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Recent evidence reveals that predictive models accounting for the interactions between near-bed coherent flow structures, ripple morphology and particle dynamics in wave-induced flows are needed. As a first step towards a fundamental understanding of the complex nature of ripple regime sand transport, a detailed study of the role of coherent flow structures on erosion is required. Without this information it will remain impossible to predict accurately sediment erosion rates, nor understand the role that coherent flow structures have on other processes such as seabed morphological evolution and the transport and dispersion of pollutants and nutrients. This information is crucial if we wish to understand how the increased threat of climate change and the resulting rise in sea level may accelerate erosion.
Aims and objectives
The studentship aims to understand the role of coherent flow structures in sand transport in the nearshore zone of coastal environments. The project will focus on the following objectives:
Characterizing the coherent flow structures present in the near-bed region over ripples in oscillatory flow
Understanding the effects of ripple morphology, flow depth and wave period and amplitude (as per rise and fall of the tide) on the size and dynamics of these flow structures
Linking the properties of the flow structures to particle entrainment and transport so that more suitable parameterisations of wave-induced sand transport can be developed
The project will involve an extensive set of controlled laboratory experiments in the Liverpool Coastal Flow Channel. The channel generates oscillatory flows with periods and amplitudes equivalent to full-scale wave flows and therefore enables wave-generated processes to be studied under controlled conditions. The student will use Liverpool’s new state-of-the-art Particle Image and Particle Tracking Velocimetry (PIV-PTV) system to simultaneously measure the flow field and sediment transport. The system involves using a laser and high-speed cameras to image the movement of buoyant particles and sediment within the flow. The PIV-PTV system will allow the student to identify vortex events and the role these events have on the movement of sediment. Continuous, co-located measurements of bed geometry will also be taken using compact cameras and structure-from-motion photogrammetric algorithms developed at Liverpool to provide sub-mm resolution DEMs. The laboratory programme will provide an unprecedented dataset for exploring the link between coherent flow structures, bed morphology and sediment erosion.
Formulation of key research questions based on critical review of literature, state-of-the-art knowledge, and existing datasets
Research training: in particular, familiarisation with laboratory flume experimentation, PIV and PTV, structure-from-motion photogrammetry, and data analysis techniques (time series analysis, spatial correlation analysis, double-averaging)
Laboratory work: running a series of flume experiments to explore the effects of bed morphology, flow depth and wave period and amplitude (as per rise and fall of the tide) on the interaction between sediment erosion and coherent flow structure dynamics
Data analysis: analysis of PIV, PTV and topography data, development of parameterisations that link the properties of flow structures to particle entrainment and transport
Outputs and dissemination: presentations at national and international conferences, writing reports and papers, PhD thesis
Competitive tuition fee, research costs and stipend (£14,056 tax free) from the NERC Doctoral Training Partnership “Understanding the Earth, Atmosphere and Ocean” (DTP website: http://www.liv.ac.uk/studentships-earth-atmosphere-ocean/) led by the University of Liverpool, the National Oceanographic Centre and the University of Manchester. The studentship is granted for a period of 42 months. Further details on eligibility, how to apply, deadlines for applications and interview dates can be found on the website. EU students are eligible for a fee-only award.
Frank, D., D. Foster, I. M. Sou, J. Calantoni, and P. Chou (2015) Lagrangian measurements of incipient motion in oscillatory flows, J. Geophys. Res. Oceans, 120, 244–256, doi:10.1002/2014JC010183.
Hurther, D., and P. D. Thorne (2011), Suspension and near-bed load sediment transport processes above a migrating, sand-rippled bed under shoaling waves, J. Geophys. Res., 116, C07001, doi:10.1029/2010JC006774.
Nino Y., F. Lopez, and M. Garcia (2003) Threshold for particle entrainment into suspension, Sedimentology, 50(2), 247–263.
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van der Werf, J. J., J. S. Doucette, T. O'Donoghue, and J. S. Ribberink (2007), Detailed measurements of velocities and suspended sand concentrations over full-scale ripples in regular oscillatory flow, J. Geophys. Res., 112, F02012, doi:10.1029/2006JF000614.
Williams, J. J., P. S. Bell, and P. D. Thorne (2003), Field measurements of flow fields and sediment transport above mobile bed forms, J. Geophys. Res., 108(C4), 3109, doi:10.1029/2002JC001336.