Sorted stripes orientated by wind action: Some observations from sub-antarctic marion island

Sorted stripes found on the volcanic scoria and glacial deposits of sub-Antarctic Marion Island indicate a distinct preferred orientation. Despite uniformity of slope and material, the stripes are predominantly aligned parallel to the wind. It is suggested that melting of needle ice by the early morning sun is of only limited importance in the sub-Antarctic owing to the almost continual overcast conditions. The effect of the wind is so great that in exposed situations stripes are formed on horizontal surfaces.     

Application of rock mass classifications: a case study from Ituango hydroelectric diversion tunnel,Colombia

ABSTRACT

The local site experience is a valuable component for the success of rock mass classification systems as tunnel design methods. The Ituango hydroelectric project is a very important source of information in order to evaluate the usefulness of the main rock mass classification systems. The objective of this research is to improve understanding of some important features of excavated rock mass, such as discontinuities, block size, shear strength and joint alteration, by analyzing some hundreds of data obtained during excavation cycle.

The field study included a survey of exposures after drilling and blasting rounds. Rock mass classification and support measures for each type of terrain along 1400 m tunnel were performed. The rock mass classes could be better explained if shear strength, alteration or block size is calculated. The assessment of these data allows evaluating the block fall risk, improving support and liner. A local correlation between the RMR and Q system was also obtained.     

考虑进洞高程效应的黄土隧道洞口段地震动力响应特征研究

通过大型振动台模型试验并采用Midas-GTS有限元软件进行模拟计算,研究黄土隧道洞口段在地震作用下的动力响应特征、破坏过程和地震波在模型中的传递规律,分析影响黄土隧道洞口段地震动力响应的主要因素。结果表明:边坡沿弧形开裂面的垮塌受坡脚剪切和坡顶拉裂的共同作用;边坡会对其卓越频率内的地震波产生明显放大效应,且在1/2坡高以上放大效应出现饱和现象;隧道临空面是影响隧道洞口段地震动力响应的主要因素。考虑进洞高程效应时隧道洞口段抗震设防长度可取距洞口5倍洞径范围。振动台模型试验与数值计算在位移、加速度、应力三个响应特征上吻合较好,证明二者结果合理可靠。研究成果可为隧道工程设计和地下结构抗震理论研究提供有益参考。     

黄河下游游荡段过流能力调整对水沙条件与断面形态的响应

河道过流能力与主槽形态有关,而主槽形态又取决于上游水沙条件,分析过流能力与这两者之间的关系对研究黄河下游游荡段河床演变规律有重要意义。从典型断面和河段平均两个尺度,定量分析了黄河下游游荡段1986-2015年平滩流量与水沙条件(来沙系数和水流冲刷强度)及汛前主槽形态(河相系数)之间的响应关系。结果表明:①1986年至小浪底水库运行前,游荡段淤积严重,主槽萎缩,河道过流能力急剧下降,自小浪底水库运行后,游荡段发生强烈冲刷,其断面持续趋向窄深,过流能力逐年恢复;②建立了断面和河段平滩流量与水沙条件及河相系数的幂函数关系,二者相关系数均在0.5以上,但河段尺度相比于断面尺度的相关系数至少可提高17%;③河段平滩流量与前5年汛期平均水流冲刷强度及河相系数的相关系数接近0.94,相应计算公式能较好地反映平滩流量的变化过程,为分析其他河段平滩流量的变化提供了参考方法。     

FLAC3D在隧道初期支护与原岩条件的"耦合"问题的应用

为了更深入地了解施工过程联拱隧道围岩一支护结构体系的力学响应,并优化施工工序,以便在实际施工中根据具体情况和经济、技术水平调整施工步骤,对联拱隧道支护设计进行了三维模型分析。用FLAC^3D对双联拱隧道网喷混凝土初期支护设计,进行了力学响应动态分析模拟,分析了岩体支护前后围岩的变形及应力状态,对隧道围岩的稳定性和初期支护结构的安全性做出综合评判,得出了一些非常有益的结论,用以指导襄十高速公路双联拱隧道的施工,取得了较好的经济效益。     

Drag Partition for Regularly-Arrayed Rough Surfaces

Vegetation and other roughness elements distributed across a surface can providesignificant protection against wind erosion by extracting momentum from the flowand thereby reducing the shear stress acting at the surface. A theoretical model haspreviously been presented to specify the partition of drag forces for rough surfacesand to predict required vegetation density to suppress wind erosion. However, themodel parameters have not yet been constrained and the predictive capacity of themodel has remained uncertain. A wind-tunnel study was conducted to measure thedrag partition for a range of roughness densities and to parameterise the model inorder to improve its range of potential applicability. The drag forces acting on bothan array of roughness elements and the intervening surface were measured independentlyand simultaneously using new drag balance instrumentation. A detailed measure of thespatial heterogeneity of surface shear stresses was also made using Irwin sensors. Thedata agreed well with previous results and confirmed the general form of the model.Analysis of the drag partition confirmed the parameter definition = C_R/C_S(where C_R and C_S are roughness element and surface drag coefficients,respectively) and a constant proportional difference between the mean and maximumsurface shear stress was found. The results of this experiment suggest that the definitionfor m, the surface shear stress inhomogeneity parameter, should be revised, although thetheoretical and physical reasons for including this parameter in the model appear to bevalid. Best-fit values for m ranged from 0.53 to 0.58.     

华南暴雨试验天气雷达数据处理及暴雨中尺度结构个例分析

文中描述了华南暴雨试验多普勒天气雷达数据的处理方法，建立了一套集图像显示、数据预处理及风场反演分析为一体的多普勒天气雷达的分析应用系统。该系统生成了三维正方形网格和经纬度网格的回波强度值，以及经噪声控制和速度扩展的径向速度风场，提交了华南暴雨试验雷达数据库。该文以1998年6月9日08:00(北京时) 探测数据为例，给出了华南暴雨试验区内4部多普勒天气雷达的反演雨强图和两部雷达的风场反演拼图，并进行了暴雨中尺度风场结构的初步分析。     

Effect of soil crusting on the emission and transport of wind-eroded sediment: field measurements on loamy sandy soil

Field data are reported for the horizontal and vertical flux of wind-eroded sediment on an agricultural field in northern Germany. Measurements were made during a windstorm that hit the region on 18 May 1999. The magnitude of both fluxes was significantly affected by the presence of a surface crust covering the test field. Measuring the physical crust strength at 45 locations with a torvane, the relationships between crust strength (τ) and the horizontal (F_h) and vertical (F_v) sediment fluxes were investigated. Both fluxes decreased as the surface crust became stronger. The decay behaved as an exponential function for both types of flux. The horizontal sediment flux over a crusted surface can be accurately predicted by completing Marticorena and Bergametti's [Journal of Geophysical Research 100 (1995) 16415] erosion model with a crust function. The vertical particle flux over crusted soil can be calculated by adding a similar function to Alfaro and Gomes's [Journal of Geophysical Research 106D (2001) 18075] dust production model. The study also suggests that the gradual bombardment of a surface crust by impacting particles does not immediately result in a decay of the crust's protective effect, provided that the crust has a minimum thickness. However, once the crust becomes perforated, its protective effect disappears very quickly, leading to much higher horizontal and vertical sediment fluxes than predicted for undamaged crusted soil.     

Movement of a small slipfaceless dome dune in the Namib Sand Sea, Namibia

The migration of a small slipfaceless dome dune close to the northern edge of the Namib Sand Sea has been measured by topographic survey. The dune dimensions are 45-m wide and 1-m high with a volume of 551 m³ that has been calculated as the difference between the dune's surface elevation and an interdune surface extrapolated from measurements around the edge of the dune. The migration direction, 64°, and distance moved, 90 m, are measured against stakes set out in 1976. The dune has moved about 90 m between 1976 and 1999. This is an average linear migration rate of around 4 m year⁻¹, and is equivalent to an annual sand transport rate of about 1.2 tonnes m⁻¹ year⁻¹. The calculated total potential sand flow in this part of the Namib Desert is 119 tonnes m⁻¹ year⁻¹, and the resultant potential sand flow is 63 tonnes m⁻¹ year⁻¹. The dune migration is therefore about 1% of the total potential sandflow and 2% of the resultant indicating that dune migration is only a small part of total potential sand transport. The results suggest that small slipfaceless dome dunes are very inefficient at trapping sand, and that winds blowing across the interdune in this area are undersaturated with sand.