Field measurements of snow-drift threshold and mass fluxes,and related model simulations

Field measurements were carried out to calculate the threshold friction velocity for snow saltation, and mass fluxes during snow drift. The wind was measured in three components by an ultrasonic anemometer, and the mass fluxes were determined using an optical sensor (snow particle counter), acoustic sensors (Flowcapt) and mechanical traps. The threshold friction velocity was found to be correlated to the grain size (R²=0.75). The mass flux measurements were compared with numerical simulations of snow drift, and it was demonstrated that the maximum snow transport takes place at shear stress values of roughly two times the average shear stress over 20 min. By implementing a probability distribution for the shear stress the mass flux was simulated with only the mean measured value of the shear stress as input. This procedure enables the future use of the numerical model for operational applications.     

山西地区未来3年地震强度预测

选取１９７０年至１９９７年时间段内发生在山西地区的逐年最大震级,采用门限自回归原理进行建模,经过反复对比,选取最佳模型,预测得知该区未来３年（至２０００年）发生强震（Ｍｓ＞６．０）的可能性不大。     

起算震级与衰减不确定性校正

分析表明,起算震级Ｍ０对危险性分析结果的表现为当Ｍ０取取值不够低时,场地的危险性将被低估。此种低估影响与场地的地震危险性高低潜源距场地的距离及衰减公式的形式和标准差有关。     

The concept of dominant discharge applied to two gravel-bed streams in relation to channel stability thresholds

Dominant discharge may be defined as that discharge which transports most bed sediment in a stream that is close to steady-state conditions. The concept is examined in relation to two single thread gravel-bedded streams. One stream is alluvial and free to adjust its geometry whilst in the other, channel capacity and form are partially constrained by cohesive till-banks and a heavily compacted bed. The total quantity of bedload and its calibre were measured for every flood over a six year period, so that the relationship between the grain-size of bedload and the most effective discharge could be examined in the context of thresholds for channel change. The dominant discharge concept was applicable to the alluvial stream in that the bankfull value is an effective discharge for maintaining channel capacity. The concept applied less well to the ‘non-alluvial’ stream. Although in both streams the bankfull value was exceeded for less than 0.34 per cent of the time, overbank flows are important in instigating channel change. It is only during overbank flows that the largest bed material is entrained in quantity. For within-channel flows a threshold separates flows which winnow fine matrix from those which entrain the finer bed gravels. This threshold occurred at 60 per cent bankfull. It was concluded that the dominant discharge concept can be applied to streams close to steady-state which are alluvial, competent, and free to adjust their boundaries. An important proviso is that two channel-stability domains can be recognized. These domains represent channel maintenance and channel adjustment and are defined by intrinsic thresholds in the bed material entrainment function.     

Threshold relations for the transport of sediment by overland flow on desert hillslopes

Although the Shields relation was developed for rivers, it has been applied to sediment transport by overland flow. According to the Shields relation, where the critical boundary Reynolds number Re_*c exceeds 40, the critical Shields number F_*c is independent of both Re_*c and the ratio of the critical flow depth to particle diameter d_c/D. Analyses of data collected from runoff plots in southern Arizona reveal that F_*c is positively correlated with both Re_*c and d_c/D. Thus the Shields relation does not apply to overland flow on debris-covered desert hillslopes. Multiple regression analysis is employed to develop alternative threshold relations in which critical boundary shear stress τ_c is related to D and d_c/D (R² = 0.782) and to D and S_c (critical gradient) (R² = 0.625). The computed R² values derive in large part from the spurious correlations of d_c/D and S_c with τ_c. Nevertheless, the relations may be utilized to predict τ_c. In this regard, the latter relation is likely to prove more useful than the former because S_c is generally known, whereas d_c is not. An investigation of the functional relation between τ_c and D reveals that τ_c varies approximately with D² for overland flow on the desert hillslopes under study, whereas the Shields relation predicts a linear relation (i.e. a D exponent of 1). This result is consistent with Cheng's data which show that F_*c varies with (d_c/D)^?1 where 0.4 < d_c/D < 2 and may be explained in terms of increased energy dissipation both in separation zones downslope of particles and in distortion of the water surface as d_c/D decreases. Consequently, larger values of τ_c, and hence F_*c, are required to initiate the transport of particles of a given size D as d_c decreases.     

地磁加卸载响应比与地震对应关系研究

通过对浙、沪、苏邻近 4个地磁观测点的近十年的资料,及邻近区域10个地震进行了每日Z分量日变幅加卸载响应比随时间变化和响应比月均值的相关研究。结果表明:加卸载响应比阈值P₀(Z) 和响应比月均值P_M(Z)的大小与地震的震级、震中距及异常出现的早晚并非完全呈线性相关;当响应比值大于某一阈值时,有可能发生一定震级的地震 ;按该方法结果综合分析,截止1998年9月底,未来1年左右的时间内在该研究区域或更大范围(含台湾区域)有可能发生6级左右的地震。该方法可作为分析预报手段参与中短期和短临地震趋势判断的一种新方法。     

地貌发育阶段的定量研究

文章提出用临界侵蚀积分值作为划分流域发育阶段的定量指标,是对经典理论的发展,主要表现在:1)将地貌发育阶段纳入开放系统的概念之中;2)避免高程积分法在划分发育阶段上的不确定性;3)建立地貌发育阶段与流域物质迁移之间的定量关系;4)区分自然侵蚀和加速侵蚀。用文中的临界侵蚀方程对黄土高原丘陵沟壑区分析计算,得到临界侵蚀积分值为0.30和0.70,即当侵蚀积分值小于0.30寸,流域处于侵蚀早期;当侵蚀积分值大于0.30,小于0.70时,流域处于侵蚀中期;当侵蚀积分值大于0.70时,流域处于侵蚀晚期。对于纯自然侵蚀过程,与临界侵蚀积分值相对应的年代及侵蚀模数为7万年前,7008t/km~2·a和距今7.2万年以后,7008t/km~2·a。目前黄土高原丘陵沟壑区加速侵蚀所占比例为30％。     

利用三维CT扫描技术定量计算熔岩流气泡体积的研究与实现

气泡体积算法及其计算机实现是火山熔岩流气泡古高度计应用研究中的难点之一,直接制约着古高度计算结果的精确度。本文提出了一种新的基于计算机断层成像技术的熔岩流气泡众数统计方法,即通过计算机断层成像技术对气孔状玄武岩样品进行三维重构,利用不同物质对射线吸收衰减系数的不一致性,标记重构数据中的岩石、气泡与无效部分,进而提取气泡连通区域的骨架,确定独立气泡之间的分割面,最终统计出气泡的数量与体积。考虑到玄武岩中气泡截面轮廓不规则的特点,采用逐点法有效地计算了截面面积,排除了连通区域轮廓对面积计算结果的影响。采用基于区域划分的阈值计算方法统计玄武岩样品的气泡数量与体积,解决了传统的单一阈值计算方法导致的小连通区域过度分割的问题。在上述新方法的基础上,本文设计并构建了新的气泡数量与体积的统计软件,计算程序的初步运算结果证实了研究方法的有效性与可行性。该方法有望提高熔岩流气泡数量、体积以及火山熔岩流气泡古高度计的计算精度,成为重建高原隆升历史的快速分析工具。     

Empirical models of rain-spells characteristics – A case study of a Mediterranean-arid climatic transect

Environmental (geomorphological, hydrological and ecological) processes are controlled by rainfall, particularly in the Mediterranean, semi-arid and arid regions. Rainfall was analyzed using the concept of rain-spells, i.e., a period of successive rain days preceded and followed by at least one day without rainfall. Daily data from 13 stations along a climatic transect extending from the Judean Mountains with a Mediterranean climate to the Dead Sea arid region in Israel were studied. Rain-spell characteristics (number, yield and duration), based on these data, are presented for different rainfall thresholds, which might be used for different environmental processes such as rock weathering, soil organic matter dynamics, landslides, overland flow and floods and soil erosion. Three estimation models have been developed in order to predict the mean annual Number of Rain-Spells (NRS), mean Rain-Spell Yield (RSY), and mean Rain-Spell Duration (RSD) for the mean annual rainfall and for any given rainfall threshold. These models can be used for current climatic conditions and for scenarios in which the rainfall total changes.     

塔克拉玛干沙漠腹地一次风蚀起沙观测试验

风蚀起沙对生态环境、人类健康及社会经济等具有很大的负面效应。利用美国Sensit公司生产的H11-LIN型风蚀传感器及贴地层风速梯度探测仪,对塔克拉玛干沙漠腹地塔中地区一次风蚀起沙过程进行了监测。结果表明：（1）风蚀起沙与微气象是一个互相影响互相制约的变化过程;（2）在此次观测试验过程中,我们观测到的最小起沙风速为6.0 m/s左右,同时发现起沙风速是一个变化的值,随着时间的延续呈现逐步增大的趋势,最小的起沙风速出现在风蚀开始发生的时候;（3）近地表风速廓线受沙粒运动影响明显,随着起沙量的增加,风速随高度变化的幅度增大,此时的风速廓线已不再遵循对数分布规律;（4）利用风蚀传感器测得塔中地区沙粒的临界起动摩擦速度为0.25 m/s,该值与经验公式估算的0.24 m/s非常接近,表明风蚀传感器在塔中地区具有较好的适用性;夏季塔中地区地表土壤湿度对风蚀起沙的阻碍作用可以忽略不计。 ?