A wind tunnel study of particle kinematics during crust rupture and erosion

Abstract

Sediments with protective crusts of varying type were subjected to particles fed from an upwind source during wind tunnel experiments carried out to compare their ability to resist erosion and alter the kinematics of the saltation cloud. A laser Doppler anemometer measured the distribution of particle velocity for saltators impacting each crusted surface and for particles ejected from this same surface, inclusive of ricochets. Biotic crusts grown on sand in an environmental chamber were able to withstand erosion over several hours of continuous particle impact, as compared to brittle salt crusts, which, regardless of wind speed or sodium chloride concentration, eroded fully within one half hour. Despite the appearance of deep pits and grooves on the ruptured crusts, loss of mass dominated over particle trapping on these rough surfaces. While decreasing salt concentration between 320gkg−1 and 80gkg−1 generally was found to be associated with an increase in the mass flux of particles ejected from the surface, little to no correlation with either wind speed or particle impact speed was observed. The range over which varying salt concentration affects the momentum of ejected particles is rather narrow, within 160gkg−1. Temporal changes in the velocity distribution of ejected particles with crust rupture and deflation are complicated by variations in crust strength with depth. This is especially true of biotic crusts and weak salt crusts. Fast moving particles associated with the upper 40% of the cumulative velocity distribution generally demonstrate little variation from the control surfaces, and probably represent ricochets. In comparison, those within the lower 60% of the distribution are significantly affected by crust rupture and erosion, and may include low energy rebounds as well as the entrainment of new particles loosened from within the crust. While such measurements are exceedingly rare, they are needed for the validation of physically based models of crust erosion.