A mechanism for the origin and development of the large-scale dunefield on the right flank of the lower reach of Laoha River, Northeast China

By viewing satellite imagery, a striking large-scale dunefield can be clearly perceived, with a size of nearly 63 km long and 11 km wide, and trending NE–SW, on the right flank of the lower Laoha River, Northeast China. By means of remote sensing imagery analysis and field observation as well as a comparison with a small-scale dunefield on the right flank of the lower Xiangshui River, analogous to the case of the lower Laoha River, this paper presents a new mechanism for its origin and development. The results show that:(1) the large-scale dunefield bears a tile-style framework overwhelmingly composed of transverse barchanoid ridges perpendicular to the predominant winds, and inlaid diverse blowouts.(2) The small-scale dunefield, referred to as a primary structural unit of the large one, is typical of an incipient dunefield, following the same rules of evolution as the larger.(3) A succession of barchanoid ridge chains can steadily migrate downwind in much the same manner as surface wave propagation in air or water stimulated by an incised valley, and ultimately tend to bear roughly the same wavelength and amplitude under stable climate and hydrologic regimes.(4) The first ridge chain acquires its sand source substantially from the downwind escarpments exposing the loose Quaternary sandy sediments to the air, while the ensuing ridges derive their sands dominantly from in situ deflation of the underlain Quaternary loose sandy sediments in blowouts, partly from the upwind ridges through northern elongated horns. Theoretically, the sands from riparian escarpments can be transported by wind to the downwind distal end of a dunefield after sufficient long duration.(5) The lower Laohahe region experienced probably three significant climatic changes in the past, corresponding to the three active dune belts, suggesting that once a large-scale dunefield occurs, it is nearly impossible to be completely stabilized, at least in its central portions. At present, seasonal shrinkage and stagnation of the lower Laoha River, widespread farming and afforestation in the valley, and establishing windbreaks downwind of the valley as well as surrounding the dunefield, appear to have significantly modified local flow fields and sand sources, engendering significant degradation of the dunefield.     

黄河源区玛多县沙漠化成因与发展过程

通过分析黄河源区玛多县沙漠化的成因和过程,结果表明,玛多县沙漠化成因类型有三种,分别为固定沙丘/古沙丘活化、滑塌陡坎及风蚀斑块,其形成都与多年冻土退化相关.在固定沙丘或古沙丘分布区,冻土退化导致热融沉陷,形成沉陷坑,沉陷坑边缘形成拉裂缝或陡坎,使下伏松散沙露出.在斜坡上,冻土退化形成滑塌陡坎,使底层土壤从侧面暴露.在平坦的冲积平原,差异性冻胀和融沉导致草皮拉裂,形成积水坑洼,冻土退化导致土壤变干,土壤的底层暴露.下伏风成沉积物暴露后,遭受风蚀,形成侧向凹槽,致使上部土体坍塌,使更多的风成沉积物暴露,这些过程的不断重复,使风蚀坑、陡坎、风蚀斑块扩大相连,最终形成流动沙丘、风蚀劣地、戈壁等地貌.     

浑善达克沙地风蚀坑形态特征及其影响因素

风蚀坑是沙质草原固定沙地活化的主要表现形式,其持续活化的结果将导致沙漠化扩展和生态环境恶化。采用实地测量方法,测定了129个风蚀坑走向、长度、宽度等形态指标。研究表明：① 研究区风蚀坑形态特征受NW、SW和W 3个风向的综合影响;② 风蚀坑走向呈WNW-ESE（112.5°）走向,有74.5%的风蚀坑走向（100°~120°）与SW风垂直,和W风呈20°左右的夹角,并影响风蚀坑的几何形状特征;③ 按风蚀坑长宽比值所反映的几何形状,可划分为卵圆形、长条形、狭长形和带形风蚀坑;依风蚀坑的地表形态特征,可划分为碟形、梯形、沟谷形、杯形和坡形5种风蚀坑。     

风蚀坑形态-动力学研究进展

风蚀坑是沙质海岸、湖岸及干旱、半干旱区沙质草原地区受风蚀作用形成的洼地。受风蚀坑地形的影响,坑内气流的风速、风向发生了变化,通过输沙过程的局部差异改变了风蚀坑的蚀积格局,并对风蚀坑形态进行改造。而形态的变化又反作用于近地表气流,由此产生风蚀坑形态-动力学的响应与反馈。总结并扼要评述国内外近年来风蚀坑形态-动力学及其发育演化方面的研究进展,分析现有研究的不足,以期对今后此类研究提供借鉴。     

Droplet breakup in subsea oil releases – Part 2: Predictions of droplet size distributions with and without injection of chemical dispersants

A new method for prediction of droplet size distributions from subsea oil and gas releases is presented in this paper. The method is based on experimental data obtained from oil droplet breakup experiments conducted in a new test facility at SINTEF. The facility is described in a companion paper, while this paper deals with the theoretical basis for the model and the empirical correlations used to derive the model parameters from the available data from the test facility. A major issue dealt with in this paper is the basis for extrapolation of the data to full scale (blowout) conditions. Possible contribution from factors such as buoyancy flux and gas void fraction are discussed and evaluated based on results from the DeepSpill field experiment.     

Droplet breakup in subsurface oil releases – Part 1: Experimental study of droplet breakup and effectiveness of dispersant injection

Size distribution of oil droplets formed in deep water oil and gas blowouts have strong impact on the fate of the oil in the environment. However, very limited data on droplet distributions from subsurface releases exist. The objective of this study has been to establish a laboratory facility to study droplet size versus release conditions (rates and nozzle diameters), oil properties and injection of dispersants (injection techniques and dispersant types). This paper presents this facility (6 m high, 3 m wide, containing 40 m³ of sea water) and introductory data. Injection of dispersant lowers the interfacial tension between oil and water and cause a significant reduction in droplet size. Most of this data show a good fit to existing Weber scaling equations. Some interesting deviations due to dispersant treatment are further analyzed and used to develop modified algorithms for predicting droplet sizes in a second paper (Johansen et al., 2013).     

基于ALOS数据的浑善达克沙质草地风蚀坑空间格局及特征分析

以2008年的ALOS卫星多光谱波段（空间分辨率10 m）和全色波段（空间分辨率2.5 m）融合后的影像数据及地面调查数据为数据源,借助ArcGIS和Fragstats3.3软件,采用景观格局指数方法,从类型和斑块水平上定量分析了浑善达克沙质草地风蚀坑的空间格局和特征。结果表明,浑善达克沙质草地风蚀坑面积占景观面积的6.1%,密度为0.026个·hm-2,分布范围广且呈局部聚集分布;根据不同发育阶段,风蚀坑形状分为碟形、椭圆形、槽形和不规则形;风蚀坑斑块面积与形状指数、分维数之间存在显著正相关关系,面积的增加,可使风蚀坑形状复杂化趋势明显。复杂形状风蚀坑主要由若干个独立的风蚀坑扩大并联结形成;相对于宽度与深度,风蚀坑面积与长度间存在更显著正相关;浑善达克沙质草地风力侵蚀基准面较高,风蚀坑深度一般不超过5 m;风蚀坑长度与宽度之间、宽度与深度之间存在相互制约关系。     

Morphodynamics and climate controls of two aeolian blowouts on the northern Great Plains,Canada

Blowouts are the most regionally pervasive active aeolian landform on the northern Great Plains of North America. This study reports a long‐term investigation into the morphological development of two adjacent blowouts in a continental dune field. The blowouts were monitored for a decade in the Bigstick Sand Hills of southwestern Saskatchewan, Canada. Topographic changes were determined from dense arrays of erosion pins in each blowout (1 per 4 m², n = 171; and 1 per 16 m², n = 150). Pin measurements were made 16 times between May 1994 and May 2004. Over the decade both blowouts expanded and more than doubled in volume. Differences in form–flow interactions have caused the larger of the two blowouts to deposit more than a metre of sediment within the deflation basin, and the smaller blowout to erode by more than a metre. A negative feedback effect was triggered when the larger blowout reached a critical size around 1994 (60 m × 36 m × 8·1 m, length × width × height) when sediment was no longer eroded from the deflation basin. A positive feedback in the smaller blowout continues to facilitate erosion from the deflation basin. Monthly observations since 2002 indicate that aspect plays an important role in the development of these blowouts by creating a spatial asymmetry in sediment availability. Sediment is more readily available throughout the year on south‐facing slopes, which receive greater insolation than north‐facing slopes and are often drier and more frequently thawed in this cold‐climate environment. Comparisons between climate data from a remote meteorological station 45 km to the southwest and sediment transport indices developed from the erosion pin data produced very few correlations significant at the 95 per cent confidence level. Nevertheless, the signs of the correlation coefficients indicate that sediment erosion and deposition in both blowouts respond similarly to the following climate variables recorded at the remote station: (i) the amount of precipitation, (ii) the transport capacity of the wind and (iii) transporting winds from a directional wedge between 180 and 330°. Taken altogether, the results from this study highlight the importance of climate and feedback effects in blowout development that may be extended to other blowouts in continental and coastal settings. Copyright © 2006 John Wiley & Sons, Ltd.     

青海省贵南县土地沙漠化驱动机制

贵南县木格滩流沙区是共和盆地最大的连续沙带,当地现代沙漠化是由古沙丘活化形成的,但古沙丘活化过程尚不明确。通过实地考察、遥感卫星影像解译、DEM高程数据分析、沉积地层分析及粒度测试,对贵南县土地沙漠化驱动机制从微观层面进行了研究。结果表明：古沙丘表面发育良好的草皮层及粉沙土层是古沙丘的保护层,保护层完整性的破坏,使下伏松散古风成沙暴露,遭受风蚀,形成侧向凹槽,引起草皮层或粉沙土层崩塌,使更多的古风成沙暴露。风力侵蚀与重力侵蚀交替进行,导致风蚀坑不断扩大,相互连接成片,形成流动沙丘。