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A wind tunnel investigation of the influences of fetch length on the flux profile of a sand cloud blowing over a gravel surface
Author(s) -
Dong Zhibao,
Wang Hongtao,
Liu Xiaoping,
Wang Xunming
Publication year - 2004
Publication title -
earth surface processes and landforms
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.294
H-Index - 127
eISSN - 1096-9837
pISSN - 0197-9337
DOI - 10.1002/esp.1116
Subject(s) - fetch , wind speed , aeolian processes , geology , wind tunnel , roughness length , flux (metallurgy) , shear velocity , wind profile power law , atmospheric sciences , geometry , meteorology , geomorphology , mechanics , mathematics , turbulence , physics , chemistry , oceanography , organic chemistry
Abstract Wind tunnel tests were conducted to examine the fetch effect of a gravel surface on the ux prole of the sand cloud blowing over it using typical dune sand. The results suggest that the ux prole of blown sand over a gravel surface differs from that over a sandy surface and is characterized by a peak ux at a height above the surface while that over a sandy surface decreases exponentially with height. The ux prole 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 coefcients. The signicance of the coefcients in the function could be dened in accordance with the fetch length of the gravel surface and wind velocity. Coefcient c represents the peak ux height and increases with both wind velocity and fetch length, implying that the peak ux height is related to the bounce height of the particles in the blowing sand cloud. Coefcient d shows a tendency to increase with both wind velocity and fetch length. The sum of a and b , representing the peak ux, 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 − U t / U )( ρ / g ) U 3 , where Q is the sand transport rate, U is the wind velocity, U t 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 coefcient 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 ux 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 ux 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 ux height, average saltation height and transport rate. Copyright © 2004 John Wiley & Sons, Ltd.