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Grain size characteristics of ripple crests and troughs under different velocity and duration of blowing wind

Huiru Li, Hong Cheng, Jifeng Li, Bo Liu, Chenchen Liu, Wei Liu, Ruxing Wang, Xueyong Zou
State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing Normal University, Beijing, China

Aeolian sand ripples are usual aeolian landform in sand seas. Variations in sand grain size are closely related to aeolian processes mechanics. Many researches have focused on shape and formation of sand ripples, but few on the difference between grain size at crests and troughs, especially their variations with velocity and duration of blowing wind. To understand the grain size variations, we used a sediment of 158μm in mean diameter (d) to simulate formation of aeolian sand ripples in a wind tunnel under four friction velocities (u*=0.42, 0.51, 0.59, and 0.68 m/s) with different blowing time (t). The samples, which were collected from ripple crests and troughs at seven sites (x=0.2, 1.0, 2.0, 4.0, 6.0, 8.0, and 10.0m) for determining the particle size distribution, were performed using a Malvern Mastersizer 2000. The results showed that: (i) d at ripple crests was always larger than that at troughs at the same site, the biggest value of difference which was 132 μm appeared at site of 1 m. (ii) d at ripple troughs was slightly affected by test conditions with a narrow range of 146-164μm. (iii) for a given blowing time, d at ripple crests was slightly affected by friction velocities except that d increased in linear function (d=344.27+34.54u*) when x=1.0 m; for a given friction velocity, d at ripple crests was slightly affected by blowing time except that d increased in power function (d=a+tb, where a and b is fitted parameters) when x=1.0, 2.0, and 4.0 m. (iv) the probability cumulative curve of sand size satisfied for a three-stage structure in most cases, and showed that the component of suspension, saltation and creep were kept stable in 0.3-3.0, 0.9-3.0 and 96.5-98.9%, respectively. Only in the case of u*=0.42 m/s and t=15 min, the probability cumulative curve of crest sand size satisfied for a more complex structure, and showed that the component of creep reached 44.0% at site of 1 m. Although these results require additional validation, they still provide new information to understand grain size characteristics of aeolian sand ripple.