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Dynamics of sand around tidal sand banks and the stratigraphic record

Department of Earth and Environmental Sciences

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Dr N Mitchell , Dr R Jerrett Applications accepted all year round

About the Project

Sand banks in macrotidal estuaries are highly dynamic. As their heights and locations change rapidly and continuously, sand banks are hazards to marine shipping and errors on nautical charts can lead to ships running aground. In areas where land-based sources of sand for construction are limited, they can be useful so commercial sands have been extracted by dredging of some coastal banks in the past. However, tidal sand banks also reduce the energy of ocean waves passing over them, so they protect beaches and cliffs of adjacent coasts from more extreme erosion. Whether sand extraction ultimately affects coastal erosion or not has been a controversial issue. Despite their importance, there is little knowledge of detailed changes in sand bank morphology and the effects of those changes on adjacent coasts.
In this project, you will gain access to a unique dataset of 13 high-resolution multibeam echo-sounder surveys of Nash Sands, a major sand bank in the macro-tidal Bristol Channel. Collected over the years 2002-2010 for a consortium of aggregates companies, the data reveal how dunes migrated around the bank and how the bank morphology changed. You will be able to develop new understanding based on these data in several areas, such as:
1) By measuring dune movements between successive surveys, you will derive dune translation velocities and associated fluxes of sand. Working with Simon Neill (Bangor University) and Lucy Bricheno (National Oceanography Centre, Liverpool), you will use a model of tidal currents and waves, from which you will predict the sand transport flux to compare with the fluxes derived from the multibeam sonar data.
Interactions between waves and currents are known to be an important driver of dune migration. To better understand this non-linear process, a coupled environmental prediction model will be used. The student will therefore have access a state-of the art supercomputing facility, and data generated in collaboration with the UK Met Office.
This ultimately will reveal the relative importance of waves and tidal currents in moving the sand. This project can also reveal important deficiencies in models based on bedload calculations, as differences between model predictions and those derived from multibeam sonar data could be due to suspended load transport.
2) Using the multibeam data and a further 10 years of single-beam data from the previous decade, you will be able to investigate wave propagation to the beaches of the South Glamorgan coast. Comparing those wave properties with changes in the amount of sand covering the coastal rock platforms in aerial imagery, you will assess the impact of sand bank elevation changes on coasts.
3) The movements of dunes and of the sand bank itself represent the development of ephemeral stratigraphy - successive seabed surfaces are buried, forming cross-stratification and other internal sedimentary structures. You will use the multibeam sonar data to work out the time-progressive development of stratigraphy. This will be an exciting development as multibeam-based stratigraphy promises to fill a major gap in our understanding of how stratigraphy develops. While outcrops on land can provide great detail of strata, we have very poor understanding of the timescales of their development or environmental conditions. In contrast, information from oceanographic bottom landers can provide excellent temporal detail on how sediment is deposited along with its environmental conditions. Multibeam sonar data usefully provide both good areal coverage and high-resolution information over periods when the environmental conditions are known.
The project will be carried out in collaboration with Marinespace Ltd., an environmental consultancy with expertise related to aggregate extractions and their environmental impacts. Rob Langman of Marinespace will be an external supervisor for the project. Through carrying out this work, you will gain some knowledge of applied as well as academic research. The project should be suitable for students wishing to develop or extend their capabilities in data science, computing, marine environmental assessment, marine geophysical surveying and research. It has the potential to lead to careers in a range of academic and commercial work in geophysical and hydrographic survey, and environmental consultancy.


Aldridge, J.N., Parker, E.R., Bricheno, L.M., Green, S.L. and Van Der Molen, J., 2015. Assessment of the physical disturbance of the northern European Continental shelf seabed by waves and currents. Continental Shelf Research, 108, pp.121-140.
Bricheno, L.M., Wolf, J. and Aldridge, J., 2015. Distribution of natural disturbance due to wave and tidal bed currents around the UK. Continental Shelf Research, 109, pp.67-77
Grant, W.D. and Madsen, O.S., 1979. Combined wave and current interaction with a rough bottom. J. Geophys. Res., 84: 1797-1808.
Lewis, H.W., Castillo Sanchez, J.M., Arnold, A., Fallmann, J., Saulter, A., Graham, J., Bush, M., Siddorn, J., Palmer, T., Lock, A. and Edwards, J., 2019. The UKC3 regional coupled environmental prediction system. Geoscientific Model Development, 12(6), pp.2357-2400.
Lewis, M.J., Neill, S.P. and Elliott, A.J., 2015. Interannual variability of two offshore sand banks in a region of extreme tidal range. J. Coastal Res., 31: 265-275.
Neill, S.P., Scourse, J.D., Bigg, G.R. and Uehara, K., 2009. Changes in wave climate over the northwest European shelf seas during the last 12,000 years. Journal of Geophysical Researches, 114: paper C06015, doi:06010.01029/02009JC005288.
Neill, S.P., Scourse, J.D. and Uehara, K., 2010. Evolution of bed shear stress distribution over the northwest European shelf seas during the last 12,000 years. Ocean Dynamics, 60: 1139-1156.
Schmitt, T. and Mitchell, N.C., 2014. Dune-associated sand fluxes at the nearshore termination of a banner sand bank (Helwick Sands, Bristol Channel). Cont. Shelf Res., 76: 64-74.
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