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Title: Bedload transport of sand-gravel mixtures in coastal and shelf-sea environments
Author: McCarron, Connor
ISNI:       0000 0004 9358 4200
Awarding Body: Bangor University
Current Institution: Bangor University
Date of Award: 2020
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Shelf-sea environments are areas of significant economic importance as a source of food, minerals and energy (i.e. renewables and hydrocarbons). The sediments on the seabed of shelf-seas can also act as a sink for bacteria, pathogens, heavy metals, or as carbon reservoirs. A common feature of shelf-sea environments is the presence of sand and gravel mixtures as a result of the contemporary reworking of sediment deposited by fluvial and glacial processes. In such mixtures, the sediment is transported dominantly via bedload transport which promotes seabed morphodynamics. The ability to accurately predict the mobility of sediment has extensive applications for a wide range of shelf-sea stakeholders. The process of predicting the bedload transport of mixtures of sand and gravel is complicated, however, by the selective entrainment of differently sized grains. This selective entrainment is referred to as the hiding-exposure (HE) effect which describes the process in which large grains more exposed to the flow require a lower bed shear stress to become mobile, whereas small grains hidden in the interstitial spaces of the large grains require a higher bed shear stress to become mobile, relative to uniform sediment of a similar size. A better understanding of the HE effect will improve the ability to predict the bedload transport of sand and gravel mixtures, thus facilitating stakeholders and consumers of shelf-sea resources. In this work, the HE effect was quantified for a range of sand-gravel mixtures through a series of flume experiments. The HE effect was shown to increase the threshold of motion of sand by up to 75%, and decrease the threshold of motion for gravel by up to 64%. The level to which the HE effect modifies the mobility of the sand and gravel fractions was found to be dependent on the proportion of gravel present in the mixture. As a result of this work, a new corrective formulae was proposed to account for the influence of the HE effect on the mobility of sand and gravel mixtures. The newly proposed HE correction extends the applicability of previous corrective formulae to bimodal sediment mixtures, reflective of shelf-sea environments, and more accurately predicts the strength of the HE effect. The influence of the HE effect on the mobility of sediment, the resulting bed morphodynamics and bed composition, was quantified through a series of sensitivity tests using both a wave-driven, and a current-driven, morphodynamic model. In the wave-driven model, bedload transport rates of the sand and gravel fractions were reduced by up to 10% and increased by up to 47%, respectively, in simulations with the HE correction. A tipping point was identified at 20% around which either hiding or exposure effects were dominant. Below 20% gravel, the increased gravel mobility and unaffected sand mobility resulted in an enhanced bed mobility. Above 20% gravel, the reduced mobility of the sand resulted in dampened mobility. In current-driven model, the HE effect had a stronger influence on the bedload transport rates of the sand and gravel fractions which were reduced by up to 76% and increased by up to 1547%, respectively, but no tipping point was identified. The inclusion of the HE effect in both models, however, resulted in the general dampening of bed morphodynamics with smaller elevation changes, reduced migration rates, and bed compositions which were less heterogeneous that that observed in simulations without the HE correction. The hiding effects were therefore interpreted as having the dominant influence. The HE effect did not have an observable influence on bed morphodynamics, however, if the background shear stress was either not sufficient to mobilise the sand fraction, or was sufficiently large that the gravel fraction is already mobile. In the wave-driven model, this resulted in a cross-shore variability in the influence of the HE effect on bed morphodynamics with increased influence in the shoaling zone and no observable effect in the swash zone. In the current-driven model, the HE effect modified the extent across which bed morphodynamics were observed. The reduction in bed morphodynamics observed in the numerical modelling was reflected in the results of further flume tank experiments which quantified the influence of sand and gravel mixtures on the development and mobility of ripples. In sand-gravel mixtures, although the results indicated that the initial development of ripples was quicker than in pure sand, the ripples took longer to reach their equilibrium dimensions. The final dimensions and migration rates of the ripples were reduced in the sand-gravel mixtures compared to those developed in sand, and decreased with the increasing fraction of gravel in the bed. The reduction in the dimensions and migration rates of the ripples was interpreted as being a result of the reduced mobility of the sand due to the hiding influence of the gravel fraction. The HE effect has been shown to have an appreciable impact on the mobility of sand-gravel mixtures, the resulting seabed morphodynamics and bed composition. The HE effect should therefore be considered in numerical feasibility studies such as those assessing the sustainability of aggregate resources, the impacts of resource extraction or the associated infrastructure on seabed morphodynamics. The HE effect will also have implications for habitat predictive modelling and studies predicting pollutant dispersal and carbon fluxes. These are only a limited number of examples of the activities which will be affected by selective entrainment of sand and gravel in a mixture as a result of the HE effect. Due to the presence of sand-gravel mixtures on shelf-seas globally, the implications of these results are likely wider ranging than is permissible to explore in this work.
Supervisor: Van Landeghem, Katrien ; Baas, Jacobus Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Hiding-exposure effect ; Mixed sand-gravel ; Threshold of Motion ; Bedload Transport