Geography and Environment

Staff

Dr Annie Ockelford PhD (Glasgow)

Photo of Dr Annie Ockelford

2015- : Lecturer in Physical Geography, University of Brighton

2014-15: Post-Doctoral Research Associate (PDRA), University of Hull

2011- 2013: Post-Doctoral Research Associate (PDRA), Department of Geography, Loughborough University 

2007- 2011: PhD, Department of Civil Engineering, University of Glasgow

Professional responsibilities

  • Chair, Outreach Committee for the British Society for Geomorphology
  • Fellow, Higher Education Academy
  • Fellow, Royal Geographical Society
  • Royal Geographical Society Ambassador
  • STEM ambassador
  • Nuffield Research Placement Provider and Project Assessor

My research area concentrates on the analysis of river bed stability and structure using high resolution, high accuracy data capture methods.  Specifically;

1)    CHORUS - CHaracterisation Of Riverbed sUrfaceStructure

Funded by NERC (2010-2013) Professor Stephen Rice (Loughborough), Dr Mark Powell (Leicester), Dr Annie Ockelford (Loughborough), Dr Thao Nguyen (Leicester), Dr Nick Tate (Leicester), Professor Jo Wood (City), Professor Ian Red (Loughborough)

Changes to the sediment boundary in gravel bed rivers to accommodate both the overlying fluid flow regime and the upstream sediment supply fundamentally changes the overall stability of the surface.  Small scale processes such acting at the grain scale cause little to no change to the surface grain size distribution yet can still have significant effects upon the overall stability (i.e. the resistance to entrainment) of the surface.  Stabilisation by large scale processes such as river bed armouring occur over vast areas of the sediment bed and act to significantly change both the surface grain size distribution together with the surface characteristics.  Yet, to date describing this structure and explicitly linking this development with changes to the overlying hydraulic signatures and sediment transport characteristics has been severely limited.  As such a series of flume experiments run at Loughborough will attempt to address this deficiency to specifically examine how sediment grade, sediment supply characteristics and hydrograph shape affect the development of surface structures in gravel-bed rivers.

2)    Quantifying the Effect of Porosity on Vertical Momentum Exchange in Gravel-Bed Rivers

Dr Annie Ockelford (Loughborough), Professor Stephen Rice (Loughborough), Dr James Cooper (Sheffield), Dr Mark Powell (Leicester)

Momentum exchange is a key control of fine sediment ingress, pollutant exchange spawning success and hyporheic flows with porosity having a significant influence on this exchange processes.  Previous attempts to examine this exchange have been undertaken in an engineering context where experimental conditions lack the complexities of natural river beds.  Consequently it has not been possible to isolate the influence of bed porosity from that of bed topography. This on-going research uses bed surface casts to directly compare between the hydraulics of porous and non-porous beds of the same topography.  This will allow us for the first time, to isolate the influence of bed porosity, across sorting and structure gradients.

3)    The Effect of Stress History on River Bed Stability

Funded by EPSRC (2007-2010 held by Haynes) Dr Annie Ockelford (Loughborough), Dr Heather Haynes (Herriot Watt)

Historically the inter-flood period has been disregarded from investigations as it was deemed that the stability of non-cohesive beds could only be altered by above threshold flows capable of sediment transport. However, more recent data demonstrates that periods of sustained low flow alter the timing of sediment re-mobilisation due to sediment bed restructuring; this effect has been termed ‘stress history’.  My PhD research concentrated on two main themes (i) analysing the grade dependent controls on river bed response to stress history and (ii) analysing the underpinning mechanisms responsible for this stress history response using laser displacement scanning.  Results showed   increasing the antecedent flow duration significantly increased the river bed stability such that the critical shear stress increased by up to 25% where uniform beds are more responsive to antecedency than graded beds. Laser based analysis reveals that vertical settlement, localised changes to bed roughness, pockets of more pronounced development of hiding effects, and particle repositioning are all mechanisms by which the bed reorganises under an applied sub threshold flow. However, the different bed grain size distributions cause significant differences in the importance of each mechanism in determining the magnitude of stress history induced bed stability.  Access to my PhD thesis can be found from the following weblink;

http://theses.gla.ac.uk/2577/

4)    Using Novel Data Acquisition Techniques to Describe Bed Surface Porosity

Funded by BSG (2011) Dr Annie Ockelford (Loughborough), Dr Heather Haynes (Herriot Watt), Dr Elisa Vignaga (Scottish Water)

The vertical structure of river beds varies temporally and spatially in response to hydraulic regime, sediment mobility, grain size distribution and faunal interaction. Implicit are changes to the active layer depth and bed porosity.  Whilst measurements of the bed surface are increasingly informed by quantitative and spatial measurement techniques (e.g. laser displacement scanning), material opacity has precluded the full 3-D bed structure analysis required to accurately define the surface-subsurface transition. This research aimed to provide an improved definition of the surface-subsurface transition depth by using vertical bed porosity profiles to directly compare between 3D MRI scanning and 2.5D laser scanning. Results showed that the surface sub-surface transition occurred where variability in the vertical profile was 0 gained from the 3D MRI data.   This mapped well onto where 2.5D laser based derivations suggested the surface sub-surface transition occurred and provides robust validation for use of surface-based laser scanning techniques.

Powell, M, Ockelford, A, Rice, S.P., Hillier, J, Nguyen, T, Reid, I, Tate, N and Ackerley, D (2016) Structural properties of mobile armors formed at different flow strengths in gravel-bed rivers. Journal of Geophysical Research: Earth Surface, 121(8), 1494–1515.

Ockelford, A and Haynes, H (2012) The impact of stress history on bed structure Earth Surface Processes and Landforms, 38 (7). pp. 717-727. 

Haynes, HOckelford, AVignaga, E and Holmes, W (2012) A new approach to define surface/sub-surface transition in gravel beds Acta Geophysica, 60 (6). pp. 1589-1606.

Ockelford, AVignaga, EHaynes, H, Holmes, W and Bertram, D (2011) Comparison of 2D laser scanning with 3D MRI scanning; Analysis of graded gravel bed porosity In: Proceedings of the 34th IAHR 2011 Balance and Uncertainty:water in a changing world., Brisbane, Australia. 26th June - 1st July 2011.

Ockelford, AHaynes, HHaynes, R and Hodge, R (2010) Using high resolution laser scanning to indicate mechanisms of stabilisation under varying sub threshold flow exposures In: proceedings of the 17th Congress of Asia and Pacific Division of International Association of Hydraulic Engineering and Research (IAHR-APD 2010), Auckland, New Zealand February 2010.

Haynes, H and Ockelford, A (2009) A Comparison of Time-Induced Stability Differences Between a Framework-Supported and a Matrix-Supported Gravel:Sand Mixture In: 33rd IAHR World Congress Water Engineering for a Sustainable Future, Vancouver, Canada 9th-14th August 2009.

Ockelford, A and Haynes, H (2008) The Effect of Grain Size Distribution Modality on the Relationship Between Stress History and Entrainment Threshold In: River flow 2008 :proceedings of the 4th International Conference on Fluvial Hydraulics, Cesme-Izmir, Turkey September 2008.