Quantification and modelling of bedform dynamics in unsteady flows

Lead Research Organisation: University of Hull
Department Name: Geography, Environment and Earth Science

Abstract

The beds of most alluvial river channels are not flat, but comprise a series of undulating sedimentary accumulations termed 'bedforms' that include ripples and dunes. These bedforms exist over a range of scales, and are constantly moving and changing their shape, size and form in response to changes in flow discharge. These bedforms are the primary roughness elements that provide resistance to the water flow. The response of bedforms to a changing discharge is therefore critical for predicting flood inundation levels. Changes in flow discharge are more rapid than changes in the bedforms, such that bedforms are commonly out of equilibrium with the flow. This is very important as the vast majority of our bed-phase diagrams (stability field predictors that relate flow velocity and sediment size to the bedform types likely to be present), morphodynamic simulations, and numerical model predictions assume simplified bed morphologies that are based on equilibrium bed states and constant discharges. Consequently, many feedbacks within our models and predictions are either ignored or highly simplified. This is a significant shortcoming as it is these models that are used, especially in more populated and urban areas, to meet demands on safety against flooding, navigation, hydropower, aggregate mining and water supply. The astute management of these rivers is paramount, putting high demands on accuracy in design, implementation and monitoring. If such models are to be improved, then new fundamental understanding is required of the processes that underlie the dynamics of bedform adjustment to unsteady flow and ways of integrating such knowledge into modelling practice. As a step towards this goal, there is a need to link hydraulic controls, the response of sediment transport processes and morphological adjustment, and the changes in form drag and bed resistance to a range of unsteady flows. Once established, these relations can be used to help improve our understanding of these dynamic processes and predict better the river stage for a set of given discharge changes. This project will delineate these processes using a combination of (i) novel laboratory investigations in a state-of-the-art flume that will quantify the flow structure and sediment transport over fixed and mobile beds as stage varies, (ii) intense fieldwork during flood events in the Mississippi River that will map and quantify changes in bed morphology, flow structure and sediment transport, and (iii) development and application of an innovative numerical model of unsteady flow over a deformable 3D boundary. This modelling work will ensure that the results are generic and have a wider appeal, notably in the improvement of models that provide flood predictions and inform environmental management decisions. All data and output will be made freely available via scientific outlets but also through public dissemination events, the internet and via a GoogleEarth based XML interface.

Publications

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Burpee Alexander P. (2015) GRAIN-SIZE CONTROLS ON THE MORPHOLOGY AND INTERNAL GEOMETRY OF RIVER-DOMINATED DELTAS in JOURNAL OF SEDIMENTARY RESEARCH

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Hardy R (2014) Modelling time dependent flow fields over three dimensional dunes in Proceedings of the International Conference on Fluvial Hydraulics, RIVER FLOW 2014

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Reesink A (2018) The adaptation of dunes to changes in river flow in Earth-Science Reviews

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Simmons S (2012) Field methods for imaging suspended sediment dynamics and fluxes with a multi-beam echosounder in 11th European Conference on Underwater Acoustics 2012, ECUA 2012

 
Description The work is still ongoing, however some important results are emerging. For example, flume runs have shown that sedimentary structures, such as cross-stata and planar bedding, are known to be produced by a range of bedform related processes and that underpin a range of sedimentary environment reconstructions (that are based on scaling laws and stability diagrams produced from rather simple laboratory flume based fluvial research) do not account for flow variability. This variability can make certain processes to be dissimilar, particularly the difference in velocity profile shape and the position of the velocity maximum in the flow. The novel laboratory research we have conducted has tested our understanding of fluvial bedform stability and scaling, whilst also facilitating an examination of bedforms produced by velocity profiles approaching those in density driven flows.
Results from a tilting, recirculating flume with a medium coarse sand show that there is a stronger relationship between the position of the downstream velocity maximum in the vertical profile and the equilibrium bedform geometries produced than for the flow depth. This demonstrates that using flow depth to define dune bedform scaling, which is the case in many stability diagrams and models, is perhaps not optimal. The experiments produced a variety of dune shapes and sizes which were quantified when at equilibrium.
Progressive suppression of the velocity maximum toward the bed in each of the experimental runs eventually increased the bed shear stress to levels sufficient for production of an upper stage plane bed whilst depth and depth-averaged velocity were in the middle of the dune regime for most bedform stability diagrams. This has implications for not only how density-driven flows produce bedforms but also for a range of fluvial flows, as any alteration of the velocity profile shape will induce a change in either the shape or type of bedform produced. Such profile shape changes commonly occur during flood pulses and/or from significant secondary circulation, such as at channel bends or within confluences, impacting our understanding of many aspects of bedform scaling in fluvial systems.
Exploitation Route Use for new scaling relations with further experiments. Impact is already been achieved via collaborations detailed in the impact narrative.
Sectors Energy,Environment,Transport

URL http://www.bedform.co.uk
 
Description We have been working with the US Geological Survey on flood propagation on the Mississippi and its modelling. This impact is at an early stage. We have also been working with ADM on shipping at low flows and predicting the height of dunes to assist in the efficient shipping of grains along the Mississippi.
First Year Of Impact 2013
Sector Agriculture, Food and Drink,Energy,Environment,Transport
Impact Types Societal,Economic

 
Description ERC Consolidator
Amount € 2,200,000 (EUR)
Funding ID GEOSTICK 
Organisation European Research Council (ERC) 
Sector Public
Country European Union (EU)
Start 04/2017 
End 11/2022