Bedload transport of sediment mixtures in shelf seas

Connor McCarron

In my PhD project I am investigating the transport of sand and gravel mixtures in shelf seas through bedload transport. When analysing shelf seabed morphodynamics, it is largely ignored that the seabed commonly consists of sand and gravel mixtures, often a result of reworked paleo-glacial deposits. Advances in predicting the transport of mixed sediments focus on rivers, and the sustainability of aggregate extraction or feasibility of offshore installations are assessed via bedload transport formulae from riverine environments. These bedload transport predictors, however, cannot account for offshore variations in bedload transport directions or the record-breaking heights of sediment waves on mixed seabeds world-wide. In addition, to account for the presence of sand and gravel mixtures, a counter-intuitive effect known as the hiding-exposure effect needs to be considered where less energy is required to mobilise larger “exposed” grains and more energy to mobilise smaller “hidden” grains compared to uniform mixtures of similar sizes. This changes the efficiency of the flow to mobilise different grain size fractions and remains to be fully quantified which is one of the aims of my project.

The industry sponsor of my PhD project is Llanelli Sand Dredging Ltd (LSDL), a wholly owned subsidiary of Royal Boskalis Westminster N.V., one of the world’s leading dredging and marine contactors. LSDL and Boskalis were interested in the effect the presence of sand-gravel mixtures may have on the recovery and therefore sustainability of aggregate extraction activities. To provide insight into this effect, LSDL provided repeat, high-resolution, bathymetric datasets collected using multi-beam echo-sounder systems (MBES), and sedimentological data from one of their aggregate sites in the Bristol Channel for analysis in my PhD project. This data was collected in accordance with the license agreement between LSDL, the Welsh Assembly Government (WAG) and the Crown Estate, through routine monitoring surveys completed to monitor if there were any impacts on the morphology of the seabed and sediment composition at the site.

In September 2015, I had the opportunity to take part on a research cruise on behalf of LSDL with SEACAMS (Sustainable Expansion of the Applied Coastal and Marine Sectors) to collect additional data as part of LSDL’s routine monitoring. This involved 5 days on board Bangor University’s research vessel, the RV Prince Madog, in the Bristol Channel. During the research cruise we collected sediment samples, multi-beam and sidescan sonar data across the dredge site. Repeat seismic datasets were also collected in order to look at the history of the bedforms present on the seabed, and also to test a seismic system, newly acquired by SEACAMS at the time.

So far, the effects of aggregate extraction have been analysed across the site and presented at the European Marine Sand and Gravel Group’s (EMSAGG) 2015 conference. The data has also been analysed along with other datasets across the Irish Sea to experimentally parameterise bedload transport in sand and gravel mixtures and inform flume tank experiments. This has allowed preliminary quantification of the hiding-exposure” effect which eventually will be used to adjust existing bedload transport formulae for implementation into sediment transport models at the National Oceanography Centre, Liverpool (NOC-L) and feed back into improving the ability to predict sediment transport on shelf seas, and therefore sustainability of aggregate extraction and feasibility of offshore installations.