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In spite of significant efforts towards three-dimensional modelling of estuarine cohesive sediment transport in the past decade (e.g. Baugh and Manning, 2007; Kombiadou and Krestinitis, 2012; Ramirez-Mendoza et al., 2014), studies investigating both the estuarine spatial scale and timescales of seasons and over remain very scarce. In such three-dimensional process-based models, cohesive behaviour impacts two critical model components: particle settling rate (due to flocculation and hindered settling for example), and bed sediment resuspension and deposition (Amoudry and Souza, 2011a).
The studentship aims to increase the predictive ability of estuarine SPM at seasonal to decadal timescales and will focus on the following objectives:
derive novel formulation(s) for settling rate applicable across seasons,
implement formulations for bed resuspension and deposition that include bioturbation, consolidation and armouring,
test, calibrate, and validate model against in situ estuarine observations,
determine mechanisms controlling transport of estuarine cohesive sediments over seasons to decades.
The studentship will combine use of in-situ observations and three-dimensional baroclinic modelling, and will focus on a case study of the Dee Estuary where cohesive sediments are abundant even though sand is also present. The project will build upon previous modelling and observational studies in the Dee Estuary (e.g., Bolanos et al., 2013; Amoudry et al., 2014; Ramirez-Mendoza et al., 2014). An extensive series of datasets have been collected in the Dee, which span several years from 2006 to 2009 and several seasons (winter and late spring), and which still remain to be analysed in a holistic way. Each dataset offers comprehensive month-long observations of cohesive and mixed sediment transport processes. The data include measurements of SPM size, near-bed currents and turbulence, full water column hydrodynamics, and near-bed sand transport. Previous modelling efforts have highlighted some key physical processes in the estuary, such as the advection of a spatial gradient in suspended concentration gradient (Amoudry et al., 2014) and flocculation (Ramirez-Mendoza et al., 2014). The principal shortcoming of these recent modelling studies is their short-term focus (days to weeks). The implication is that is still remains difficult to bring together the conceptual understanding at these timescales and the longer term insight gained from analyses of sediment cores in which the net effects of estuary mixing and hydrodynamic sorting are aggregated (Rahman and Plater, 2014). The extension of the model to longer time scales will aim to bridge that gap.
In addition to new expressions for resuspension, deposition and settling rates, inter-comparisons against other recent methods and relevant formulations will also be pursued. The new formulations developed during the studentship will aim to incorporate a fast timescale component due to hydrodynamics and turbulence related processes (e.g. turbulence-induced flocculation) and a slow timescale component due to seasonal variability. Formulations for bed resuspension and deposition will build upon existing layered bed structure in coastal ocean models (e.g., Warner et al. 2008; Amoudry and Souza, 2011b) and the algorithms of Sanford (2008). These have only been tested under idealised conditions to date, and this project would therefore provide the first thorough model validation against in situ measurements. Formulations for settling rates will provide unified parameterisations of turbulence-induced flocculation, valid both across different wave-current regimes and across seasons. These will be derived following analyses of observational data of floc size and turbulence (e.g. Ramirez-Mendoza et al., 2014; Ramirez-Mendoza, 2015), and may involve the use of new machine learning techniques such as genetic programming and/or artificial neural networks. A validation dataset of particle size will be obtained within a 4-dimensional framework from dated saltmarsh cores within the Dee estuary, providing an overall chronology of net sediment accretion patterns through time.
Formulation of key research questions based on critical review of literature, state-of-the-art knowledge, and available observational and modelling methods.
Research training: familiarisation with data analysis and numerical modelling techniques.
Development and numerical implementation of new formulations for resuspension, deposition, and settling rates.
Model testing, validation against observations, sensitivity analysis.
Synthesis: Analysis of numerical results towards determining mechanisms controlling estuarine sediment transport and for comparison with long-term conceptual understanding.
Outputs and dissemination: presentations at national and international conferences, writing reports and papers, PhD thesis.
In addition to the DTP training, the student will receive training in mathematical and physical analytical techniques, data analysis, and numerical modelling.
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.
Amoudry, L.O., Ramirez-Mendoza, R., Souza, A.J., and Brown, J.M., 2014. Modelling based assessment of suspended sediment dynamics in a hypertidal estuarine channel. Ocean. Dyn., 64 (5), 707-722.
Amoudry L.O. and Souza A.J. (2011a) Deterministic coastal morphological and sediment transport modeling: a review and discussion. Rev. Geophys., 49, RG2002, 1-21, doi:10.1029/2010RG000341
Amoudry L.O. and Souza A.J. (2011b) Impact of sediment-induced stratification and turbulence closures on sediment transport and morphological modelling, Cont. Shelf Res., 31 (9), 912-928
Baugh, J.V. and A.J. Manning (2007) An assessment of a new settling velocity parameterisation for cohesive sediment transport modeling, Cont. Shelf Res., 27 (13), 1835-1855.
Bolanos R, Brown JM, Amoudry LO, Souza AJ (2013) Tidal, riverine and wind influences on the circulation of a macrotidal estuary. J. Phys Oceanogr. 43 (1), 29-50
Fitzsimmons et al. (2011), Treatise on Estuarine and Coastal Science, 4.04, Elsevier.
Kombiadou K, and Krestenitis NY (2012) Fine sediment transport model for river influenced microtidal shelf seas with application to the Thermaikos Gulf. Cont. Shelf Res., 36, 41–62.
Rahman, R. and A.J. Plater (2014) Particle-size evidence of estuary evolution: A rapid and diagnostic tool for determining the nature of recent saltmarsh accretion, Geomorphology, 213, 139–152.
Ramirez-Mendoza. R. (2015) Flocculation controls in a hypertidal estuary, Ph.D. Thesis, University of Liverpool.
Ramirez-Mendoza, R, A.J. Souza and L.O. Amoudry (2014) Modelling flocculation in a hypertidal estuary, Ocean Dynamics, 64 (2), 301-313
Sanford L.P. (2008) Modeling a dynamically varying mixed sediment bed with erosion, deposition, bioturbation consolidation and armouring, Computers & Geosciences, 34, 1263-1283.
Warner, J.C., Sherwood, C.R., Signell, R.P., Harris, C.K., Arango, H.G. (2008) Development of a three-dimensional, regional, coupled wave, current, and sediment transport model. Comput. Geosci. 34, 1284–1306.
Winterwerp J.C. & W.G.M. van Kesteren (2004) Introduction to the physics of cohesive sediment in the marine environment., Elsevier.