How does changing bed topography during floods effect turbulence and flow resistance in rivers?


  • Supervisors: Dr James Cooper, University of Liverpool
    Dr Jonny Higham, University of Liverpool


  • External Supervisors:

  • Contact:

    Dr James Cooper, University of Liverpool, james.cooper@liverpool.ac.uk

  • CASE Partner:

Application deadline: 10 January 2020

Introduction:

The accurate estimation of flow resistance in rivers is of fundamental importance for predicting flood risk and erosion because it dictates how much energy is extracted from the flow by the channel. Thus, flow resistance governs flow properties such as mean flow velocity and water depth, and dictates the energy available for the flow to entrain, transport and deposit sediment. The key challenge is providing an accurate evaluation of this resistance during flood events when the bed morphology rapidly changes and affects how much energy is extracted from the flow. 

Generally, in rivers, a representative surface grain or roughness size is used to account for the effects of grain roughness on turbulence and flow resistance1,2. However, gravel river bed surfaces have systematic patterns of grain arrangement that change according to flow and transport conditions3,4. Our hypothesis is that a representative grain or roughness size therefore does not contain sufficient information to determine fully the influence of bed topography arrangement on flow resistance. Thus during a flood event the link between flow resistance and grain and roughness size does not hold. If this hypothesis is true, then gaining a new understanding of the physical link between grain arrangement and the generation of turbulence on flow resistance5,6 could be key to developing new river flow models for predicting flood risk and erosion.

Project Summary:

Using detailed measurements of hydrodynamics over gravel beds in a laboratory flume, the project will achieve the following objectives:

  1. Characterizing the evolution, geometry and stability of turbulent flow structures during flood events
  2. Understanding the effects of changing grain arrangement on these flow properties, and thus on the exchange of fluid momentum between the bed surface and the overlying flow
  3. Linking the geometrical and dynamic properties of the flow to grain arrangement so that more suitable parameterisations of flow resistance can be developed 

The student will use Liverpool’s state-of-the-art Particle Image Velocimetry (PIV) system to measure the flow field in great detail6. The system involves using a laser and high-speed cameras to image the movement of buoyant particles within the flow in 3D. The PIV system will allow the student to identify turbulent events and the role these events have on the exchange of momentum between the bed and the overlying flow, and thus on flow resistance. Co-located measurements of bed geometry will also be taken using Structure-from-Motion photogrammetry to provide sub-mm resolution DEMs. 

The new dataset collected from the experiments will be invaluable for the advancement of state-of-the-art 3D modelling approaches, as it will provide essential high-resolution measurements in the boundary layer. This will provide a unique source for model calibrations, helping to inform the direction of future model formulation.

References:

1 Bathurst J.C. (1985) Flow resistance estimation in mountain rivers, Journal of Hydraulic Engineering, ASCE, 111, 4, 625-643.

2 Aberle J. and Smart G.M. (2003) The influence of roughness structure on flow resistance on steep slopes, J. Hydraul. Res., 41, 3, 259-269.

3 Cooper J.R. and Tait S.J. (2009) Water-worked gravel beds in laboratory flumes: a natural analogue?, Earth Surface Processes and Landforms, 34, 384–397, doi: 10.1002/esp.1743.

4 Mao L., Cooper J.R., and Frostick L.E. (2011) Grain size and topographical differences between static and mobile armour layers, Earth Surface Processes and Landforms, 36, 10, 1321-1334, doi:10.1002/esp.2156.

5 Hardy, R.J., Best, J.L., Lane, S.N. & Carbonneau, P. (2010) Coherent flow structures in a depth-limited flow over a gravel surface: The influence of surface roughness. Journal of Geophysical Research: Earth Surface, 115, F03006.

6 Cooper, J. R., Ockleford, A., Rice, S.P. and Powell, D.M. (2018) Does the permeability of gravel river beds affect near-bed hydrodynamics?. Earth Surface Processes and Landforms, 43(5), 943-955, doi:10.1002/esp.4260.

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