A wind tunnel investigation of the influences of fetch length on the flux profile of a sand cloud blowing over a gravel surface

Abstract

AbstractWind tunnel tests were conducted to examine the fetch effect of a gravel surface on the flux profile of the sand cloud blowing over it using typical dune sand. The results suggest that the flux profile of blown sand over a gravel surface differs from that over a sandy surface and is characterized by a peak flux at a height above the surface while that over a sandy surface decreases exponentially with height. The flux profile of a sand cloud over a gravel surface can be expressed by a Gaussian peak function: q = a + b exp (−0·5((h − c)/d)2), where q is the sand transport rate at height h, and a, b, c and d are regression coefficients. The significance of the coefficients in the function could be defined in accordance with the fetch length of the gravel surface and wind velocity. Coefficient c represents the peak flux height and increases with both wind velocity and fetch length, implying that the peak flux height is related to the bounce height of the particles in the blowing sand cloud. Coefficient d shows a tendency to increase with both wind velocity and fetch length. The sum of a and b, representing the peak flux, increases with wind velocity but decreases with fetch length. The average saltation height derived from the cumulative percentage curve shows a tendency to increase with both the fetch length and wind velocity. For any fetch length of a gravel surface the sand transport equation is expressed as Q = C(1 − Ut/U)(ρ/g)U3, where Q is the sand transport rate, U is the wind velocity, Ut is the threshold velocity measured at the same height as U, g is the gravitational acceleration, ρ is the air density, C is a proportionality coefficient that decreases with the fetch length of the gravel surface. At a given wind velocity, the sand transport rate over a gravel surface is only 52–68 per cent of that over a sandy surface. The flux rate in true creep over a gravel surface increases with wind velocity but decreases with the fetch length, whereas the creep proportion (the ratio of creep flux to the sand transport rate) decreases with both the wind velocity and fetch length. Two‐variable (including fetch length and wind velocity) equations were developed to predict the peak flux height, average saltation height and transport rate. Copyright © 2004 John Wiley & Sons, Ltd.