How does the development of particle scale structure control river scale morphology?

Lead Research Organisation: Durham University
Department Name: Geography

Abstract

The transport of sediments is a key process in the global geological cycle, a cornerstone of aquatic ecosystems and has a multi-billion pound impact on agricultural, industrial and urban, flood- and erosion-risk hazards. Understanding and being able to predict the stability of gravel river beds is important for multiple reasons: changes in river bed shape will change the channel capacity and thus affect flood risk; the bed stability affects the amount of sediment that can be moved by the flow, which will have impacts on the downstream channel morphology and dynamics; the river bed is a habitat for many species, and thus changes in the river bed will have implications for the river ecosystem; and in order to manage and restore rivers effectively, channels need to be designed with a known level of stability.

However, current ability to predict sediment entrainment and thus river bed stability is limited by our understanding of the factors that affect sediment movement. Grain size is typically accounted for, but other factors such as sediment structure (the way in which individual sediment grains are packed together in 3D) and the role of fine sediments in cementing grains together are not. Furthermore, these factors vary spatially across the bed of a river, producing a spatial pattern of areas that are more or less easily entrained, i.e. a template of erodibility. We hypothesis that this spatial pattern of erodibility plays an important role in controlling both the shape of the river bed, and how this shape changes under different flow conditions.

We will test this hypothesis by quantifying, for the first time, the development of 3D sediment structure in both a field and a laboratory environment using high energy CT-scanning. These data will allow us to identify causal relationships between the different controls and sediment structure. The application of this technique to large numbers of samples from both field and laboratory settings will provide a significant and unique dataset for understanding the structure and production of 3D bed sediments.

Using an existing theoretical framework, we will use the data from both the flume and field data to produce relationships that can be used to predict sediment structure, and consequently the erodibility of the bed, from the controlling factors of sediment input and flow. This relationship will be implemented within a numerical modelling framework in which we will upscale from the field and flume to represent additional range of channel and flow conditions. We will work with end-users to ensure that the new knowledge is transferred effectively into guidance for policy and operational activity within the river management community.

Planned Impact

The beneficiaries from this research are:

1. UK and international users of morphological models ISIS-Sediments and Delft3D, and Civil Engineering companies Halcrow and Deltares
A key outcome of this research will be the development of relationships quantifying the impact of fine sediment delivery and bed morphology on sediment stability. As part of the Pathways to Impact, we will facilitate the implementation of our relationships into the widely-used morphological models ISIS-Sediments (see letter of support from Halcrow) and Delft3D (see letter of support from Deltares). We will also undertake testing and validation of ISIS-Sediments. This will produce a significant improvement in the predictive capability of these models.

2. Flood and erosion-risk management personnel in the Environment Agency (EA), Scottish Environment Protection Agency (SEPA), and other conservation bodies
The improved understanding of sediment transport that this project will provide is essential for assessing the impact of sediment transport on flood risk, and thus flood and erosion risk management. As outlined in the Pathways to Impact, we will engage with key end-users from an early stage in the project. Key project partners the EA, Halcrow and Deltares (see letters of support) will be tasked to create a network list for key end uses of this research in order to interact with a wide end-user base and to tailor the dissemination to existing requirements. The user list will include SEPA and conservation bodies. These end users will benefit in the following ways:

a) Project website:
This will be used to disseminate the research findings, publications, datasets and updated model code.

b) Improvement of ISIS-Sediments and Delft3D
Users of these morphological models will benefit from their enhanced predictive capability.

c) Key project outcomes disseminated by EA via a guidance note and Flood and Coastal Erosion Risk Management (FCERM) Research news
Research findings will be disseminated to key users via the EA, enabling them to implement more effective flood and erosion-risk management.

d) Knowledge exchange workshop
A knowledge exchange workshop on Sediment Management will be held for UK practitioners. This will broaden out from the specifics of the project results (though incorporating the new research) to cover aspects aimed at producing a foundation for new policy and practice in sediment management, including case studies from the USA, and Netherlands

e) International dissemination via sediment management for practitioners workshops
At no cost to the project, international practitioners will benefit from incorporation of the project outputs into training courses currently run by the project's Visiting Researchers.

3. Scientists from a range of disciplines
The datasets and findings produced by this research will be of direct interest to scientists from a range of disciplines including fluvial geomorphology, river management and restoration, aquatic ecology, CT scanning and sedimentology. See 'Academic Beneficiaries' for further information.

Publications

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Description The aim of the grant was to use CT scanning to quantify the structure of the sediment that comprises river beds. This information is important to understand the stability of these beds, and the data will enable us to improve models of sediment transport. During the grant we extracted samples of river bed sediment from flume experiments and from our field site. During the grant we developed a method to process the CT scans of these samples and to extract the properties of the sediment grains that we are interested in. The data collection was more difficult than we anticipated because the samples have a high density and require a particularly long scan time. The high desnity also meant that the scans were difficult to process because of the low contrast between sediment grains and other material. We finally developed a dataset of over 1000 individual sediment grains from across ~20 different samples. We also developed a new theoretical model of grain entrainment, that uses our new 3D data to estimate the force required to entrain any individual sediment grain. By combining the data and the model we looked for the grain properties that best predicted how stable any individual sediment grain is. We found that grain protrusion (i.e. the amount that a grain sticks up) was most important. This is in contrast to previous work that has foucussed on instead on grain pivot angle. We are now considering how we can make our findings more widely applicable, e.g. developing easily applicable techniques to measure grain protrusion and hence bed stability.
Exploitation Route We held an impact workshop in July 2016, which was attended by the Environment Agency and other organisations. We originally planned to use our data to produce a sediment transport model, which could be applied within morphodynamic models of river channels, however this has not been possible within the project duration.
Sectors Environment

URL http://www.nercpatches.org/
 
Description Impact workshop 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact We ran a workshop for members of the Environment Agency and other environmental consultancies. This presented our work, and broader issues around sediment in rivers. On the following day we attended a field day with the EA.
Year(s) Of Engagement Activity 2013,2017