ABSTRACT: The Shields number is a granular bed state parameter.

CGRG Bibliography of Canadian Geomorphology
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Abstract

The Shields number can be interpreted as a fundamental measure of the state of a granular surface under shear flow, signifying the intensity of bed material transport over the surface. The ‘critical’ Shields number is interpreted as a measure of the threshold condition at which individual grains begin to be entrained or, in practice, the state beyond which rare entrainment events rapidly become more common. In fact, the Shields number is a measure of the aggregate state of the granular bed that reduces to a measure of individual grain mobility only under conditions of no intergranular constraint. Hiding, imbrication and more complex structural arrangements of granular stream beds always produce some degree of constraint and, in channels beyond about 3% gradient, constraint is essential if individual grains are to remain in the channel under flows that just submerge the grain. Indirect evidence for the Shields number as a bed state parameter is available from a large number of observations. How to assign a particular critical Shields number to a given bed is, however, by no means clear. By what mechanisms do bed configurations develop that lead to grain constraint? This question poses a problem in granular mechanics in a restricted active layer or, near threshold, essentially on a rough 2D plane. Such circumstances have been much less investigated than 3D granular flows. Modification of the bed state so that individual grains become more reluctant to move has been generally characterized as ‘bed settlement’ but details remain almost entirely open for investigation. Differential mobility of different grain sizes and interference between mobile and static grains are important aspects of the problem.

Bibliography of Canadian Geomorphology