Controls on the aerodynamic roughness length and the grain‐size dependence of aeolian sediment transport

Abstract

Estimates of the wind shear stress exerted on Earth's surface using the fully rough form of the law‐of‐the‐wall are a function of the aerodynamic roughness length, z0. Accurate prediction of aeolian sediment transport rates, therefore, often requires accurate estimates of z0. The value of z0 is determined by the surface roughness and the saltation intensity, both of which can be highly dynamic. Here we report field measurements of z0 values derived from velocity profiles measured over an evolving topography (i.e. sand ripples). The topography was measured by terrestrial laser scanning and the saltation intensity was measured using a disdrometer. By measuring the topographic evolution and saltation intensity simultaneously and using available formulae to estimate the topographic contribution to z0, we isolated the contribution of saltation intensity to z0 and document that this component dominates over the topographic component for all but the lowest shear velocities. Our measurements indicate that the increase in z0 during periods of saltation is approximately one to two orders of magnitude greater than the increase attributed to microtopography (i.e. evolving sand ripples). Our results also reveal differences in transport as a function of grain size. Each grain‐size fraction exhibited a different dependence on shear velocity, with the saltation intensity of fine particles (diameters ranging from 0.125 to 0.25 mm) saturating and eventually decreasing at high shear velocities, which we interpret to be the result of a limitation in the supply of fine particles from the bed at high shear velocities due to bed armoring. Our findings improve knowledge of the controls on the aerodynamic roughness length and the grain‐size dependence of aeolian sediment transport. The results should contribute to the development of improved sediment transport and dust emission models. © 2018 John Wiley & Sons, Ltd.

Size‐Independent Susceptibility to Transport in Aeolian Saltation

“…Further work is thus needed to better understand how the surface PSD and other factors, such as the configuration of coarse particles (e.g., Gillette & Stockton, ; Greeley et al, ; Iversen et al, ) and availability of loose sediment (e.g., Gillette & Chen, ; Sharp, ), determine both bulk and size‐selective saltation thresholds. Improved techniques for obtaining high‐resolution size‐resolved saltation measurements, such as recently presented by Field and Pelletier (), could improve our understanding of saltation over mixed sediment beds.…”

Studying the spatial and temporal changes in aeolian sand transport in a wind tunnel using 3D terrestrial laser scanning

“…Specifically, many researchers have proven that both the height and wavelength of the ripples increased with increasing erosion duration and wind velocity (e.g., Andreotti et al, 2006;McKenna Neuman & Bédard, 2017;Sharp, 1963). This suggests that the shear velocity and aerodynamic roughness length calculated from the wind profile should be affected by the changes of RMSH (Field & Pelletier, 2018;Wang et al, 2018).…”

A fully predictive model for aeolian sand transport

“…This roughness length is defined as the equivalent grain roughness replacing any type of roughness yielding the same overall bed roughness (same velocity profile). It is most logic to assume that the effective roughness (k s ) of the sandy beach surface is related to the:  grain roughness effects of static and saltating particles (Owen, 1964;Farrell and Sherman, 2016;Field and Pelletier 2018);  height of the micro bed morphology (ripples), Field 2016, Field andPelletier, 2018);  size of shells or shell fragments/clusters which are sometimes abundantly available, particularly at nourished beaches.…”