| Shrub communities,which are widely distributed in desert-oasis transitional zones,are important natural windbreaks and sand fixation protective barriers of oases.They are important in improving the oasis environment.Nebkhas are vegetated dunes(also called shrub-coppice dunes),developed by accumulation of aeolian sand within and around shrubs.They are primarily distributed in arid and semiarid regions around the world.Because of their importance in aeolian processes and geomorphology,their development and dynamics,morphological characteristics,surface flow features,and vegetation characteristics have received much attention from researchers.However,interactions between sand transport,vegetation geometry and dune formation have not been well studied,and few details pertaining to wind flow and sand movement over nebkhas have been reported in the literature.As a typical nebkha,Nitraria nebkhas(formed by Nitraria tangutorum and Nitraria sphaerocarpa)are widely distributed in the ecotone between oasis and desert,desert edges,and dry lake basins in arid Northwest China.Owing to their prostrate characteristics,multi-branch growth habits,and ability to produce offspring through clonal propagation when their stems are buried by sand,Nitraria shrubs have a strong capacity to capture wind-driven sediments and form Nitraria nebkhas.In the present study,we add detail to our knowledge of wind field and sand movement over nebkhas with distinct morphological characteristics.The study area was Minqin County in Gansu Province,China,in the lower reaches of the Shiyang River Basin and eastern Hexi corridor.We address the development of nebkhas under the same wind energy and sand supply,through field experiment and computational simulation.The results show the following.(1)Height,width and total length are substantially interrelated for all three development stages.Increases in one of these three dimensions are accompanied by those in the other two,ensuring that the bottom projection of the nebkhas can always maintain an elliptical or oval shape.With the increase of height and width,the nose and tail become gradually symmetric in length and shape,and this tendency is demonstrated in the field by an outline varying from parabolic shapes to elliptic cones.(2)The internal structure of a stabilized nebkha is characterized by gradually clearing strata with height.Strata were distinguished by a clay layer.The character of sand particle diameter in different strata was not clear and the nebkhas were mainly composed of very fine sand,fine sand,and sand(?63–?500 um).From the windward to leeward side of the nebkha,mean diameter of sand particles decreased from 195.0 um(windward)to 172.7 um(middle)and 149.9 um(leeward).(3)During a gale from the northwest,airflow patterns at longitudinal and transverse lines of the nebkhas were affected by bleed flow through shrubs and displaced flow around or above them,and were complicated by their interaction.Although wind speed along the longitudinal line of three nebkhas declined to their minimum at the upside of the leeward side.Followed by an immediate acceleration,the variation of wind speed measured along the transverse line of the three Nitraria nebkhas was more sophisticated,owing to variation of the dominant flow around or above them.This was bleed flow through the shrubs to separation flow and vortices,and then separated and reattached flow on their leeward side.Horizontal sand flux decreased on the windward side and increased on the leeward side along a longitudinal line,approached the value measured at the windward side toe at the point of the leeward side toe for all three nebkhas.Differences among them were remarkable,which were caused by the change of obstacles from shrubs to solid inner,and by variation of the wind field.(4)During the gale from the northwest,the three Nitraria nebkhas mainly experienced a period of deposition,and their solid inners continued to enlarge,but deposition thickness was different.Furthermore,the contributions of solid inners of the particular nebkhas caused variation in their integral porosity.The leeward toe of the Nitraria nebkha in the first stage was eroded,while deposition occurred at the same position for the other two nebkhas.Because of its large body,more energy was accumulated in front of the windward slope and was dominated by separated and reattached flow on the leeward side,the Nitraria nebkha in the stable stage not only was eroded at the toe of the windward side and its top,but also had the smallest mean deposition thickness.During a gale from the east,the three Nitraria nebkhas experienced a different period of erosion/deposition,and major deposition on the windward side and erosion on the leeward side were different.(5)The analysis shows that wind direction variation between northwest,northeast and east in the study area is the reason for the formation of sediment layers that were mainly composed of coarse sand particles.Additionally,sheltering of the Nitraria shrubs formed and conserved the sediment layers on the windward side.The clay layer deposited and developed after sandstorms is key to enlarging and uplifting of the leeward side of the nebkhas,as is the wind speed.(6)The airflow field and sediment transport patterns of the nebkhas are affected by the change in the dominant flow around or above them.In the early stage of the nebkha,bleed flow through the shrubs is dominant,which governs nebkha evolution.Then,separation flow and vortices formed by interaction between flow passing through shrubs and that bypassing the irregular-shaped solid inner play a leading role in the transition phase of nebkhas,between their early and stabilizing stages.Finally,separated and reattached flow formed by interaction between flow passing through shrubs and that bypassing the solid inner with conical shape becomes dominant during the stabilizing stage.In general,the variation and ultimate morphology of the Nitraria nebkha are determined by interactions between morphology,airflow,sediment transport,and sediment erosion/deposition. |
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