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 ofshelf-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 mixturesas a result of the contemporary reworking of sediment deposited by fluvial and glacialprocesses. In such mixtures, the sediment is transported dominantly via bedload transportwhich promotes seabed morphodynamics. The ability to accurately predict the mobility ofsediment has extensive applications for a wide range of shelf-sea stakeholders. The processof 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 referredto as the hiding-exposure (HE) effect which describes the process in which large grains moreexposed to the flow require a lower bed shear stress to become mobile, whereas small grainshidden in the interstitial spaces of the large grains require a higher bed shear stress to becomemobile, relative to uniform sediment of a similar size. A better understanding of the HE effectwill improve the ability to predict the bedload transport of sand and gravel mixtures, thusfacilitating stakeholders and consumers of shelf-sea resources.In this work, the HE effect was quantified for a range of sand-gravel mixtures through a seriesof flume experiments. The HE effect was shown to increase the threshold of motion of sand byup to 75%, and decrease the threshold of motion for gravel by up to 64%. The level to whichthe HE effect modifies the mobility of the sand and gravel fractions was found to be dependenton the proportion of gravel present in the mixture. As a result of this work, a new correctiveformulae was proposed to account for the influence of the HE effect on the mobility of sandand gravel mixtures. The newly proposed HE correction extends the applicability of previouscorrective formulae to bimodal sediment mixtures, reflective of shelf-sea environments, andmore accurately predicts the strength of the HE effect.
The influence of the HE effect on the mobility of sediment, the resulting bed morphodynamicsand 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, bedloadtransport rates of the sand and gravel fractions were reduced by up to 10% and increasedby up to 47%, respectively, in simulations with the HE correction. A tipping point wasidentified 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 bedmobility. 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 ratesof the sand and gravel fractions which were reduced by up to 76% and increased by up to1547%, respectively, but no tipping point was identified.The inclusion of the HE effect in both models, however, resulted in the general dampeningof bed morphodynamics with smaller elevation changes, reduced migration rates, and bedcompositions which were less heterogeneous that that observed in simulations without the HEcorrection. The hiding effects were therefore interpreted as having the dominant influence.The HE effect did not have an observable influence on bed morphodynamics, however, ifthe background shear stress was either not sufficient to mobilise the sand fraction, or wassufficiently large that the gravel fraction is already mobile. In the wave-driven model, thisresulted in a cross-shore variability in the influence of the HE effect on bed morphodynamicswith increased influenced in the shoaling zone and no observable effect in the swash zone. Inthe current-driven model, the HE effect modified the extent across which bed morphodynamicswere observed.
The reduction in bed morphodynamics observed in the numerical modelling was reflected inthe results of further flume tank experiments which quantified the influence of sand and gravelmixtures on the development and mobility of ripples. In sand-gravel mixtures, although theresults indicated that the initial development of ripples was quicker than in pure sand, theripples took longer to reach their equilibrium dimensions. The final dimensions and migrationrates of the ripples were reduced in the sand-gravel mixtures compared to those developed insand, and decreased with the increasing fraction of gravel in the bed. The reduction in thedimensions and migration rates of the ripples was interpreted as being a result of the reducedmobility 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 effectshould therefore be considered in numerical feasibility studies such as those assessing thesustainability of aggregate resources, the impacts of resource extraction or the associatedinfrastructure on seabed morphodynamics. The HE effect will also have implications forhabitat 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 byselective entrainment of sand and gravel in a mixture as a result of the HE effect. Due to thepresence of sand-gravel mixtures on shelf-seas globally, the implications of these results arelikely wider ranging than is permissible to explore in this worK.