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The Effect of Patch Size on Grain-Scale Roughness Parameterization in Fluvial Environments

Author(s): Jane Groom, Stephane Bertin, Heide Friedrich

Linked Author(s): Heide Friedrich

Keywords: Roughness, parameterization, scales, microtopography, fluvial.

Abstract: Several disciplines use the term roughness with little clarity. Although there is a general consensus that surface roughness is synonymous of the topography (or structure) of a surface, in fluvial environments, roughness is important due to the interactions between sediment, flow and ecology. River roughness is affected by the surface morphology, composition and structure, acting at different scales superimposed onto one another. There can also be confusion between the term roughness and flow resistance. It is important to differentiate between the two terms and ensure roughness is used as an input parameter for flow resistance equations, rather than a synonymous term. We use the term grain-scale roughness to describe the microtopography of a surface, therefore representing the topography as a result of individual grains. Detailed measurements of the surface were gathered through the collection of digital elevation models (DEMs) of patch-scale fluvial surfaces both in the laboratory and the field, obtained using stereo-photogrammetry. Moving-window detrending applied to the DEMs allowed isolating grain-scale roughness from the underlying larger-scale bedforms. Grain-scale roughness of the gravel patches was derived from the detrended DEMs using a series of surface metrics including skewness, kurtosis, standard deviation, inclination index (degree of imbrication) and roughness lengths obtained from second-order structure functions. These metrics (or roughness parameters) each represent different attributes of the surface. The data presented highlights the spatial variability of grain-roughness parameters at the patch-scale, indicating for a true representation of roughness, an adequate patch size for the surface must be obtained. Due to surface complexity, it can be concluded that a single roughness parameter cannot adequately describe roughness across a surface, even at the patch-scale. Developments to roughness parameterization are still required; however a more defined use of the term is available in the patch-scale fluvial application


Year: 2017

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