A Fully Quantified Hiding-Exposure Effect for Different Sand-Gravel Mixtures Reflective Of Coastal And Shelf Sea Environments.

Predicting seabed dynamics remains an area of great uncertainty and the accuracy of current sediment transport models are limited, in part due to the variable seabed composition. In this research, the ability of the flow to entrain different fractions of sand-gravel mixtures has been quantified through a comprehensive series of laboratory experiments underpinned by an integrated study of offshore geophysical and sedimentological data. Initial results indicate that in sand and gravel mixtures typically found offshore, the critical shear stress needed to mobilise sand fractions increases by up to 17% and it decreases by up to 8% for gravel fractions, compared with uniform sediments of the same size. This hiding-exposure effect varies in relation to the sediment composition and we have measured how it affects bedform morphodynamics. Ripples formed in sand-gravel mixtures are spatially more regular in terms of sinuosity than those formed in pure sand with reduced heights and wavelengths by up to 66% and 28% respectively. Traditional models for predicting bedload transport rely heavily on empirical models that are based on idealized conditions, where grain sizes are assumed to be uniform. This is not reflective of many coastal and shelf sea environments, and our fully quantified hiding-exposure effect for different sediment mixtures found in the natural environment will help to better predict seabed mobility. Our results should be considered when assessing the risks involved in introducing changes to seabed dynamics, thus facilitating a wide range of development activities in coastal and shelf environments.