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School of Geography

Dynamics of flow resistance and sediment entrainment in gravel-bed rivers

Project Outline
Shallow flows over partially- or marginally-inundated gravel surfaces are often complex and three-dimensional. The assumptions underlying conventional flow resistance equations do not apply under such conditions. Spatial variability of gravel beds is observed natural gravel bed rivers; homogeneous aggregations (‘patches’ and ‘micropatches’) have been reported at a variety of scales. This patchiness is shaped by the spatial variability of riverine processes and, in turn, effects sediment entrainment. Thus, changes in the magnitude, frequency, direction or duration of forces exerted by a moving fluid will modify surface morphology through (de-)stabilisation of bedforms, fine sediment entrainment (Vericat et al., 2008), armouring, imbrication, etc. 

Changes in the structure and composition of the bed will affect near-bed hydraulics and functional relations between flow intensity and sediment transport.  Given the relatively poor performance of such relationships (Gomez and Church, 1989), a greater understanding of this feedback is essential to improve forecasts of bed stability and channel adjustments to rapid climate and land-use change, as well as likely ecological responses. The interaction of near-bed flows with rough, patchy, gravel substrates will be a function of particle size distribution, bed arrangement, particle shape and orientation, packing, spacing and sorting, the presence of particle clusters, etc., more appropriately characterised as a random field of elevations (Aberle and Nikora, 2006). The consequent non-unique relationship between surface morphology and grain-size has prompted many flume studies to apply similar statistical approaches of characterising gravel-bed roughness (e.g. Robert, 1988; Nikora et al., 1998; Nyander et al., 2002).

This project aims to evaluate the two-way interaction between near-bed hydraulics and bed morphology in a field setting, thereby avoiding issues surrounding the representativeness of laboratory flumes. Combining surface roughness analysis with measurement of three-dimensional velocity fields (using Acoustic Doppler Velocimeters, for example) permits investigation of the hydrodynamics associated with surface roughness. High bed shear stresses can be simulated in the field using basic flow manipulations and the resulting effect on bed morphology analysed using through-water terrestrial laser scanning (Smith et al., 2012). Such an understanding will have direct implications for the setting of environmental flows within modified discharge regimes, providing an essential hydraulic and geomorphological basis to subsequent ecological surveys. 

Skills
Training in research skills and discipline specific skills on hydraulics and process geomorphology will be provided as part of the River Basin Processes and Management research clusters in the School of Geography. Full training in Terrestrial Laser Scanning techniques will be provided, though it is anticipated that the successful candidate will have a background in spatial analysis and some GIS, GPS and field surveying experience. This project involves data collection in the field, though no specific location is identified.

Student profile
The prospective student should have, or expect to receive, a first class BSc degree, or a distinction at Masters level, in an appropriate discipline. They should have interests and experience in most, if not all, of the following topics: spatial analysis, geomorphology, surveying and hydraulics.

Enquiries
Informal enquiries should be directed to Mark Smith at m.w.smith(at)leeds.ac.uk.
Enquiries relating to the application process and funding can be sent to Jacqui Manton (j.manton(at)leeds.ac.uk )

References

  • Aberle, J. and Nikora, V. 2006. Statistical properties of armoured gravel bed surfaces. Water Resources Research 42, W11414. DOI:10.1029/2005WR004674
  • Gomez, B. and Church, M. 1989. An assessment of bed load formulae for gravel bed rivers. Water Resources Research 25, 1161?1186.
  • Nikora, V.I., Goring, D. and Biggs, B.J.F. 1998. On gravel-bed roughness characterisation. Water Resources Research 34, 517?527.
  • Nyander, A., Addison, P., McEwan, I. and Pender, G. 2002. River bed surface roughness analysis using 2-D wavelet transform-based methods. 15th ASCE Engineering Mechanics Conference, Columbia University, New York, USA.
  • Robert, A. 1988. Statistical properties of sediment bed profiles in alluvial channels. Mathematical Geology 20, 205?225.
  • Smith, M.W., Vericat, D. and Gibbins, C. 2012. Through-water terrestrial laser scanning of gravel beds at the patch scale. Earth Surface Processes & Landforms 37, 411?421. doi: 10.1002/esp.2254
  • Vericat, D., Batalla, R.J. and Gibbins, C.N., 2008. Sediment entrainment and depletion from patches. Water Resources Research 44, W11415, doi:10.1029/2008WR007028.