The hiding-exposure effect revisited: A method to calculate the mobility of bimodal sediment mixtures

Connor J Mccarron,Katrien J.J Van Landeghem,Jaco H Baas,Laurent O Amoudry,Jonathan Malarkey

doi:10.1016/j.margeo.2018.12.001

Connor J Mccarron, Katrien J.J Van Landeghem + Show 3 more

Open Access

https://doi.org/10.1016/j.margeo.2018.12.001

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Abstract

Predicting seabed mobility is hampered by the limited accuracy of sediment transport models when the bed is composed of mixed sediments. The hiding-exposure (HE) effect modifies the threshold of motion of individual grain classes in sediment mixtures and its strength is dependent on the grain size distribution. However, an appropriate method of predicting this effect for bimodal sediment mixtures remains to be developed. The prototypical example of a bimodal mixture is that consisting of a well-sorted sand and gravel for the fine and coarse fractions respectively. Through a comprehensive series of laboratory experiments, the HE effect has been quantified for a full range of sand-gravel mixtures from pure sand to pure gravel, the choice of which has been underpinned by an integrated study of offshore geophysical and sedimentological data found in coastal and shelf seas. In the sand–gravel mixtures used in the present study the critical shear stress needed to mobilise the sand and gravel fractions increased by up to 75% and decreased by up to 64%, respectively, compared to that needed to mobilise well-sorted sediment of similar size. The HE effect was found to be dependent on the percentage of gravel (coarse mode) present in the bimodal mixture, whereby the effect for the mixture is the weighted sum of the HE effect for the fine and coarse modes.

Highlights

Sediment composition and transport are amongst the main controlling factors on seabed functioning in coastal and shelf seas
Sand-gravel mixtures are common in coastal and shelf sea environments globally (Holland and Elmore, 2008) and the Irish Sea is representative of these environments from which a good coverage of sediment samples is available from the British Geological Survey (BGS), Natural Resources Wales (NRW), the Geological Survey of Ireland (GSI) and the Marine Institute (MI) (Fig. 1)
Pure sand with di = 0.15 mm requires a shear stress of 0.20 N m−2 to become mobile compared with 0.95 N m−2 for a sandgravel mixture with fg = 0.5, which is equivalent to a 75% increase in τcr,i

Summary

Introduction

Sediment composition and transport are amongst the main controlling factors on seabed functioning in coastal and shelf seas. The presence of structures associated with, for example, renewable energy and hydrocarbon extraction, can cause disturbance to natural sediment transport dynamics. This may have undesirable consequences, including damage of pipelines or subsea cables and scouring of the bed around man-made structures James, 2002; Malham et al, 2014; Diesing et al, 2017; Ling et al, 2017), and carbon cycling is controlled by the heterogeneity of seafloor biota (Snelgrove et al, 2018).

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