Grain-size characteristics of aeolian deposits in the shelterbelt of railway at Shapotou
沙坡头铁路防护体系内风沙沉积的粒度特征

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.