Study on the Longitudinal Dune Surface Activity in Gurbantunggut Desert

本文利用DETI可移动测风系统对古尔班通古特沙漠半固定沙垄进行了实地观测，结合同期多向集沙和阶梯式积沙试验，系统研究了典型沙垄表面的风速变化和沙面活动规律。得出以下主要结论：（1）垄顶风速放大率随入射角的增大呈指数关系递增，而背风坡风速占垄顶风速的比率，则随入射角的增大呈线性递减的趋势。当入射角从5°变化到85°时，垄顶风速放大率由1增至2.3，背风坡上部风速占垄顶风速的比率则由0.8降至0.2，可见入射角大小控制着沙垄表面风速变化的幅度。（2）大角度入射气流速度变化主要受控于沙垄形态，以爬坡所致的迎风坡气流被压缩、聚合造成的流速逐渐增大为其显著特征，背风坡则以回旋涡流造成的风速剧降为特点；小角度入射气流速度变化主要受控于地表植被状况，以爬坡所致的流速增大作用相对较弱，背风坡形成的弱涡流场不会造成风速的剧降。（3）沙垄底部和下部风速廓线的对数拟合关系较好，两坡上部和沙垄顶部风速廓线发生偏折。气流小角度入射时，迎风坡中部和上部受小半灌木层和灌木层阻滞作用的影响，在40cm和100cm的对应高度上廓线向左偏折，背风侧风速降低幅度不大，在40cm左右的高度上形成弱涡流中心；气流大角度入射时，沙垄各部位廓线斜率明显增大且未发现对应的偏折，背风侧上部各层风速大幅度降低，可见地形的影响占据首位。（4）垄顶风速放大率的存在，大大增加了沙垄顶部的输沙率和活动性，从某种程度上讲是古尔班通古特沙漠长期以来在垄顶部广泛存在流动带的动力因素。特别是在大角度入射的风力作用下，这种效果就更为显著。若以坡脚风速代表区域近地面风速的大小，以中角度入射的平均风速放大率1.7来进行推算，那么在初始风速大于2.9m/s时，垄顶风速即可大于5m/s而产生风沙活动。（5）沙面活动区域主要集中于沙垄顶部约20m宽的范围，垄间和两坡下部鲜有风沙活动。当起沙风从沙垄两侧交错入射时，气流不断地将流动带内的沙物质从一侧搬运到另一侧，因而形成了流动带两侧蚀积量较大的特征；流动带中部作为输沙的床面，虽然输沙量较大但其表面平坦、没有固着沙物质的植被，因此蚀积量较小。（6）各风速级别下，沙垄顶部输沙率的垂线分布均符合指数分布规律，1cm以下的输沙率集中在0.7~3.57g/cm2.min-1之间，18~20cm处的输沙率数值几乎为零；各层输沙率均随风速的增大而增大。输沙量主要分布在0~5cm内，风速为6~14m/s时，5cm以下的输沙量约占输沙总量的77%左右；约90%的输沙量集中在10cm以下。风向相同时，输沙总量随风速的增大而增大，但底层含沙量百分比逐渐减小，上层含沙量逐渐增加。（7）垄顶输沙势与多向集沙之间具有良好的相关性。利用二者的对应关系，初步推测1 VU的输沙风能在1m的过沙宽度上可以产生25kg的输沙量。如果能充分利用研究区的气象资料，即可建立单个沙垄实际输沙量与风速、风向的定量关系，这将对古尔班通古特沙漠的防沙治沙工作具有重要理论和实践意义。Field measurement of airflow velocity at five heights over semi-fixed longitudinal dune surface was made in Gurbantunggut Desert. In the meantime, we use two types of sand traps to carry out the wind erosion and deposition experiment in order to understand the effects of wind direction to the airflow and surface activity on this type of dune. The primary conclusion as flows: （1）There exists a strong linear relationship between wind velocity and angle of incidence. The magnification of wind velocity at the top of dune rise up with the angle’s increase, while the velocity on lee slope as a percentage to the top of the dune reduce with the angle’s increase. The magnification rise from 1 to 2.3 and the percentage reduce from 0.8 to 0.2 when the incidence angles rise from 5° to 85°. The change of wind velocity with what range mainly depends on the angle of incidence. （2）The trait of wind with big incidence angle was mainly dominated by the shape of dune, and that with small angles mainly dominated by the character of vegetation. The airflow velocity increased up at the toss slope because of the air flow aggregation, and decreased down in the upside lee slope. Both speedup and speed-down is weak when the wind incidence with small angle. （3）The wind speed profile in interdune was sameness with lower slope and both of them have a logarithmic increase in velocity with height, and the relationship does not exist at the upper slope and the top of the dune. Effected by the vegetation, when the airflow incidence with small angels, the wind speed profile at the middle and the upper of the toss slope are flexural in the height of 40cm and 100cm and the reduce of the velocity in the lee slope is tiny. According to the character of the speed profile, we primary predicate that the center of the whirlpool is about at 0.4m high from the sand surface. When the airflow incidence with big angels, the increase extent of velocity on windward slop is more than small one, and the fall of velocity in lee slope is more than small angels too. （4）Cite the wind speed in the lower slope is 2.9m/s and incidence with a middle angle as an example: the magnify rate is about 1.7 and when the airflow reach the top of the dune, the speed can increase to 5m/s which make the sand move. To some extent, magnify rate is the dynamical factor to the existence of the mobile strip, especially in large angle incident. （5）The mobile strip on the top of the dune is mainly about 20m, while there are scarcely any activity in the interdune and lower slope. When the wind blowing from the two side of the dune alternately, the sand is taken from one side to another, thus the activity in the verge of the mobile strip is acuteness. Although the activity in the middle of the strip is acuteness too, there is a little deposit here because of the limited vegetation. （6）There exists a strong linear relationship between the rate of sand transporting and height in different wind speed. The range of mass flux below 1cm is 0.7 to 3.57g/cm2.min-1, and there almost no sand in the height of 18 to 20cm. The rate of sand transporting increase with the velocity, and about 77% sand below in 5cm, 90% below in 10cm. The sand collected by the traps increase with the velocity as the wind blow from the same direction, and the proportion of quantity which in the top layer also increase with the velocity. （7）A strong relationship exists between drift potential and mass flux in the top of the dune. Base on this relationship, we primary predicate that the wind energy scale with 1VU can make 25kg sand move in 1m of the mobile trip. If we can use the windy data which collected in the study area, and establish the ration relationship between mass flux and wind data, this will be available to the protection in Gurbantunggut desert

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