Alternating Processes of Distribution of Grain Size over Barchan Dunes：A Case Study of Accumulated Sand-belts of Oasis Edge of Hexi Desert Area of Gansu Province

Barchan dunes is a common dune type.There are a lot of barchan dunes at accumulated sand-belts of the oasis edge of Hexi desert area of Gansu.Characteristic of grain size were analyzed by comprehensive investigation and sampling in the Hexi Corridor desert area，and repeated sampling and determination in Minqing desert area.The surface layer of crescent dunes and crescent dunes was mainly composed of fine sand and medium sand at 0-5 cm depth.The sand grains in Gulang sandy area were finer and grain sizes were quite different from those of Jinchang，Linze，Jinta and Minqin dunes.The grain size of the main wind direction （NW） process from the bottom of windward slope to the top of dune changes from coarse to fine; the bottom of windward slope has the most coarse sand，followed by the middle of the windward slope.The grain size of reverse wind direction （SE） process from the bottom of the windward slope of dune to the top of dune changes from coarsening to fining and then to coarsening.The bottom of windward slope has the most coarse sand，followed by the middle of windward slope.The standard deviation and variation coefficients of dune grain size in the main wind direction process were larger than those in reverse wind direction process.The two processes of main wind direction and reverse wind direction lead to the alternate change of grain size in each part of the dune.Because of the different of seasons observed，two different conclusions about the distribution of grain size of windward slope of crescent dune were drawn.