沙坡头铁路防护体系内风沙沉积的粒度特征

包兰铁路沙坡头段铁路防护体系内,由于下垫面性质发生变化,流沙区至人工植被带地表沉积物中表层土壤的黏粒和粉沙含量显著增大,平均粒径减小,颗粒分选性显著变差。防护体系内表层沉积物中的粉沙含量与地表相对高程反相关,黏粒含量与地形起伏没有直接关系,颗粒平均粒径、分选性与地表相对高程正相关。在土层垂直方向上,表层0~5cm内土壤粉沙含量较高,颗粒分选性差;5cm以下深度内土壤粉沙含量减少,颗粒分选性变好。在防护体系内沙丘地貌的不同部位,沙尘沉积速率不同。据此估算表明防护体系内固定沙丘迎风坡顶部沉积速率为0.147 cm/a左右;背风坡和丘间地平均堆积速率大于0.588cm/a。

Grain-size characteristics of aeolian deposits in the shelterbelt of railway at Shapotou

By sampling along a representative section across the shelterbelt of Baotou-Lanzhou Railway at Shapotou area,grainsize characteristics of aeolian deposits in the shelterbelt and the influences of landform were studied.The total 20 sampling points covered various landform sites such as the windward and leeward slopes of sand dunes,dune crests,and inter-dune depressions.Three of which were located on the surface of a mobile dune at the upwind edge of the shelterbelt,and the other points were located in the straw checkerboard and artificial vegetation belt.The results show that clay and silt contents in surface soil increase greatly and mean grain-size decreases from moving sand area to the artificial vegetational belt due to the changes of the surface properties,and sorting becomes poor.It seems that silt contents decrease with relative height of surface in the shelterbelt;mean grain-size and sorting increase with surface height.Statistics further show that mean grain-size,sorting coefficient and silt content were well linearly related to the relative elevation.Dependence relation between clay content and landform was not observed.The possible cause is that clay material in the deposition was mainly sourced from general dust events.The influence of the landform on dust deposition is very limited.Clay sediments tend to cumulate in the shelterbelt under the protection of the straw checkerboard and artificial vegetation but hardly accumulate in moving sand area due to intensive aeolian activities.Vertically,silt content is high and sorting is poor in the surface 5 cm layer;below this depth silt content decreases and sorting becomes well.The grain-size profile varies among landforms.Estimated aeolian deposition rates around the crest of fixed dunes average 0.147 cm per year,versus more than 0.588 cm on the leeward side and in inter-dune depressions.As a whole,this study revealed the changing regulation of grain-size characteristics along the direction of NNW-SSE in the shelterbelt of Baotou-Lanzhou Railway at Shapotou area,demonstrated the relations between grain-size characteristics and landform,as well as the relations between deposition rates and landform.The results objectively reflected the effects of the landform on aeolian deposition in the shelterbelt.