| Wind is the primary control the formation of aeolian geomorphology. The wind which causes sand movement is almost always turbulent and the velocity magnitude and direction of each point are fluctuating over time, so revealing the wind velocity fluctuation is the theoretical basis of further studying wind and sand transport. This paper systematically analyzes the characteristics of wind velocity fluctuation of the slopes in the lower reaches of the Laoha River in sand-windy weather, which is based on the statistical analysis of the features of sand-driving wind in the lower reaches of the Laoha River from 2006 to 2013 and uses wind speed data obtained from field observations on the slopes of both sides along the downstream area of the Laoha River in the spring of 2013, achieving the goal of having a further understanding of the interaction mechanism of wind and valley slopes, and then providing a theoretical basis for making clear the formation of riverine dune belts in this area. The results are as follows:(1) The spring (from March to May) is the main phase of sand-drift activity in lower reaches of the Laoha River. During the phase of sand-drift activity, the directions of sand-driving wind were mainly WNW、NW; the drift potential (DP) is 70.50 VU; the resultant drift potential (RDP) is 37.89 VU, belonging to intermediate wind energy environment; the resultant drift direction (RDD) is 108.3°; the wind direction variability (RDP/DP)is 0.54, belonging to intermediate ratio and sharp bimodal wind regime.(2) The characteristics of wind velocity fluctuation at valley slope of the lee side in the lower reaches of the Laoha River:the point of 0.25 m at the slope top is significantly affected by vegetation and sand saltation movement, and except this, as the height from the earth’s surface increases, the value and the intensity of wind velocity fluctuation will increase accordingly, and the maximum value is 4.04 m·s-1 and the relative value (gustiness) is basically stable. However, the value and the relative value (gustiness) of wind velocity fluctuation are unstable at the slope foot of the leeward wind owing to the presence of "secondary air-flow" resulted from the mutation of the terrain of valley slope. As the height from the earth’s surface increases, the intensity of wind velocity fluctuation will gradually increase and the maximum value is 2.98 m·s-1. The value and the intensity of wind velocity fluctuation at the slope top are greater than the ones at the slope foot the leeward wind whereas the relative value is less than the one at the slope foot of the lee slope.(3)The characteristics of wind velocity fluctuation at the windward slope in lower reaches of the Laoha River:either at the foot or the top have the larger the distance between the surface of the wider range of wind velocity fluctuation. Whether at the slope foot or at the slope top, the higher the height from the earth’s surface is, the wider the range of wind velocity fluctuation is, and the maximum value is 4.49 m·s-1. Besides, the intensity of wind velocity fluctuation at both places will increases with the increase of height. While if the height is over 0.5m, the relative value (gustiness) of wind velocity fluctuation is basically at a stable level, but the intensity at the top will relatively decrease due to the disturbance from the escarpment.(4) It is found that the values of wind velocity fluctuation of valley slopes are very great in sand-windy weather conditions by comparing the characteristics of wind velocity fluctuation of two slopes in the lower reaches of the Laoha River and this phenomenon should be paid much attention to. In the special background of atmospheric circulation, the wind velocity fluctuation at valley slopes is not only related to the inherent air vortex diffusion but also is deeply influenced by the sand movement on the overlying surface, vegetation and especially airflow changes on the surface of valley slopes caused by changes in the terrain of valley slopes. |
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