Southeastern Edge Of The Tengger Desert Sand Flow Characteristics

Aeolian sediment is a kind of movement characterized by the group particles above the surface, which can not only be used to describe the condition of erosion and accumulation near the surface and to investigate into the formation and development principles on the issue of the aeolian landforms, but also be one of the most significant theory regarding the research of aeolian landforms and the establishment of sand prevention and control engineering. Therefore, research into the issue on aeolian sediment plays a fundamental role in the aeolian science field. Field observations conducted as the direct measurements of aeolian transport over the sand surface and dunes have been at the very foundation of providing a thorough understanding of sand transport since the spatial and temporal changes in aeolian transport, surface moisture or treatment and the fetch length etc. are basic factors that control the sediment transport of the dune in the wild. Aeolian transport indicates the result of wind-surface interactions, and hence relies strongly on variations in the characteristics of the sediment surface. In addition, the spatial and temporal changes in transport rate over a dune are main factors which control the morphology of the dune. We obtain the direct observation of aeolian transport on the plot with the different surface treatments and over a developing transverse dune at the Aeolian Experiment Site in the southeastern part of China’s Tengger Desert in order to preliminary investigate into the principles about aeolian transport on the three plots with different surface and over a transverse dunes on which we established four observation points.We compare the effects on aeolian transport that the different surface treatment has by means of conducting field observations of aeolian transport using LDDSEG sampler to collect the sediments within lm height in three80m×80m plots with different surface treatments including gravel-covered sand, enclosed shifting sand. and open (unprotected) shifting sand. To do this, we analyze the transport rate and the flux density profile above each surface and the observation results show as follows:(1)Transport rate. The transport rates above the three plots with different surface treatment mainly increase proportionally with the cube of the mean wind velocity and with the maximum wind velocity during the observation period as a whole, but we obtain that the transport rate was more strongly correlated with sand drift potential. Furthermore, transport rates above the plot with open shifting sand are greater than those above the plots with enclosed shifting sand and the gravel-covered plot.(2)Flux density profiles. The flux density profiles for all three surface treatments follow the exponential decay law that was proposed by most researchers previously to describe the characteristics of saltation flux density profiles. Apart from that, the flux density profiles for enclosed shifting sand and the open shifting sand are very similar, but that both differ highly from that for the gravel-covered sand on which the flux density shows that transport rate decays more slowly with increasing height, producing flux density profiles with a higher average saltation height. Therefore, the surface treatments influence both saltation of sand particles and the flux density profile above the surface.(3) Coefficients of flux density function. Coefficients of the exponential decay function describing the flux density profiles are defined as a function of the wind velocity and the surface. Coefficient a bears relationships with the transport rate, and the coefficient b represents the relative decay rate of flux density with increasing height and the average saltation height. The greater the b value, the more slightly the flux density at the different observation points over the transverse dune decays with increasing height, which implies that the greater the wind speed, the more sand particles will move in the upper air. Moreover, we know that value b largely depends on both the surface treatments and the wind speed as well.The transport rate as a function of distance varies during the formation of the dune. We perform the direct observation at four points on the transverse dune:the toe, mid-stoss, crest, and leeward foot of a developing dune so that the variation principles of transport rate and flux density profiles over a developing dune can be investigated. The results indicate that:(1)Transport rate. Due to the dune observed being the initial stage, the transport rate of the transverse dune during the research does not reach its equilibrium value, which can be defined as a "relaxation" process that occurs before saturation of the sediment transport rate. The transport rate versus wind speed relationship is complicated over a developing dune compared with the relationships over flat surface and over dunes that are in equilibrium with the local wind. Therefore, the variation in the transport rate as a function of distance varied constantly during the field observation. Trend lines for transport rate over the transverse dune manifest that the transport rate tends to increase with respect to wind speed and that generally increased from the toe to the crest above the stoss slope of the dune as well. Meanwhile, the transport rate for the toe is similar to that for the leeward foot of the dune.(2)The relationships of crest/toe ratio for transport rates. The crest/toe ratio for transport rates seems to depend greatly on dune height. In present study, the crest/toe ratio for transport rate over the2.5m tall transverse dune was much lower than those calculated for taller dunes. Contrary to the previous conclusion based on field observations that the crest/toe ratio for transport rate decreases with increasing wind velocity measured at the toe, we find that this ratio increased with increasing wind speed.(3)Flux density profiles. Flux density profiles of the transverse dune at different points show that sediments near the surface are much more than higher air, and flux density generally decreases as a function of height. Flux density profiles for different points above the dune at different wind speeds are well described by the exponential decay law, as has been proposed for saltation flux density profiles.(4)Coefficients of the flux density profiles function. Coefficients in the flux density profile function can be defined in the light of the transport rate and wind speed. However, the dependence of relative decay rate with height and average saltation height on wind speed was weaker than that observed in a wind tunnel and above a flat surface.