Analysis of small‐scale gravel bed topography during armoring

Abstract

[1] In evaluating the resistance of sediment particles to entrainment by the action of the flow in a river, the grain geometry is usually characterized using representative sizes. This approach has been dictated, initially by lack of physical insight, but more recently by the lack of analytical tools able to describe the 3-D nature of surface grain organization on water-worked sediment beds. Laboratory experiments are presented where mixed grain size beds were mobilized under a range of hydraulic and sediment input conditions. Detailed bed topography was measured at various stages. Statistical tools have been adopted which describe the degree of surface organization on water-worked sediment bed surfaces. The degree of particle organization and the bed stability can be evaluated in relative terms using the properties of the probability density distribution of the bed surface elevations and in absolute terms using a properly defined 2-D structure function. The methods described can be applied directly to natural water-worked surfaces given the availability of appropriate bed surface elevation data sets.

Statistical properties of fresh snow roughness

“…Such a methodology considers roughness as a random field of elevations and makes use of structure functions as the main statistical tool to capture the characteristic parameters which retain the most significant properties of roughness. The random field approach has proved to be extremely helpful for the characterization of various rough surfaces including gravel beds in rivers [Nikora et al, 1998;Robert, 1991;Marion et al, 2003;Aberle and Nikora, 2006], Martian topography [Nikora and Goring, 2005], and also, snow covered surfaces [Rees, 1992;Rees and Arnold, 2006;Arnold and Rees, 2003;Löwe et al, 2007].…”

Probabilistic description of grain resistance from simultaneous flow field and grain motion measurements

“…Field observations by Buffington et al [1992] and Church and Hassan [1998] and laboratory evidence [Marion et al, 2003] suggested that the resistance of grains to entrainment from naturally formed gravel beds is not related simply to grain weight, as described by the surface grain size distribution, but to some other property of the bed, possibly the arrangement of the surface grains and topography of the bed surface. These observations suggest that the use of models that link entrainment simply to grain size, such as Shields', may over predict sediment mobility because they ignore the contribution to bed stability caused by surface grain arrangements observed in water-worked deposits.…”

Water‐worked gravel surfaces: High‐order structure functions at the particle scale

“…To what extent this difference depends upon physical mechanisms involved, the scale of consideration, grain shape and size distribution is not clear. The available information on water-worked gravel surfaces of streambeds mainly includes 1-D and 2-D correlation functions, semivariograms, and the second-order structure functions of bed elevations [Furbish, 1987;Robert, 1988Robert, , 1990Clifford et al, 1992;Bergeron, 1996;Nikora et al, 1998;Butler et al, 2001;Marion et al, 2003;Smart et al, 2004]. All these measures represent the second-order statistics and are equivalent, in a practical sense, to each other.…”

“…However, natural water-worked gravel surfaces often demonstrate deviations from the Gaussian distribution [e.g., Nikora et al, 1998]. Using extensive laboratory experiments Marion et al [2003] showed that deviations from normality increase with bed development. These results suggest that if the bed description is limited to the second-order statistics then some important information on bed surface structure can be missed.…”