Xunming Wang

The flux profile of a blowing sand cloud: a wind tunnel investigation

Abstract The flux profile of a blowing sand cloud, or the variation of blown sand flux with height, is the reflection of blown sand particles that move in different trajectories, and also the basis for checking drifting sand. Here we report the wind tunnel results of systematic tests of the flux profiles of different sized sands at different free-stream wind velocities. The results reveal that within the 60-cm near-surface layer, the decay of blown sand flux with height can be expressed by an exponential function: q h = a exp(− h / b ), where, q h is the blown sand transport rate at height h , a and b are parameters that vary with wind velocity and sand size. The significance of coefficient a and b in the function is defined: a represents the transport rate in true creep and b implies the relative decay rate with height of the blown sand transport rate. The true creep fraction, the ratio of the sand transported on the surface ( h =0) to the total transport varies widely, decreasing with both sand size and wind speed. The flux profiles are converted to straight lines by plotting sand transport rate, q h , on a log-scale. The slope of the straight lines that represents the relative decay rate with height of sand transport rate decreases with an increase in free-stream wind velocity and sand grain size, implying that relatively more of the blown sand is transported to greater heights as grain size and wind speed increase. The average saltating height represented by the height where 50% of the cumulative flux percentage occurs increases with both wind speed and grain size, implying that saltation becomes more intense as grain size and/or wind velocity increase.

The blown sand flux over a sandy surface: a wind tunnel investigation on the fetch effect

Abstract Detailed wind tunnel tests were conducted to examine the fetch effect of a sandy surface on a sand cloud blowing over it. The results suggest that the fetch length of a sandy surface has a significant effect on both the vertical flux profile and total horizontal flux. The sand flux over a sandy surface increases with height in the very near surface layer, but then decays exponentially. In agreement with the widely accepted conclusion, the decay function can be expressed by q=aexp(−h/b), where q is the sand flux at height h. Coefficient a that tends to increase with wind speed implies the influence of wind, while coefficient b that defines the relative decay rate shows the influence of both the fetch and wind. The relative decay rate increases with fetch when the fetch length is short, then becomes constant when the fetch reaches a certain length. The threshold fetch length over which the relative decay rate keeps constant increases with wind speed. The average saltation height generally increases with fetch. Both the relative decay rate and average saltation height show that the fetch effect on the flux profile becomes more significant when the wind speed increases. The total sand transport equation for the total fetch can be expressed by Q=C(1−Ut/U)2U3(ρ/g), where Q is the total sand transport rate, U and Ut are the wind velocity and threshold wind velocity at the centerline height of the wind tunnel, respectively, g is gravitational acceleration, ρ is the density of air, and C is a proportionality coefficient that increases with the fetch length, implying that the total sand flux increases with the fetch length.

Aerodynamic roughness of gravel surfaces

Abstract The interactions between surface winds and gravel surfaces that can be characterized by aerodynamic roughness length have important implications for sediment mobilization, transport, sedimentation and the development of desert pavements (‘gobi’ deserts). Wind tunnel results of systematic tests carried out to determine the aerodynamic roughness length of gravel surfaces of different size and coverage at a range of free-stream wind velocities are reported. The aerodynamic roughness of gravel surfaces is a function of gravel size, gravel coverage and free-stream wind velocity. The 1/30 law proposed by Bagnold, based on Nikuradses results of water pipe flow, or taking the aerodynamic roughness length as a fixed percentage of roughness element height is not supported for gravel surfaces. The aerodynamic roughness of gravel surfaces decreases with free-stream wind velocity. The variation of aerodynamic roughness with gravel coverage can be described by quadratic curves. Maximum aerodynamic roughness length occurs at gravel coverage range above 40–75%, increasing with gravel size. Multivariate models based on free-stream wind velocity and gravel coverage have been developed for predicting the aerodynamic roughness of different gravel-sized surfaces. The maturity of the roughness effects of gravel surfaces increases with gravel coverage. In general, the roughness effects are fully developed when the gravel coverage is over 15%.

Grain size characteristics of dune sands in the central Taklimakan Sand Sea

Abstract Compared with the sand grain size of the other sand seas, central Taklimakan Sand Sea has some of the finest sands seen globally. The dunes are composed of fine and very fine sands with a diameter between 2.00 and 4.00 φ (0.25–0.063 mm). There are differences in the grain size distributions for different dune types. Mean grain size of the compound/complex crescent dune sands is 3.08 φ; the compound dome dunes 3.21 φ; the compound/complex linear dune sands between 2.63 and 3.41 φ; and the star dunes 2.81 φ. From the northern edge of the sand sea towards southern edge, the components of fine and very fine sands increase, closely related to the wind regimes, time scale for dune development, and underlying sediments. Compared with the foregoing, there are some differences for the compound/complex linear dunes in the centre of Taklimakan. Though the pattern of coarser crests does exist, the sorting parameters indicate that the sands on the west flank of compound/complex linear dunes are better sorted than that on the east flank and crest. Sampling on the superimposed dunes developed on the surface of the complex linear dunes suggests that the patterns of finer crest, coarser crest all existed in central Taklimakan. Under the conditions of low energy wind regime when fine sand is available, the frequency of the pattern for finer crest and better sorting will increase. Sampling on the simple crescent dunes developed on the interdunes suggests that the simple dunes are developed originally with nearly the same grain size distributions and parameters. With the development of the dunes and evolution of the dune morphology, the grain size distributions of the dune sands varied under the actions of the wind regimes, time scale and underlying sediments.

Geomorphology of sand dunes in the Northeast Taklimakan Desert

Three types of sand dunes exist in the Taklimakan Desert, namely compound/complex crescent dunes and crescent chains, compound dome dunes and compound/complex linear dunes. Besides these three compound/complex types, single simple dunes are also distributed throughout the sand sea. The compound/complex linear dunes are developed under acute bimodal wind regimes. Though the ratios of the resultant drift potential (RDP) and the drift potential (DP) are the same as that near the border and adjacent area of the sand sea, the compound/complex crescent and dome dunes are developed, respectively, because of divergence of the sand available, the stress of the sand-moving winds and the time scales of dune formation. The sand supply for the dunes is not from Lopo Nor in the east as previous studies suggested but mainly from local alluvial or lacustrine deposits. The grain size component does not correlate evidently to the morphology parameters of the sand dunes. Analyses of the DP and drift direction suggest that the northeast Taklimakan is an area of low wind energy and the resultant drift direction (RDD) coincides well with the distribution, morphology and scales of the dunes.

Monitoring sand dune advance in the Taklimakan Desert

The migration rate of sand dunes is important for the design of the sand-control system in the Taklimakan Desert. Sand dune movement was monitored in a sample plot along the desert-crossing highway by means of a topographical survey in late 1991, 1992 and 1993. The results reveal that most of the morphometric parameters of the dunes are not as well correlated as they are generally supposed to be. The mean advance rate of the dunes was 7.29 and 5.56 m year−1 in 1992 and 1993, respectively. The advance direction is towards the SW, approximately in accordance with the local resultant wind direction. The advance rate of the dunes was controlled by the local wind regime and dune morphometry. However, the relationships between advance rate and the morphometric parameters of the dunes are variable for the immature dunes. The small scale and immaturity of the dunes, insufficient sand supply, and complex wind modes are responsible for the complex dune parameter relationship, involving frequent changes of dune morphology, as well as changes of dune advance rate.

Sand sea activity and interactions with climatic parameters in the Taklimakan Sand Sea, China

An analysis is undertaken of the temporal and spatial variability of climatic parameters including precipitation, potential evaporation and wind regimes that control the aeolian activity in the Taklimakan Sand Sea. Mobility index is calculated according to the mode that Lancaster (J. Arid Environ. 14 (1988) 233) suggested. Our results show that there are apparent temporal and spatial variations for this mobility index. The annual aeolian activity varies in phase with the number of windy days, precipitation and potential evaporation. Dune development is highly consistent with distribution of the mobility index. The mobility index shows that the Taklimakan Sand Sea experienced high aeolian sand activity during the 1960s and mid-1980s, and then related sand transport was reduced from the mid-1980s to the late 1990s. The mobility index cycle cannot be obtained because the available meteorological data cover only short periods. Such an approach will require longer research combined with field observations of sand movement.

Dynamic processes of a simple linear dune--a study in the Taklimakan Sand Sea, China

Samples of dune sands, surveys of the morphology and field measurements of wind velocity and direction of a simple linear dune in Taklimakan Sand Sea show that the airflow and sand flux vary with the change of wind direction on the dune surface. Decrease of the airflow stress on the lee flank does not result in much decrease of the sand flux because of the low threshold shear velocities and the airflow conditions. There are no significant relations between the sand flux on the lee flank and the angle of incidence of the airflow. The low threshold shear velocities and the maintenance of the sand flux at the lee flank are the main mechanisms keeping the linear shape of the dunes. Measurements of the sand flux shows that it reaches a maximum on the crest of the dune. The grain size of the transported sands has some differences compared to that of the dune surface. The sands transported are finer than that on the dune surface, but better sorted under the influence of the medium to low wind activity. The field experiment results exhibit that it is possible for the dunes to be shaped as linear dunes during the processes of accumulation and elongation.

Nebkha (coppice dune) formation and significance to environmental change reconstructions in arid and semiarid areas

Environments in arid and semiarid regions are extremely sensitive to climate changes. High wind activity in these regions has resulted in an extensively developed arid geomorphology, but past environmental changes are poorly understood because of the absence of relatively high-resolution proxies. The accumulation characteristics of nebkhas, which have developed extensively in these regions, can be used as a method of reconstructing environmental changes. Here we summarized recent advances in research on the formation, development, and sediment characteristics of nebkhas and their significance to environmental changes in arid and semiarid regions. Based on the studies of our colleagues, we suggested that research on nebkha formation can provide distinct clues about environmental changes in arid and semiarid regions; however, continued studies are needed.

Effects of aeolian processes on nutrient loss from surface soils and their significance for sandy desertification in Mu Us Desert, China: a wind tunnel approach

Mu Us Desert, a region with high aeolian activity, is at extremely high risk of sandy desertification. Using surface soil samples collected from Mu Us Desert of northern China, we evaluated the effects of aeolian processes on nutrient loss from surface soils by employing wind tunnel experiments. The experiments were conducted using free-stream wind velocities of 14, 16, 18 and 22 m/s. Our results showed that the fine particles (<50 μm in diameter; 12.28% of all transported materials) carrying large nutrient loadings were exported outside the study area by aeolian processes. After the erodible fine particles were transported away from the soil surfaces at low wind velocity (i.e. 14 m/s), the following relatively high wind velocity (i.e. 22 m/s) did not have any significant effect on nutrient export, because the coefficients of variation for soil organic matter, total phosphorus, total nitrogen and available potassium were usually <5%. Our experimental results confirmed that aeolian processes result in a large amount of nutrient export, and consequently increase the risk of sandy desertification in arid and semi-arid ecosystems.