Aerodynamic entrainment threshold: effects of boundary layer flow conditions

Experimental data are presented demonstrating the influence of boundary layer flow conditions on aerodynamic entrainment of grains in the absence of intersaltation collisions. New methods are proposed for (1) the unambiguous determination of aerodynamic threshold for any grain population and (2) approximation of the probability density function (PDF) distributions of threshold shear velocity for aerodynamic entrainment. In wind tunnel experiments, the orderly spatial development of flow conditions within a developing boundary layer over the roughened surface of a flat plate constrains the aerodynamic threshold condition in terms of both mean and fluctuating values. Initial grain dislodgements and subsequent erosion from narrow strips of loose, finely fractionated ballotini were recorded photographically as wind speed was increased. Boundary layer parameters, including average threshold shear velocity (U_*t), were calculated using the momentum integral method. Direct observations show that sporadic oscillation of grains preceded dislodgement. At slightly higher velocities most grains rolled over their neighbours before entering saltation. Initial entrainment in spatially semi-organized flurries of 50 or more grains was followed by quiescent periods at airflow velocities close to threshold. These observations provide strong circumstantial evidence linking both the nature and spatial pattern of initial grain motions to sweep events during the fluid bursting process. For each grain fraction, values of U_*t were found to span an unexpectedly wide range and to decrease downwind from the leading edge of the plate as turbulence intensity increased. A probabilistic entrainment model is applied to the aerodynamic threshold condition so as to incorporate the effects of changing turbulent flow regimes over the plate. Analysis of strip erosion curves gives both an objective definition of the threshold condition and usable approximations of the PDF for U_*t required by the model and for future stochastic treatment of the threshold condition.     

Aerodynamic grain‐size distribution of blown sand

Aeolian sand entrainment, saltation and deposition are important and closely related near surface processes. Determining how grains are sorted by wind requires a detailed understanding of how aerodynamic sand transport processes vary within the saltating layer with height above the bed. Grain‐size distribution of sand throughout the saltation layer and, in particular, how the associated flux of different grain size changes with variation in wind velocity, remain unclear. In the present study, a blowdown wind tunnel with a 50 cm thick boundary layer was used to investigate saltating sand grains by analyzing the weight percentage and transport flux of different grain‐size fractions and the mean grain size at different wind velocities. It was found that mean grain size decreases with height above the sand bed before undergoing a reversal. The height of the reversal point ranges from 4 to 40 cm, and increases with wind velocity following a non‐linear relationship. The content of the finer fractions (very fine and fine sand) initially increases above the sand bed and then decreases slightly with height, whereas that of the coarser fractions (medium and coarse sand) exhibits the opposite trend. The content of coarser grains and the mean grain size of sand in the saltation layer increase with wind velocity, indicating erosional selectivity with respect to grains in multi‐sized sand beds; but this size selectivity decreases with increasing wind velocity. The vertical mass flux structure of fine sand and very fine sand does not obey a general exponential decay pattern under strong wind conditions; and the coarser the sand grain, the greater the decrease rate of their transport mass with height. The results of these experiments suggest that the grain‐size distribution of a saltating sand cloud is governed by both wind velocity and height within the near‐surface boundary layer.     

The initiation of particle movement by wind

When air blows across the surface of dry, loose sand, a critical shear velocity (fluid threshold, ut), must be achieved to initiate motion. However, since most natural sediments consist of a range of grain sizes, fluid threshold for any sediment cannot be defined by a finite value but should be viewed as a threshold range which is a function of the size, shape, sorting and packing of the surface sediment. In order to investigate the initiation of particle movement by wind a series of wind-tunnel tests was carried out on a range of pre-screened fluvial sands and commercially available glass beads with differing mean sizes and sorting characteristics. A sensitive laser-monitoring system was used in conjunction with a high speed counter to detect initial grain motion and to count individual grain movements. Test results indicate that when velocity is slowly increased over the sediment surface the smaller or more exposed grains are first entrained by the fluid drag and lift forces either in surface creep (rolling) or in saltation (bouncing or hopping downwind). As velocity continues to rise, larger or less exposed grains may also be moved by fluid drag. On striking the surface saltating grains impart momentum to stationary grains. This impact may result in the rebound of the original grain as well as the ejection of one or more stationary grains into the air stream at shear velocities lower than that required to entrain a stationary particle by direct fluid pressure. As a result, there is a cascade effect with a few grains of varying size initially moving over a range of shear velocities (the fluid threshold range) and setting in motion a rapidly increasing number of grains. Results of the tests showed that the progression from fluid to dynamic threshold, based on grain movement, can be characterized by a power function, the coefficients of which are directly related to the mean size and sorting characteristics of the sediment.     

Continual erosion of bare rocks after the Wenchuan earthquake and control strategies

The newly bared rocks created by the Wenchuan earthquake are undergoing continual intensive erosion in the form of detachment and movement of individual grains. Grain erosion is defined as the phenomenon of breaking down bare rocks under the action of insolation and temperature change, detachment of grains from the rockwalls by wind, flow down of grains on the slope under the action of gravity, and accumulation of grains at the toe of the mountain, forming a deposit fan. The Wenchuan earthquake, which occurred in Sichuan on May 12, 2008, caused thousands of avalanches and landslides and left scars on slopes and a huge area of bare rocks. Grain erosion causes flying stones, injured humans and resulted in numerous slope debris flows. The process of grain erosion and strategies to limit the erosion were studied by field investigations and field experiments. According to these field investigations and field studies, the most serious grain erosion occurs in spring and early summer when it is very dry. Rocks are broken down to grains under the action of insolation and temperature change. Then, wind blows the grains from the bare rock down slope. Experimental results showed that the amount of grains blown down by wind per area of rock surface per unit time is proportional to the fourth power of the wind speed. However, the size of the grains blown down by wind increases linearly with the wind speed. An experiment proved that grain erosion can be controlled with two moss species. Moss spores were mixed with clay suspension and splashed on bare rocks. The moss species germinated on the rock surface in one month and greened the bare rocks in two months. The moss layer protected the rocks from insolation and mitigated the effects of temperature change, thus effectively mitigated grain erosion.     

Laboratory measurements of pivoting angles for applications to selective entrainment of gravel in a current

Important to grain entrainment by a flowing fluid is the pivoting angle of the grain about its contact point with an underlying grain. A series of experiments has been undertaken to determine how this angle depends on grain shape (rollability and angularity), on the ratio of the size of the pivoting grain to those beneath, and on factors such as imbrication. The experiments involved gravel-sized spheres (ball-bearings and marbles), natural pebbles selected for their approximately triaxial ellipsoid shapes, and angular crushed basalt pebbles. The pivoting angles for these grains were measured on an apparatus consisting of a board which can be progressively inclined, the angle of the board being equal to the pivoting angle at the instant of grain movement. The pivoting angles of spheres showed reasonable agreement with a theoretically derived equation, showing much better agreement than in previous studies which utilized sand-sized spheres. A series of measurements with spheres ranging from sand to gravel sizes reveals that the pivoting angles decrease with increasing particle size. Our results are therefore consistent with the earlier studies limited to sand-size spheres. The cause of this size dependence is unknown since moisture and electrostatic binding can be ruled out. Similar size dependencies are also found for the ellipsoidal pebbles and angular gravel. The experiments with ellipsoidal pebbles demonstrated a strong shape dependence for the pivoting angle, being a function of the ratio of the pebble's smallest to intermediate axial diameters. This ratio controls the grain's ability to roll and pivot; with small ratios of these diameters the pebbles tended to slide out of position, whereas with ratios closer to unity (circular cross-section) true pivoting took place and the angles were smaller. Experiments with flat pebbles placed in an imbricated arrangement yielded much larger angles than when the pebbles lay in a horizontal position, the pivoting angle being increased approximately by the imbrication angle. The angular crushed gravel also required high pivoting angles, apparently due to interlocking of the grains resulting from their angularity. Other factors being equal, the measurements of pivoting angles demonstrate that the order of increasing difficulty of entrainment is spheres, ellipsoidal grains, angular grains, and imbricated grains. The results obtained here make possible the quantitative evaluation of these shape effects on grain threshold, as well as evaluation of the selective entrainment of grains from a bed of mixed sizes.     

Successive aeolian saltation: studies of idealized collisions

As observed by Bagnold and experimentally reconfirmed by other workers, the impact angles of saltating grains are remarkably constant over a wide range of conditions, lying between 10° and 16°. It can be shown that successive saltation contains a mechanism which very effectively confines impact angles to that range. This control mechanism is most effective at windspeeds less than about 15–30 m s^-1, depending on grain diameter and mass. The control mechanism is evaluated from model calculations of grain populations saltating over a level bed consisting of a layer of loose grains. The grains are assumed to be spherical and uniform in size and mass, also rigid and perfectly elastic. The model also describes distributions of maximum height of grain paths and of lift-off-angles. Compared to other processes involved in aeolian saltation, successive saltation is the only process with a high probability of transferring energy from horizontal into vertical grain movement. This fact, together with the calculations presented, strongly suggests that successive saltation plays a major role in saltation in air. Successive saltation of uniform grains is theoretically impossible if the ground over which saltation occurs is tilted by about 15° against wind direction. Values of tilt angles in this range are observed in nature as stoss-side angles of dunes and ripples, leading to the concept that stoss-sides are tilted up by deposition until successive saltation is subdued.     

A physical statistical approach to erosion

Erosion is a complex process consisting of many components such as surface runoff, impact of raindrops, wind forces, soil and rock mechanics, etc. Trying to integrate all these processes into a physical model seems to be hopeless. In order to understand the variety of natural shapes and patterns produced by erosion we present an integrated statistical approach. Our model is based on simple physical constraints for the separation of amalgamated particles (abrasion) and for the movement of loose particles (denudation) and on the laws of statistics. After some simplifications, we obtain a nonlinear system of partial differential equations which is solved using finite volume techniques. The model is suitable for the formation of different types of rill systems and the episodic behaviour of erosion processes, a kind of self-organized criticality. Besides effects of inhomogeneities, e.g. the formation of terraces can be investigated.     

Thresholds of aeolian sand transport: establishing suitable values

This paper assesses the practical use and applicability of the time fraction equivalence method (TFEM; Stout & Zobeck, 1996) of calculating a wind speed threshold for sand grain entrainment in field situations. A modification of the original method is used and is applied to 1 Hz measurements of wind speed and sand transport on a beach surface. Calculated grain entrainment thresholds are tested in terms of the percentage of sand transport events that they explain. It was found that the calculated thresholds offered a poor representation of the occurrence of saltation activity, explaining only about 50% of the measured transport events. Results are discussed in terms of system response time, wind speed measurement height, undetected events and sampling period. A shear velocity threshold for grain entrainment was also calculated, but this also failed to explain a high proportion of the sand transport events. The best results (67–91% of transport events explained) were found by calculating a threshold based on time‐averaged (≈ 40 s) wind velocity measurements. The applicability of a single threshold to a natural grain population is discussed. A natural surface is likely to possess a range of thresholds varying over short time scales in response to parameters such as grain rearrangement and changes in moisture conditions. The results show that calculated thresholds based on 40 s time‐averaged data consistently explain a high proportion of the recorded sand transport events. This is because such a time‐averaged approach accounts for higher frequency variability inherent in the sand transport system.     

Air density effects on aeolian sand movement: Implications for sediment transport and sand control in regions with extreme altitudes or temperatures

Aeolian sand transport results from interactions between the ground surface and airflow. Previous research has focused on the effects on sand entrainment and mass transport of surface features and wind velocity, but the influence of air density, which strongly constrains airflow characteristics and the resulting sand flow, has not been widely considered. In the present study, entrainment, saltation characteristics and transport rates were examined at nine experimental sites ranging in elevation from ?154 m below sea‐level (Aiding Lake) to 5076 m above sea‐level (Tanggula Mountain pass on the Qinghai–Tibetan plateau). At each site, a portable wind tunnel and high‐speed camera system were set up, and the friction wind velocity, threshold friction velocity and sand flow structure were observed systematically. For a given volumetric airflow, lower air density increases the wind velocity. Low air density also creates a high threshold friction velocity. The Bagnold wind erosion threshold model remains valid, but the value of empirical parameter A decreased with decreasing air density and ranged from 0·10 to 0·07, the smallest values reported in the literature. For a given wind velocity, increased altitude reduced total sand transport and creeping, but the saltation rate and saltation height increased. The present results provide insights into the fundamental mechanisms of the initiation and transport of sand by wind in regions with an extreme temperature or altitude (for example, alpine deserts and low‐lying lake basins) or on other planets, including Mars. These results also provide theoretical support for improved sand‐control engineering measures. The data and empirical equations provided in this paper improve the ability to estimate threshold and transport conditions for wind‐blown sand.     

Applications of grain-pivoting and sliding analyses to selective entrapment of gravel and to flow-competence evaluations

Models of sediment threshold by grain pivoting or sliding over underlying particles are examined in order to explore their application to evaluations of selective entrainment of gravel by flowing water. Of special interest is whether such process-based models provide satisfactory evaluations of flow competence and the movement of large clasts by floods. A detailed derivation is undertaken, focusing first on the fluid flow and forces at the particle level. The resulting threshold equation for the particle-level velocity is then modified to yield the mean entrainment stress for the flow as a whole. This approach is appropriate for considerations of selective entrainment of grains of varying sizes within a deposit, the sorting being due to their relative projection distances above the bed and the dependence of their pivoting angles on grain size and shape. The resulting threshold equations contain a number of coefficients (e.g. drag and lift) whose values are poorly known, but can be constrained by requiring agreement with the Shields curve for the threshold of grains in uniform deposits. If pivoting coefficients based on laboratory measurements with tetrahedral arrangements of particles are used in the models, smaller degrees of selective sorting are predicted than found in the field measurements of gravel entrainment. However, if reasonable modifications of those coefficients are made for expected field conditions, then the models yield good agreement with the data. Sliding models, where sorting is due entirely to projection distances of the grains above the bed, yield somewhat poorer agreement with the field data; however, the sliding models may have support from laboratory experiments on gravel entrainment in that the data and theoretical curves have similar concave trends. The existing measurements lack documentation of the mechanisms of grain movement, so it is not possible to conclusively determine the relative importance of grain pivoting versus sliding. In spite of such uncertainties, the results are encouraging and it is concluded that pivoting and sliding models for grain entrainment do have potential for field computations of selective entrainment and flow competence.     

A simple model for the prediction of the deflation threshold shear velocity of dry loose particles

A very important parameter in aeolian equations is the deflation threshold shear velocity, which quantifies the instant of particle motion. In this paper, a simple model is presented for the prediction of the threshold shear velocity of dry loose particles. It has the same functional form as the widely used models of Bagnold (1941) and Greeley & Iversen (1985), but differs in its treatment of the so‐called threshold parameter. As the new expression was based on the moment balance equation used by Greeley & Iversen, it includes a function for the aerodynamic forces, including the drag force, the lift force and the aerodynamic moment force, and a function for the interparticle forces. The effect of gravitation is incorporated in both functions. However, rather than using an implicit function for the effect of the aerodynamic forces as in the Greeley & Iversen model, a constant aerodynamic coefficient was introduced. From consideration of the van der Waals' force between two particles, it was also shown that the function for the interparticle cohesion force is inversely proportional to the particle diameter squared. The model was calibrated on data reported by Iversen & White (1982). The new expression compared, at least for terrestrial conditions, very well with the Greeley & Iversen model, although it is much simpler. It was finally validated with data from wind‐tunnel experiments on different fractions of dune sand and sandy loam soil aggregates. The soil aggregates were treated as individual particles with a density equal to their bulk density. The good agreement between observations and predictions means that, when predicting mass transport of particles above a given soil, minimally dispersed particle‐size distributions should be considered rather than the granulometric composition of the soil.     

Inter‐annual variability of southerly winds in a coastal area of the Atacama Desert: implications for the export of aeolian sediments to the adjacent marine environment

The analysis of the aeolian content of marine cores collected off the coast of the Atacama Desert (Mejillones Bay, Chile) suggests that marine sediments can record inter‐annual to inter‐decadal variations in the regional southerly winds responsible for particle entrainment at the surface of the nearby desert. However, the establishment of a simple and direct correlation between the sediment and wind records is complicated by the difference of time scales between the erosion and accumulation processes. The aim of this work is to: (i) assess the inter‐annual variability of the surface winds responsible for the sand movements; and (ii) determine whether the integration over periods of several months completely smoothes the rapid changes in characteristics of the transported and deposited aeolian material. To accomplish this aim, 14 years of 10 m hourly wind speed, measured at the Cerro Moreno (Antofagasta) Airport between 1991 and 2003 and at the Orica Station between 2000 and 2004, were analyzed. For each year, the wind speed statistical distribution can be represented by a combination of two to three Weibull functions. Winds of the lowest Weibull mode are too weak to move the sand grains at the surface of the pampa; this is not the case for the intermediate mode and especially for the highest speed mode which are able to erode the arid surface and transport particles to the bay. In each individual year of the period of study, the highest speed mode only accounted for a limited number of strong erosion events. Quantitative analysis of the distribution of the friction velocities and of their impact on erosion using a saltation model suggests that, although all wind speeds above threshold produce erosion events, values around 0·45 m sec^?1 contribute less to the erosion flux. This gap allows separation of the erosion events into low and high saltation modes. The correlation (r = 0·997) between the importance of the third Weibull mode and the extent of higher rate saltation indicates that the inter‐annual variability of the erosion at the surface of the pampa, as well as the transport of coarse particles (>100 μm), are directly related to inter‐annual variations in the prevalence of the strongest winds. Finally, a transport and deposition model is used to assess the possible impact of the wind inter‐annual variability on the deposition flux of mineral particles in the bay. The results suggest that inter‐annual differences in the wind speed distributions have a quantifiable effect on the intensity and size‐distribution of this deposition flux. This observation suggests that a detailed analysis of the sediment cores collected from the bay could be used for reconstructing the inter‐annual variability of past winds.     

Grain scale simulations of loose sedimentary beds: the example of grain-bed impacts in aeolian saltation

Sediment transport by wind is one of many processes of interest to the geomorphologist in which grain to grain contacts play an important role. In order to illustrate the modelling of collections of frictional, inelastic sedimentary grains with the particle dynamics method (PDM), we use the grain impact process in aeolian saltation as a specific example. In PDM, all the forces on each particle are evaluated at a sequence of small time-steps, and the Newtonian equations of motion are integrated forward in time. Interparticle forces at grain contacts are treated as springs with prescribed stiffness (normal force) and by a Coulomb friction law (tangential force); particle inelasticity is represented by spring damping. The granular splash resulting from saltation impacts is assessed for sensitivity to the choice of grain properties, and the integration time-step. We find that for the range of impact speeds and impactor masses relevant to aeolian settings, grain splashes are relatively insensitive to grain stiffness, grain inelasticity and grain friction, and that the pattern of ejection from the bed is largely controlled by bed microtopography. A large set of impact realizations involving a variety of impact points on a small set of target beds is used to collect the appropriate statistics for describing the stochastic splash process. The splash function representing these statistics is then available for use in calculations over longer time-scales, such as the evolution of the saltation curtain. The details given here will enable the interested reader to adapt PDM modelling to other types of clastic sedimentary systems.     

Transport by wind of granular materials of different grain shapes and densities

Aeolian transport rates were measured for three sands: a quartz sand (relative density 2.68, sphericity 0.73), a shelly sand (relative density 2.64, sphericity 0.59, carbonate content 67%), and granular aluminium oxide (relative density 3.95, sphericity 0.67). Low sphericity depresses the transport rate, particularly at high wind speeds; high density also does so but the effect is more marked at low transporting wind speeds. The sand of low sphericity undergoes longer saltations than the other materials, but is dislodged less frequently than they are and is transported less freely in a given wind. Of the more spherical materials, the denser was the least effective at extracting energy from the wind for grain transport in the range of our experiments. Both of the more spherical materials showed evidence of a fairly sudden transition of transport behaviour at modest wind speed. It is inferred that this marks the transfer of the function of grain dislodgement from direct wind action to inter-granular collision. The transition did not occur in experiments on the material of lower sphericity. For a given transport rate, wind speed near the bed is highest for the grains of low sphericity (by a considerable margin) and lowest for the more compact quartz grains. Propensity for transport by wind is greatest for the quartz sand, less so for the heavy material and least for the material of low sphericity.     

Influence of the saffman force,lift force,and electric force on sand grain transport in a wind–sand flow

Quasi-horizontal trajectories of salting sand grains were found using high-speed video-recording in the desertified territory of the Astrakhan region. The sizes and displacement velocities of the saltating sand grains were determined. A piecewise logarithmic approximation of the wind profile in a quasi-stationary wind–sand flow is suggested, which is consistent with the data of observations and modeling. It was established that, in the regime of stationary saltation, the wind profile in the lower saltation layer of the wind–sand flow depends only slightly on the wind profile variations in the upper saltation layer. The vertical profiles of the horizontal wind component gradient in a quasi-stationary wind–sand flow were calculated and plotted. It was shown using high-speed video recording of the trajectory of a sand grain with an approximate diameter of 95 μm that the weightlessness condition in the desertified territory of the Astrakhan region in a stationary wind–sand flow is satisfied at a height of approximately 0.15 mm. The electric parameters of a wind–sand flow, which can provide for compensation of the force of gravity by the electric force, were estimated. In particular, if the specific charge of a sand grain is 100 μC/kg, the force of gravity applied to the sand grain can be compensated by the electric force if the vertical component of the electric field in a wind–sand flow reaches approximately 100 kV/m. It was shown that the quasi-horizontal transport of sand grains in the lower millimeter saltation layer observed in the desertified territory can be explained by the joint action of the aerodynamic drag, the force of gravity, the Saffman force, the lift force, and the electric force.     

The variability of critical shear stress, friction angle, and grain protrusion in water-worked sediments

The erodibility of a grain on a rough bed is controlled by, among other factors, its relative projection above the mean bed, its exposure relative to upstream grains, and its friction angle. Here we report direct measurements of friction angles, grain projection and exposure, and small-scale topographic structure on a variety of water-worked mixed-grain sediment surfaces. Using a simple analytical model of the force balance on individual grains, we calculate the distribution of critical shear stress for idealized spherical grains on the measured bed topography. The friction angle, projection, and exposure of single grain sizes vary widely from point to point within a given bed surface; the variability within a single surface often exceeds the difference between the mean values of disparate surfaces. As a result, the critical shear stress for a given grain size on a sediment surface is characterized by a probability distribution, rather than a single value. On a given bed, the crtitical shear stress distributions of different grain sizes have similar lower bounds, but above their lower tails they diverge rapidly, with smaller grains having substantially higher median critical shear stresses. Large numbers of fines, trapp.ed within pockets on the bed or shielded by upstream grains, are effectively lost to the flow. Our calculations suggest that critical shear stress, as conventionally measured, is defined by the most erodible grains, entrained during transient shear stress excursions associated with the turbulent flow; this implies a physical basis for the indeterminacy of initial motion. These observations suggest that transport rate/shear stress relationships may be controlled, in part, by the increasing numbers of grains that become available for entrainment as mean shear stress increases. They also suggest that bed textures and grain size distributions may be controlled, within the constraints of an imposed shear stress and sediment supply regime, by the influence of each size fraction on the erodibility of other grain sizes present on the bed.     

毛乌素沙地东南部边缘不同地质成因类型土地沙漠化粒度特征及其地质意义

首次对毛乌素沙地东南部边缘沙漠—黄土地带的土地沙漠化进行系统的地质学分类,提出了沙地内部就地起沙型沙漠化、河流谷地就地起沙型沙漠化、风化残积就地起沙型沙漠化和风沙侵入型沙漠化等四种土地沙漠化类型。通过深入分析不同地质成因类型土地沙漠化的粒度特征,探讨了不同地质作用对土地沙漠化的贡献及其对土地沙漠化防治方面的重要意义。研究表明,研究区的土地沙漠化以就地起沙型为主,河流的水力搬运是沙漠化物质搬运的主要途径,风力作用则是对河流湖泊沉积物的进一步分选和再沉积。最后,提出了不同类型的土地沙漠化防治对策建议。     

黄河源区玛多县沙漠化成因与发展过程

通过分析黄河源区玛多县沙漠化的成因和过程,结果表明,玛多县沙漠化成因类型有三种,分别为固定沙丘/古沙丘活化、滑塌陡坎及风蚀斑块,其形成都与多年冻土退化相关.在固定沙丘或古沙丘分布区,冻土退化导致热融沉陷,形成沉陷坑,沉陷坑边缘形成拉裂缝或陡坎,使下伏松散沙露出.在斜坡上,冻土退化形成滑塌陡坎,使底层土壤从侧面暴露.在平坦的冲积平原,差异性冻胀和融沉导致草皮拉裂,形成积水坑洼,冻土退化导致土壤变干,土壤的底层暴露.下伏风成沉积物暴露后,遭受风蚀,形成侧向凹槽,致使上部土体坍塌,使更多的风成沉积物暴露,这些过程的不断重复,使风蚀坑、陡坎、风蚀斑块扩大相连,最终形成流动沙丘、风蚀劣地、戈壁等地貌.     

A wind tunnel study of the parameters for aeolian sand transport above a wetted sand surface using sands from a tropical humid coastal region of southern China

In this study, wind tunnel tests were performed to determine the relationships between sediment transport, the surface moisture content, and wind velocity using beach sands from a tropical humid coastal area of China. The variation in the properties of the creep proportion, relative decay rate as a function of height, and average saltation height in the flux profile were determined. Sand transport was measured using a standard vertical sand trap. The creep proportion (i.e., the proportion of the particles that move along the surface rather than undergoing saltation) and relative decay rate decreased and more particles were ejected to higher positions as moisture content and wind velocity increased. The creep proportion ranged between 0.12 and 0.33, and averaged 0.22. The creep proportion and relative decay rate decreased abruptly at moisture contents between 0.587 and 1.448%; the latter value was close to 1.591%, the moisture content at a matric potential of ?1.5 MPa. This moisture content limit may indicate a change in the form of soil water from adsorbed films on particle surfaces to capillary forces created by inter-particle water bridges. The surface moisture content therefore appears to decisively determine the degree of the restraint on particle entrainment by the wind. The average heights, below which 25, 50, 75, and 90% of sand transport occurred, increased with increasing moisture content (except at 0.231% moisture content) and wind velocity. The mean saltation height at various wind velocities increased linearly with increasing moisture content.     

Effects of bed longitudinal inflexion and sediment porosity on basal entrainment mechanism: insights from laboratory debris flows

Channel morphology and bed sediment erodibility are two crucial factors that significantly affect debris flow entrainment processes. Current debris flow entrainment models mostly hypothesize the erodible beds are infinite with uniform slopes. In this study, a series of small-scale flume experiments were conducted to investigate the effects of bed longitudinal inflexion and sediment porosity on basal entrainment characteristics. Experimental observations revealed that sediment entrainment is negligible at early stages and accelerates rapidly as several erosion points appear. Continual evolution of flow-bed interfaces changes interactions between debris flows and bed sediments, rendering the interfacial shear action involved into a mixed shear and frontal collisional action. Lower bed sediment porosity will change the spatial arrangement and orientation of particle mixture, strengthen the interlocking and anti-slide forces of adjacent sediment particles, and promote the formation of particle clusters, all of which will increase bed sediment resistance to erosion. By examining the post-experimental bed morphology, the slope-cutting amounts and topographic reliefs are determined to positively correlate with longitudinal transition angles. These high topographic reliefs may indicate the propensity of triangular slab erosion, rather than strip-shaped slab erosion, in non-uniform channels with relatively steep erodible beds. Empirical formulas are obtained that denote the relationships among bed sediment strength, channel curvature radius, and sediment porosity through a multi-parameter regression analysis. This study may aid in clarifying the complex coupling effects of spatial variations in debris flow dynamics as well as sediment erodibility and bed morphology in non-uniform channels with abundant seismic loose material.