The near-bed airflow and the movement of sand dune sediments by wind are fundamental dune geomorphological processes.This research measured the wind profiles and sand mass flux on the rounded top of a transverse dune at the southern edge of the Tengger Desert to examine how to best predict the vertical profile of sand flux.This work also tested the accuracy of previously developed models in predicting the apparent roughness length during saltation.Results show that mass flux vertical distribution over the dune top is underestimated by an exponential function,overestimated by a power function,but closely matches the predictions made using the LgstcDoseRsp function.Given suitable values ofα,βandγaccording to the grain size composition,S?rensen equation with the peaked shape of the mass transport curve will well predict the dimensionless mass flux qg/ρu*3against dimensionless shear velocity u*/u*t.The modified Charnock model works best of the previously published models tested,with an R2of 0.783 in predicting the enhanced roughness over the moving sand surface,as opposed to an R2of0.758 for the Owen model and an R2of 0.547 for the Raupach model.For the rounded dune top in this study,C m=0.446±0.016. 相似文献
A theoretical equation was developed to express the time variation of drainage density in a basin or geomorphic surface: Di(t, T) is the drainage density at time T on the i-th basin or geomorphic surface, which was formed at time t; β(τ) is a factor related to the erosional force causing the development of the rivers of the basin or surface at time τ; δi is the maximum drainage density; and Di is the initial drainage density on the i-th geomorphic surface or basin. The equation is based on the assumption that the drainage density increases with time until it reaches a specific upper limit δi(t)), the maximum drainage density, which is related to certain physical properties of the basin. The equations for various dated basins or geomorphic surfaces can be combined into one modified equation if the same relative erosional forces have acted on those basins or surfaces (β(t) = β(t) and if the basins or surfaces have the same physical properties δi(t) = δi(t), (Di = D0). The application of this equation to coastal terraces and glacial tills shows that the model is compatible with observed drainage densities on various dated basins or surfaces. 相似文献
On the pumice flow deposits of the Asama volcano, Japan, many salts such as halite (NaCl), gypsum (CaSO4·2H2O), hexahydrite (MgSO4·6H2O) and mirabilite (Na2SO4·10H2O) crystallize at the base of south-facing valley cliffs. The zone of salt efflorescence and of resulting polygonal rind correspond to the zones of notch formation and high water content. The main conditions for salt crystallization and polygonal rind formation are: (1) the existence of groundwater containing a high concentration of Cl?, SO, Ca2+, Mg2+, and Na+; (2) a valley cliff material with a high capillary action and small tensile strength; and (3) low humidity and a high ground-surface temperature derived from the direct incidence of sunshine. Given the right conditions, salt weathering can occur not only in the arid regions but also in humid, temperate inland regions. 相似文献
The increase of soil mass flux with distance downwind, the fetch effect for wind erosion, has been observed and reported on since 1939. This model incorporates the following three mechanisms. (1) The ‘avalanching’ mechanism in which one particle moving downwind would dislodge one or more particles upon impact with the surface. The result of a chain of such events is an increase of mass flux with distance. (2) The ‘aerodynamic feedback’ effect, suggested by P. R. Owen, in which the aerodynamic roughness height is increased by saltation of particles; the resulting increased momentum flux increases saltation. These increases define a positive feedback loop with respect to distance downwind. (3) The ‘soil resistance’ mechanism, which is largely an expression of the change with distance of threshold velocity. Change of threshold velocities may be caused by inhomogeneities of the soil or progressive destruction of aggregates and crust in the direction of saltation fetch. An experiment was run in March 1993 at Owens Lake to test this model. Detailed measurements of wind profiles and mass fluxes were taken on a line parallel to the wind direction. These data support the proposed three-mechanism model. 相似文献
Wind tunnel simulations of the effect of non-erodible roughness elements on sediment transport show that the flux ratio q/qs, shear velocity U*, and roughness density λ are co-dependent variables. Initially, the sediment flux is enhanced by kinetic energy retention in relatively elastic collisions that occur at the roughness element surfaces, but at the same time, the rising surface coverage of the immobile elements reduces the probability of grain ejection. A zone of strong shearing stress develops within 0·03 to 0·04 m of the rough bed because of a relative straightening of velocity profiles which are normally convex with saltation drag. This positive influence on fluid entrainment is opposed by declining shear stress partitioned to the sand bed. Similarly, because the free stream velocity Uf is fixed while U* increases, velocity at height z and particle momentum gain from the airstream decline, leading eventually to lower numbers of particles ejected on average at each impact. When the ratio of the element basal area to frontal area σ is approximately equal to 3·5, secondary flow effects appear to become significant, so that the dimensionless aerodynamic roughness parameter Z0/h and shear stress on the exposed sand bed Ts decrease. It is at this point that grain supply to the airstream and saltation drag appear to be significantly reduced, thereby intensifying the reduction in U*. The zone of strong fluid shear near the bed dissipates. 相似文献
HCHO is ubiquitous and important chemical constitutes in the troposphere. The concentrations of the HCHO (aq) in the rainwater were measured in the Guiyang city, southeastern of China from May 2006 to April 2007 and 153 discrete samples were collected. Rainwater (N = 151) HCHO (aq) concentrations ranged from lower than method detection limit (MDL) to 40.2 µmol/L with a volume weighted mean value of 7.4 ± 8.8 µmol/L. The strong correlations between HCHO (aq) and HCOO? (r = 0.69, n = 137), HCHO (aq) and nss‐ (r = 0.74, n = 137), HCHO (aq) and (r = 0.67, n = 137), HCHO (aq) and (r = 0.74, n = 133) suggest the significant influence of the anthropogenic input for the HCHO (aq) levels. The concentration levels of rainwater HCHO (aq) was inversely proportional to the amount of rainfall, indicating the below‐cloud process is the most important mechanism for rainwater HCHO (aq) scavenging processes. More than 70% of the HCHO (aq) wet deposition took place during the early stage of the rainfall. According to the air mass back‐trajectory analysis, the rainwater with industrial back‐trajectories coming from the north had the highest levels of HCHO (aq) while the rainwater with the green‐covered or marine back‐trajectories from the southeast had the lowest concentrations, and this indicate the HCHO (aq) originated from urban or industrial regions served as an important source of the rainwater. The annual HCHO (aq) wet deposition flux was calculated as 6.96 mmol/m2 per year and the total deposition flux was estimated as 24.35 mmol/m2 per year, 71.4% of which was dominated by dry deposition. 相似文献
Quantifying aeolian sand transport rates relies upon the computation of the near-surface shear velocity (u*) determined from velocity profiles of the wind. While it has been recognized that various conditions, such as saltation, surface roughness, surface slope and atmospheric conditions, have an effect on the velocity profile, it is commonly assumed that measurements made above the surface will be representative of the near-surface shear velocity. Airflow and temperature data collected over a flat substrate at White Sands National Monument in New Mexico, however, show the significant effects that atmospheric conditions have on velocity profiles. During the day, when solar insolation is heating the surface, atmospheric conditions in the lowest several metres become unstable, resulting in enhanced convection and vertical mixing so that the velocity gradient changes little with height. As a result, the shear stress in this region of vertical mixing lessens, while the near-surface shear stress is increased because the higher wind speeds are now nearer the surface. At night, the near-surface atmospheric conditions are stable, thereby reducing convection and vertical mixing, resulting in stratified airflow and increased shear velocity away from the surface. Unless this atmospheric effect is accounted for, estimates of sediment transport rates may be in error by as much as a factor of 15 times when wind speeds are near threshold velocity. At wind speeds approaching 10 ms1, at 5m above the surface, this error in computing sediment transport is reduced to a factor of only two to three times, and may be within the range of measurement error. 相似文献