Terrestrial sensitivity to abrupt cooling recorded by aeolian activity in northwest Ohio, USA

Lower slopes of the Sandia Mountains are characterized by granitic corestone topography and weathering-limited slopes with thin grusy colluvium and weakly developed soils. In contrast, thick soils with illuvial clay and pedogenic carbonate have developed below aplite outcrops. Aplite is resistant to chemical decomposition, but physically weathers to blocky clasts that enhance surface roughness and erosional resistance of colluvium, promoting accumulation of eolian fines. Thick B horizons on aplite slopes indicate limited erosion and prolonged periods of stability and soil development. Accretion of eolian material limits runoff and prevents attainment of a steady-state balance between soil production and downslope transport.     

Modelling controls on aeolian dune‐field pattern evolution

A second‐generation, source‐to‐sink cellular automaton‐based model presented here captures and quantifies many of the factors controlling the evolution of aeolian dune‐field patterns by varying only a small number of parameters. The role of sediment supply, sediment availability and transport capacity (together defined as sediment state) in the development and evolution of an aeolian dune‐field pattern over long time scales is quantified from model simulations. Seven dune‐field patterns can be classified from simulation results varying the sediment supply and transport capacity that control the type and frequency of dune interactions, the sediment availability of the system and, ultimately, the development of dune‐field patterns. This model allows predictions to be made about the range of sediment supply and wind strengths required to produce the dune‐field patterns seen in the real world. A new clustered dune‐field pattern is identified from model results and used to propose an alternative mechanism for the formation of superimposed dunes. Bedforms are hypothesized to cluster together, simultaneously forming two spatial scales of bedforms without first developing a large basal dune with small superimposed dunes. Manipulation of boundary conditions produces evolving dune fields with different spatial configurations of sediment supply. Trends of spacing and crest length increase with decreasing variability as the dune field matures. This simple model is a valuable tool which can be used to elucidate the dominant control of aeolian sediment state on the construction and evolution of aeolian dune‐field patterns.     

Field measurement and scaled-down wind-tunnel model measurement of airflow field over a barchan dune

Airflow is measured over a barchan dune in the field and over a scaled-down model in a wind tunnel. The change of the flow speed over the stoss side is represented by the change of speed-up ratio. According to the field measurement, the wind profiles within 0-3m above the stoss can be divided into two segments. The lower segment, about 0.66 m thick, is the inner-boundary layer, within which the friction velocities derived from the wind profiles increase from the upwind inter-dune region to the upper stoss, and then decrease near the dune top. This change, together with the changes of airflow field, speed-up ratio and sand flux, is related to the morphological change and contributes to the stable shape and height of a barchan dune. In the wind tunnel, airflow varies in a similar way as in the field, with the speed-up ratios constantly higher than 1.0 and increasing along the stoss slope. While the segmentation of wind profiles also occurs in the wind tunnel, friction velocities derived from the wind profiles decrease along the stoss, indicating a very thin inner-boundary layer above the wind tunnel model where the detailed wind-speed change becomes difficult to measure using the present instruments.     

Terrestrial photogrammetric techniques applied to the control of a parabolic dune in the Liencres dune system,Cantabria (Spain)

The spatial–temporal variations of a dune system can be determined by using diverse ‘geomatic’ methodologies: geodesy, global positioning system (GPS) and photogrammetry. In the case of the Liencres dune system, a study will be carried out using the ‘close‐range’ photogrammetry technique and the topography technique (total station and GPS). In order to determine the dynamic of the dune system it is necessary to repeat the process of study after a specific interval of time. For this reason, three dimensional data should be available in two different time periods, between which the displacement of the object of analysis (the front portion of the dune) will be significant enough to evaluate its magnitude. This work analyses the viability of photogrammetry for the determination of the spatial–temporal changes of a coastal parabolic dune. Two factors have been analysed: first, the comparison of the photogrammetric results with the results obtained from topographic methods (total station and GPS), and second, the quantification of the displacement of the dune system. The analysis of the correspondence between the movement of different parts of the dune and the influence of the intensity and direction of the prevailing wind in the area is also desired. The dune advanced 12·15 ± 0·06 m (an average of 8·5 m/yr), and the partial implications for the dynamic of human modified processes on the natural park have been established. Copyright © 2008 John Wiley & Sons, Ltd.     

Optically stimulated luminescence dating of aeolian sand in the Otindag dune field and Holocene climate change

The dune system in Otindag sand field of northern China is sensitive to climate change, where effective moisture and related vegetation cover play a controlling role for dune activity and stability. Therefore, aeolian deposits may be an archive of past environmental changes, possibly at the millennial scale, but previous studies on this topic have rarely been reported. In this study, thirty-five optically stimulated luminescence (OSL) ages of ten representative sand-paleosol profiles in Otindag sand field are ob-tained, and these ages provide a relatively complete and well-dated chronology for wet and dry varia-tions in Holocene. The results indicate that widespread dune mobilization occurred from 9.9 to 8.2 ka, suggesting a dry early Holocene climate. The dunes were mainly stabilized between 8.0 and 2.7 ka, implying a relatively wet climate, although there were short-term penetrations of dune activity during this wet period. After ~2.3 ka, the region became dry again, as inferred from widespread dune activity. The "8.2 ka" cold event and the Little Ice Age climatic deterioration are detected on the basis of the dune records and OSL ages. During the Medieval Warm Period and the Sui-Tang Warm Period (570-770 AD), climate in Otindag sand field was relatively humid and the vegetation was denser, and the sand dunes were stabilized again. These aeolian records may indicate climate changes at millennial time scale during Holocene, and these climatic changes may be the teleconnection to the climate changes elsewhere in the world.     

横向沙丘气流平均速度变化规律的风洞模拟

在沙丘动力系统中,存在沙丘形态、气流、沙粒运移三者之间复杂的相互作用。通过风洞实验的方法,针对不同形态的6组横向沙丘模型,采用粒子图像测速系统,测量了模型沙丘周围气流水平速度和垂直速度的变化规律。实验结果表明,横向沙丘迎风坡水平气流存在1.28～1.89之间的加速率,垂直气流存在上扬趋势,这二者均有随沙丘迎风坡坡度增大而增大的趋势。在横向沙丘背风坡,由于气流的分离,水平气流速度减小并出现反向,其大小约为自由风速的17％;垂直气流速度存在下沉趋势,其最大沉速出现在气流重附点附近;背风坡气流速度的变化受沙丘迎风坡坡度影响较小,受自由风速的影响较大。沙丘对气流速度的改变在近地层较为显著,随着高度的增加地形影响逐渐减小。     

库姆塔格沙漠大尺度风场特征分析

利用美国NCEP/NCAR1979—2006年27 a地面10 m高度的日平均风场再分析-2资料以及46 a的地面常规气象站观测资料和1 a的沙漠中自动气象站观测资料,对库姆塔格沙漠地区风场进行了统计与诊断分析。结果表明：沙漠地区,一年四季北风和东北风为近地面主要存在的风向,春季风速最强,夏季次之,秋冬季最弱;在夏秋冬季节,由于北风的逐渐减弱,加上受到南侧阿尔金山和青藏高原风场的影响,南下的气流在沙漠南侧分为三支,分别流入新疆、青海和甘肃。春夏季节在沙漠北侧基本都是以东北风或者西北风吹向库姆塔格沙漠,秋冬季节,沙漠地区出现西南风。根据沙漠周围6个气象站风场风速显示,库姆塔格沙漠地区风速存在明显的周期变化,在过去的10多年中库姆塔格沙漠周围风场风速处于一个低谷期,但最近几年风速有不断加强的趋势。     

干砂强夯动力特性的细观颗粒流分析

通过引进和开发二维颗粒流程序,基于相似理论建立了可以模拟砂土地基强夯加固的细观颗粒流模型,结合小比尺室内细观模型试验,从颗粒细观力学角度入手对干砂在强夯冲击加固过程中的动力反应特性进行了数值模拟。结果表明,数值模拟结果与试验结果有较好的一致性,利用颗粒流模型可以很好地模拟干砂在冲击荷载作用下的锤底动接触应力、颗粒间动接触应力和颗粒位移场分布情况,并可以实时跟踪颗粒的变位及接触应力变化,实现从细观角度揭示干砂强夯动力响应特性,研究工作为今后砂土强夯加固宏细观机制研究提供了一条新的思路。     

钙质石灰岩中桩基轴向承载特性的有限元分析

为了研究钙质石灰岩中桩基的合理设计理论及开发相关的变形破坏数值分析方法,利用弹塑性有限元方法对以硫球石灰岩作为持力层的桩基现场轴向载荷试验进行了数值分析。有限元分析中钙质石灰岩被假定为理想弹塑性材料。结果表明,如果能在有限元解析中合理地考虑桩侧摩阻力的低减效果,弹塑性有限元计算可以较好地模拟与分析钙质砂土中桩基的轴向承载力特性。最后还对开口钢管桩基的土塞效应的发生机制进行了分析。     

大型海底、海岸和沙漠沙丘的形态和迁移特征

海底、海岸和沙漠沙丘的迁移与其波高、波长等形态参数有关。对一些代表性大型沙丘的分析表明,海底、海岸和沙漠沙丘的形态特征存在着较大的差异,在相同波长条件下,海底沙丘波高最小,海岸沙丘波高次之,沙漠沙丘波高最大。其原因主要是流体厚度的差异,其次是物质供给条件的不同:浅海地区的水深限制了沙丘向上生长,而陆地气流厚度较大,给沙丘的向上生长提供了较大的空间;沙漠环境的沉积物供给远大于海岸环境,因而导致沙丘高度的差异。沙丘形态参数经常偏离统计的波长 波高曲线,数值实验结果显示,这与推移质输运率有关。推移质输运率的沿程变化可以使形态参数系统性地偏离波长 波高曲线;推移质输运率的沿程突变可使沙丘迁移受阻,造成沉积物的垂向堆积,形成超高的沙丘。由于大型沙丘的迁移特征受控于推移质输运率和沙丘高度,因此可以通过波高设计来计算迁移距离 历时曲线,进而控制沙丘迁移动态。文中给出了一个获取迁移距离 历时曲线的算例。设计波高可通过人工篱笆、隔挡墙、沟渠、表面护层等措施而实现。