中文摘要：

为了揭示新月形沙丘顶点与沙脊线重合和分离的两种过程,对甘肃河西走廊沙区绿洲边缘的新月形沙丘进行了调查观测,并在民勤绿洲边缘新月形沙丘上进行了流场观测。结果表明：1新月形沙丘的顶点与沙脊线的重合和分离两种过程交替进行,春季≥5 m·s^-1的NW风是沙丘顶点与沙脊线重合的主要原因,夏季≥5 m·s^-1的SE风是沙丘顶点与沙脊线分离的主要原因。重合的过程也是沙丘前移的过程。2沙丘顶点与沙脊线重合时,沙丘顶点的风速大于上风向旷野风速;沙丘的顶点与沙脊线分离且当旷野风速≥8 m·s^-1时,沙丘顶点至沙脊线之间的风速小于上风向旷野风速。3从沙丘迎风坡底部到沙丘顶部,中沙增多细沙减少;沙丘各部位的细沙粒度与≥2 m·s^-1的风速正相关,中沙粒度与≥2 m·s^-1风速为负相关;沙丘各部位粒度的不确定性是由两个相反方向的风沙流过程造成的,此过程与沙丘顶点与沙脊线重合和分离两种过程的交替进行是对应一致的。

英文摘要：

There are two statuses of sand activities on the top and ridge line of barchan dunes,i. e. the superposition and separation. In order to reveal the processes of these two statuses,in this study the barchan dunes in the oasis fringe in the desert areas,Gansu Province,were investigated and observed,and the flow field of barchan dunes in the Minqin oasis fringe was also observed. The results are as follows： 1 The processes of superposition and separation of barchan dunes on the top and ridge line occurred alternately. The NW wind with wind speed≥5 m·s^-1 in spring was the main cause of superposition of barchans dunes on the top and ridge line; while the SE wind with wind speed≥ 5 m·s^-1 in summer was the main driving force of separation of barchans dunes on the top and ridge line. The superposition process was also the dune moving process; 2 The wind velocity on the top of the dunes was higher than that at the upwind open field when the top and ridge line of dunes were coincide; the wind velocity from the top to the ridge line of dunes was lower than that at the upwind open field when the top and ridge line of dunes were separated and the wind speed at upwind open field was ≥8. 0 m · s^-1; 3 The sand with particle size of0. 5- 0. 25 mm increased while that with particle size of 0. 25- 0. 05 mm reduced from the bottom of windward slope to the dune top. The sand with particle size of 0. 25- 0. 05 mm was positively correlated with the wind speed≥2 m·s~（-1）at different positions of dunes,while the sand with particle size of 0. 5- 0. 25 mm was negatively correlated with the wind speed≥2 m·s^-1. The uncertainty of particle size at different positions of dune was caused by the two sand flows with opposite directions,which was consistent with the alternative activity of the superposition or separation of the dunes on the top and ridge line.