide robust new tools, which we will release as open-source code to the scientific community, for predicting and understanding how sand-bed rivers respond to environmental change. This research will also have significant end-user and educational benefits, which we will realise by working closely with project partner HR Wallingford, and by producing a collection of high-quality learning materials and teaching resources aimed at the Geography A-level curriculum, and released via national organisations with a strong commitment to educational outreach.

Planned Impact

The fundamental science advances that will be delivered by this project have direct relevance to a wide range of agencies concerned with economic, social, management and policy issues.

Key beneficiaries of this work within the UK include the Environment Agency, environmental consultancies and independent engineering organisations, who design water and sediment management solutions to diverse problems in river and floodplain environments. Numerical models are key tools used by these agencies to predict flooding, river flows and sediment movement, and the advances in modelling made in this project will be of direct relevance to their work, both in terms of their use of models and plans for future model development. In addition, the novel data acquisition technologies that will be deployed in this research (both the UAV quadrocopter used to acquire very high resolution imagery and digital elevation models, and the acoustic instrumentation used to obtain concurrent measurements of 3D flow and sediment transport) have application within both the EA and engineering consultancies, who require similar datasets and will benefit from the methodological developments made and knowledge gained during this project. In order to capture the expertise and networks within this group of end-users we will deliver a workshop at the end of year 2, hosted by project partner HR Wallingford. The workshop will focus on technological developments for measuring stream flow, sediment flux and remote sensing (via UAV) of wet and dry zones of the river and floodplain.

Other potential beneficiaries of knowledge arising from this research include the hydrocarbon industry, which uses both process-based and probabilistic models to characterize alluvial reservoir heterogeneity, and would benefit from the potential for simulating large-scale channel belt sedimentary architecture that the models developed by this project will deliver. We will ensure that these benefits are realised by exploiting our existing links within the industry (see investigator and VR CVs) to disseminate this research through a workshop run in Houston. The workshop will be self-financing using funds from a consortium of oil companies using existing contacts.

The numerical models that will be developed during this project have wide ranging beneficiaries drawn from academia, industry and government agencies. To maximize uptake of these models, and associated benchmarking datasets, we will make these available via multiple access points, including the project web site, the NERC Environmental Information Data Centre, and the NSF-funded Community Surface Dynamics Modeling System (CSDMS) (see letter of support).

We will also produce a collection of educational materials and teaching resources aimed at the Geography A-level curriculum (years 12 and 13) and addressing themes of river channels, floods and management, and human impacts. We will release these materials via the project web site, and via organisations that actively pursue Educational outreach, including the Royal Geographical Society and British Society for Geomorphology.

The Impact Plan will be delivered through a combination of workshops, a user-friendly project website, and close liaison with Project Partners. We have a good track record of delivering on these types of activity and will be able to generate interest and uptake within industry, environmental and engineering consultancies, the press and the general public.
 
Description We have combined field monitoring and numerical modelling to quantify and understand the interplay between river bed topography, water flow and sediment transport in the sandy braided South Saskatchewan River, near Outlook (SK Province Canada). We have carried out multiple phases of field work focused on a c. 20 km stretch of river, for which we have obtained several sets of repeat aerial imagery. Field work involved carrying out ground-based differential global positioning system surveys to locate ground control points within these image sets. These data have been processed using structure from motion photogrammetric techniques and a statistical model between flow depth and image brightness. Using these techniques we have generated a set Digital Elevation Models of the river bed and floodplain topography over a range of spatial resolutions (pixels of 5cm-2m). Spatially distributed velocity and sediment transport data have also been collected using acoustic Doppler current profiler surveys. This information has been used to set up a set of two-dimensional and three dimensional numerical models, using the OpenFOAM CFD package and model code developed during the project. Numerical simulations have been carried out to investigate the role of interactions between bedform-scale and channel-scale topography as controls on flow structures, sediment transport and channel evolution. Our findings are that:

1) River bathymetry and its evolution can be mapped effectively using photogrammetric techniques
2) This provides a means of calculating rates of sediment transport and bedform migration in sand bed rivers
3) Bedforms (sand dunes) vary substantially in their size and rate of migration in both time and space
4) Bar migration rates can be used, in combination with numerical models, to provide a new means of measuring sediment flux in sand-bed rivers
5) Bedforms play an important role in influencing flow, sediment transport and the evolution of river bathymetry at larger spatial scales
Exploitation Route We have developed numerical models and field methods that could be used by environmental scientists and engineers to monitor or predict river evolution.
Sectors Environment