Influence of vegetation cover on sand transport by wind: field studies at Owens Lake,California

Field studies conducted at Owens Lake, California, provide direct measurements of sand flux on sand sheets with zero to 20 per cent cover of salt grass. Results from 12 different sand transport events show that aerodynamic roughness length and threshold wind shear velocity increase with vegetation cover as measured by vertically projected cover and roughness density (λ). This results in a negative exponential decrease in sediment flux with increasing vegetation cover such that sand transport is effectively eliminated when the vertically projected cover of salt grass is greater than 15 per cent. A general empirical model for the relation between sand flux and vegetation cover has been derived and can be used to predict the amount of vegetation required to stabilize sand dune areas. © 1998 John Wiley & Sons, Ltd.     

CAUSES OF THE FETCH EFFECT IN WIND EROSION

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.     

THE INFLUENCE OF RAINFALL ON TRANSPORT OF BEACH SAND BY WIND

This paper deals with the effect of rainfall on the process of wind erosion of beach sands and presents results from both field and wind tunnel experiments. Although sediment transport by splash is of secondary importance on coastal dunes, splash–saltation processes can move sediments in conditions where no motion is predicted by aeolian processes. The effect of raindrop impact on the movement of soil particles by wind was measured on a sand beach plain using an acoustic sediment sampler. In general, an increase of particle movement by wind at the sensor heights was observed during rainfall. Rainfall also affected the wind erosion process during and after rain by changing the cohesive conditions of the surface. The influence of the surface moisture content on the initiation of wind erosion and on the vertical distribution of transported sand particles was studied in a wind tunnel. Moisture significantly increased threshold wind velocities for the initiation of sediment transport and modified vertical sediment profiles.     

滇西实验场地质构造背景对地震波波速影响的实验研究

本实验用超声地震模型实验模拟观测,滇西地震实验场地质构造格架对地震波波速的影响。模型为1米量级。模型与原型地质的比为1∶470000。实验模型有两块,一块为均匀无构造模型,另一块为有滇西实验场主要地质构造格架模型。实验对两种模型的P波速进行了比较,给出了以模型中心为观测点以及以昆明、中甸、下关和渡口为观测点,模拟震中距为50公里、100公里、150公里和200公里圈上P波波速变化的背景场。这一实验是用物理模型模拟研究复杂地质构造格架对地震波波速影响的首次方法探讨。     

地震台阵滤波及子台分布对聚束效果的影响(Ⅱ)——子台分布及聚束效果的分析

本假设对于确定的相关系数及子台间距存在一个相干波长，将不同频率、震中距的相关性变化的原因归纳为视波长的变化．对近事件而言，几乎不存在相关系数十分理想的子台间距的区间范围．对非理想相关的台阵，应用S．Mykkeltveit等介绍的增益公式来预测台阵的增益，其效果与实际情况吻合的较好．论述了区域台阵与侦察远处事件的台阵有诸多不同之处．     

帕米尔东北侧地壳物性结构及其发震环境探讨

通过对帕米尔东北侧伽师及其周边地区两条深地震测深剖面Ｓ波资料的处理计算，结合Ｐ波研究结果，得到了S波二维地壳速度结构和波速比（vP/vS）分布．研究表明:① 塔里木块体平均地壳波速比明显高于西昆仑和天山褶皱带，显示了坚硬、稳定地壳物性特征．下地壳正常偏低的波速比（泊松比）值，说明塔里木块体的下插是该地区地壳运动的主要特征；② 天山褶皱带上地壳岩层相对较软，易于在应力作用下断裂破碎和应力能量释放，是其小震密集分布的重要构造因素；③ 伽师附近正处在下地壳C界面和壳幔边界的上隆顶部或拐点上，是强震群发生的深部构造背景．上地壳中下部的块体边界附近波速比值的交错变化和接触面复杂形态的存在，形成了伽师强震群出现的特殊构造环境．震源处波速比相对较高，剪切模量较小，可能是伽师强震群应力降偏低的主要原因．      

南海北部陆缘反“S”型构造带及其对地震活动的影响

通过地形—地貌、断裂构造、地壳结构、中新生代沉积盆地、第四纪地质特征等分析,认为南海北部陆缘存在滨岸岛链、陆坡北缘和陆坡南缘三条反“S”型构造带,它们形成于晚第三纪,较“新华夏系”和“南海系”晚。上新世末期以来,反“S”型构造带的活动方式,对南海北部陆缘第四纪地质特征影响显著,同时控制了这一区域的地震活动方式和地震带展布,其中,“滨海地震带”和“雷琼地震带”同属于滨岸岛链带,“台西滨外地震带”和“东沙—海南地震带”同属于陆坡北缘带。