Study On Development Mechanism Of Nebkha In The Basin Of Hetian River, Xinjiang

Nabkha is a common kind of biological wind deposition geomorphy in arid and semi-humid desert, and sandy coast. It has important significance in the aeolian geomorphology, monitoring and evaluating land desertification and regional environmental evolution so on.In this paper, the typical nabkha is the research object in the basin of Hetian River in Xinjiang. Two banks of Hetian River in Taklimakan Desert hinterland and the transition zone of the oasis and the desert have been investigated, according to Aeolian geomorphology, Aeolian physics, Phytogeography, Phytoecology, Sedimentation and other discipline theory. Shape data of 223 Tamarix nabkha, 120 Phragmite nabkha and 123 Alaghi nabkha have been measured, and about 500 sedimentary samples have been collected from the surface of three kinds of nabkhas, and the wind speed section and wind-sand flow at the top of the typical nabkha have been observed at the boundary zone of oasis in Luopu and Moyu county. Furthermore, In order to clarify the dynamic mechanism of nabkha growth and reflect nabkha's material composition, the wind-tunnel imitative experiment and the grain size analysis have been done. Through comprehensive analysis, the main conclusions can be summarized as follows:In the basin of Hetian River, Tamarix, Phragmite and Alaghi nabkhas are main types. Tamarix nabkhas mainly distribute in flood plain, silty flat, dry lake and marshes plains, river's banks, terraces, typical saline soil areas, interdunes low ground in moving dunes and oasis edge. Phragmite nabkhas lie in Hetian River's terraces, oasis edge, silted land and flood plains. Alaghi nabkhas scatter in the sandy land along river's bank, the land in the flood and alluvial plain which have relatively adequate moisture and moderate salinity, the region which is interfered frequently and oasis edge.The region is lower wind energy environment, and capability of transported sands is weaker, and it results that the space geometry shape of nabkhas are approximate semi-ellipsoid which have few difference in slope-length and slope-gradient of between upward and leeward sides in the basin of Hetian River. At the growth stage, Tamarix nabkha mainly appears approximately conical, and at the mature and stable stage, they are approximate hemispherical. While the whole shape of Phragmite nabkha is approximate semi-ellipsoid which have shorter and steeper upward slopeand longer and lower gradient leeward slope. The shape of Alaghi nabkha presents approximate semi-ellipsoid which have longer and lower gradient upward slope and shorter and steeper leeward slope. At the growth stage, the long axis and short axis of all kinds of nabkhas are in synergic growth. The horizontal scale and height of nabkha show significant correlation.The shrub induces the change of the flow pattern structure on the ground, and interferes with wind-sand flow so that the wind-sand flow converts into over-saturation from not-saturation status in the vortex area behind shrub. In addition, shrub blocks the wind-sand flow so that partly solid fragment material to accumulate in and behind the shrub. So the shrub plays an important role in maintaining the shape and promoting the growth of nabkha.The character of soil erosion and accumulation around shrub shows that the obvious wind erosion area distributes at the upward side of the root, and that area of sand accumulated is in the weak vortex region at the leeward side of the shrub, and that the wind erosion area lies in the air flow recovery area at the leeward side of shrub and the air flow accelerated area at the both sides. The growth area of nabkha is the area of sand accumulation.Plants at the top of different shape nabkhas all enhance the rugosity of nabkhas, retard airflow to get over nabkhas, reduce energy, eliminate common wind-erosion areas by strong wind at the top of nabkhas, and reinforce influence range of vortex at leeward side of nabkhas. These result in the change of the structural character of wind-sand flow. Plant plays a very important role in changing airflow pattern structure, promoting the dusty and sand to accumulate, and nabkhas to grow up.Sediments on Tamarix, Phragmite and Alaghi nabkhas are silty sand. Grain size distribution scopes of sediments on Tamarix , Phragmite and Alaghi nabkhas are respectively from 0.399μm to 1002.374μm , from 25.178μm to 282.507μm, and and from 0.447μm to 2000μm. The combined action of wind-sand flow, nabkha shape, shrub and sediments results that grade, average grain size and sorting of sediments change regularily on different part of nabkhas.The sand source and wind power of region and shrub affect development of nabkha. Tamarix nabkha also experiences the same stage when Tamarix shrub grows, stablizes and declines. Tamarix shrub's disturbance to wind-sand flow and the shape of nabkha's dynamic equilibrium are all different in every stage. In the Tamarix nabkha's growth stage, the Tamarix displays moderate horizontal growth, and sandand dust are rapidly deposited to form conical nabkha. In the stabilization stage, Tamarix nabkha's shape appears semispherical and is intensely effected by Tamarix shrub's dynamical action. In the declination stage, Tamarix shrub is withered, and the arc dune starts to appear at upwind side, or the shadow dune arises at leeward side.There are two types of modes of nabkha growth in the basin of Hetian River.One is induced by river migration on banks of Hetian River in Taklimakan Desert hinterland, the other is caused activity of people in the transition zone between the oasis and the desert.