Field observation on the sand transport over transverse dune's crest
Zhengcai Zhang, Zhibao Dong
Key Laboratory of Desert and Desertification, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou, China
Dune movement processes is sand transport processes, especially for the duneʹs crest. However, there are little research on sand transport over dune crest. Meanwhile, in the field conditions, wind direction maybe changed at any time, and flow dynamics over dune surface are controlled by topographic forcing and steering effects and are sensitive to changes in incident flow angle. To explain dune geomorphology (dune crest in this paper) and flow direction effect on the sand transport, 76 field measurement were done over dune crest at the Shapotou Aeolian Experiment Site (northeastern Tengger Desert, 37°33'43.78" N, 105°01'56.28" E). The transported sediment were measured by the LDDSEG vertical segmented sediment sampler designed by the Key Laboratory of Desert and Desertification, Chinese Academy of Sciences.
Our field observation indicated that roughness length calculated by wind profiles was much larger than previous study results, linear regression analysis indicated that the modified Charnock model is best for relationship between shear velocity and roughness length (Cm = 0.432, R2 = 0.997). Sand transport flux can be expressed as exponential functions and Gaussian function over dune crest. For the complex relationship between sand transport and wind conditions (mean velocity, maximum wind velocity, shear velocity, potential sand transport, and wind direction) and surface properties (roughness length, fetch length), correlation coefficient analysis indicated that sand transport rate over dune crest is positive greatly related to mean velocity (R2=0.58) and shear velocity (R2=0.57), and is almost no relationship between humidity, however, is negative related to wind direction (R2=-0.37). Wind direction affect sand transport over dune crest and the coefficients of sand transport flux, and the critical wind direction is 272°, which is close to the perpendicular line of crest about 37° in this paper.