Medium-term evolution of a gully developed in a loess-derived soil

Field surveys in the Belgian loess belt revealed the presence in many forested areas of large, permanent gully systems, most of which are currently inactive. In cultivated areas, such gullies can only be observed in cross-sectional soil profiles through hollows, as virtually all such large gullies are currently infilled with colluvium. Little is known about the spatial distribution, initiation and temporal evolution of these large, permanent gully systems on loess-derived soils. Therefore, the medium-term evolution of a gully initiated in a cultivated area on loess-derived soils southwest of Leuven (Belgium) in May–June 1986, was studied over 13 years. Two intense rainfall events created this (ephemeral) gully, which was not erased by subsequent tillage. Between June 1986 and the December 1999, eight field surveys were conducted to measure gully dimensions. During two surveys, topographic indices (e.g., slope and drainage area) were also measured. Daily rainfall for the measuring period were obtained from a rainfall station located some 10 km southwest of the gully. Analysis of rainfall data showed that no extreme rainfall event was required to initiate such large (permanent) gullies, as observed in forested areas and through cross-sectional profiles in cultivated fields in the Belgian loess belt. Return periods of the event that caused the gully varied between <1 year and 25 years, depending on the assumptions used for defining event rain intensity. Once established, length, surface area and volume of the studied gully evolved with time, cumulative rainfall or cumulative runoff, following a negative exponential relation. This accords with observations reported for gullies in Australia and the USA. This study shows that a degressive increase of gully extension, can be largely explained by the evolution of a “slope–drainage area” factor (S×A, which is proportional to stream power) with time. While gully length and gully surface area asymptotically evolve towards a final value, gully volume decreased at a given point in time. From this, it is inferred that sediment deposition will potentially infill the gully to such an extent that the farmer can drive across it. From this moment on, the combined effect of water and tillage erosion in the gully drainage area, will lead towards rapid infilling. This expected evolution of a gully in cultivated fields accords with observations of large infilled gully systems in cultivated areas in eastern Belgium. The permanent gullies observed under forest are attributed to the fact that after severe gully erosion, this area was reforested or abandoned. Therefore, the sediment source was cut off and the gully was not filled in by sediment deposition.     

试论吉拉沟的构造成因

从吉拉沟的形态、第四纪沉积物特征、人工地震剖面、电磁场特征及探槽揭露等认为：吉拉沟在深部对应于一基底隆起，该隆起被海西期断裂所限制。这些断裂在印支早期便停止了活动。在区域南北向应力场及差异压实作用下，基底隆起形成潜山型内核。其上覆中－新生界盖层形成披覆型背斜，在背斜顶部产生一系列平行于轴向的纵张裂隙。在内外营力的共同作用下，这些纵张裂隙不断发展贯通，并演变成规模巨大、近东西走向的吉拉沟。     

动态致裂过程的能量分析

本文从能量观点出发,对裂纹形成过程进行了分析,并以此为基础,讨论了各种致裂方法的原理和特点。     

Understanding soil‐pipe flow and its role in ephemeral gully erosion

The role of soil pipeflow in ephemeral gully erosion is not well understood. Experiments were conducted on continuous soil pipes to better understand the role of internal erosion of soil pipes and its relation to ephemeral gully development. Soil beds of 140 cm length, 100 cm width and 20 cm depth had a single soil pipe of different initial sizes (2, 4, 6, 8, and 10 mm diameter) extend from a water reservoir to the outlet. Experiments were run on Providence silt loam and Smithdale loam soils under a constant head of 15 cm established for 30 min. Either the tunnel collapsed or the head could not be maintained. Soil pipes that were initially 2 and 4 mm clogged instantaneously at their mouth and did not exhibit flow, whereas, pipes initially ≥ 6 mm enlarged by 268, 397, and 699% on average for the 6, 8, and 10 mm diameters, respectively. Critical shear stress values were found to be essentially zero, and erodibility values gave erosion indexes that were extremely high. The rapid internal erosion resulted in erratic flow and sediment concentrations with periods of no flow as pipes were temporarily clogged followed by surges of high flow and high sediment concentrations. Tensiometers within 6 cm of the soil pipes did not exhibit pressure increases typically associated with pipe clogging. Flow through 10 mm diameter soil pipes exhibited tunnel collapse for both soils tested. Tunnel collapse typically occurred within minutes of flow establishment suggesting that ephemeral gullies could be misinterpreted as being caused by convergent surface flow if observations were made after the runoff event instead of when flow is first established through soil pipes. Copyright © 2011 John Wiley & Sons, Ltd.     

基于DEM的黄土高原丘陵沟壑区沟谷网络节点研究

选择黄土高原3个典型地貌类型区为试验样区,基于数字高程模型(DEM)沟谷网络与沟谷网络节点自动提取技术,得到了各样区沟谷网络节点,并从地貌形态学、沟谷成因学以及水文学原理出发,对不同级别沟谷网络节点水流累积属性进行了深入分析,结合数理统计方法获得了：(1)同一沟谷系统不同分辨率沟谷网络节点水流累积量均值分级统计规律;(2)不同地貌类型样区相同分辨率沟谷网络节点水流累积量均值分级统计规律。该结论能够有效解释黄土高原丘陵沟壑区各级沟谷系统的形成机理及其空间分异规律,同时能够合理地说明沟谷系统发育与地表侵蚀、切割强度之间的相互关系。     

基于DEM的黄土地貌类型提取与制图——以黄土高原丘陵沟壑实验样区为例

以陕北绥德县刘家沟流域为实验样区 ,从地貌成因、形态和土地利用角度出发 ,分析了黄土高原丘陵沟壑区 3种典型地貌类型的区域特征 ;以不同地貌类型的区域特征为基础 ,在地理信息系统平台软件 Arc View的支持下 ,利用数字高程模型 (DEM)自动提取黄土丘陵区 3种基本地貌类型的方法和技术。研究分析结果证明 :利用 DEM自动提取地貌类型是一种快速、高效的技术方法 ,对指导退耕还林、防治水土流失和进行土地利用动态监测都具有重要意义。     

黄土高原遥感分类应用研究

Due to complex terrain of the Loess Plateau, the classification accuracy is unsatisfactory when a single supervised classification is used in the remote sensing investigation of the sloping field. Taking the loess hill and gully area of northern Shaanxi Province as a test area, a research was conducted to extract sloping field and other land use categories by applying an integrated classification. Based on an integration of supervised classification and unsupervised classification, sampling method is remarkably improved. The results show that the classification accuracy is satisfactory by the method and is of critical significance in obtaining up-to-date information of the sloping field, which should be helpful in the state key project of converting fiumland to forest and grassland on slope land in this area. This research sought to improve the appfication accuracy of image classification in complex terrain areas.     

基于扩展有限元法对土体滑坡破坏过程的模拟

边坡失稳一般经历滑裂面萌生、扩展和贯通的过程。近年发展起来的扩展有限元法（XFEM）可以合理地模拟这一过程,但在实际应用中仍未克服需要预先设定滑裂面的起始位置,以及滑裂面前端扩展方向的判断精度较低等缺陷。首先介绍滑裂面萌生和扩展的判断和程序实现机制。然后提出了确定滑裂面起始位置的方法,重点描述了其中的自动判断裂缝萌生位置的方法,即根据单元内的应力状态求得拉应力水平和剪应力水平,根据其相对关系并结合应力历史进行判断。接着介绍了联合运用扇形控制域和圆形控制域确定滑裂面前端扩展方向的新方法。该方法可提高XFEM对滑坡破坏过程的模拟精度。最后通过两个边坡失稳算例的计算分析验证所提方法的可靠性。     

青海共和盆地干热岩地热储层水力压裂物理模拟和裂缝起裂与扩展形态研究

水力压裂是青海共和盆地干热岩地热资源开发的难点技术问题之一。本文基于升级改造的大尺寸真三轴水力压裂物理模拟实验系统模拟干热岩储层高温高压环境,利用青海共和盆地露头岩心进行水力压裂物理模拟实验,揭示干热岩储层水力裂缝的起裂和扩展规律。通过物理模拟实验发现：干热岩储层裂缝起裂可以通过文中提出的起裂模型判断起裂方式和预测起裂压力;水力裂缝在岩石基质中的扩展形态简单,仅沿最大主应力方向延伸;但是水力裂缝会受到岩石中弱面的影响,发生转向沿弱面延伸,形成较复杂的裂缝形态。因此,建议在干热岩储层实际施工中,在天然裂缝发育较丰富的层段开展水力压裂,以实现复杂裂缝网络提取地热能。     

Spatial analysis of factors controlling the presence of closed depressions and gullies under forest: Application of rare event logistic regression

Although numerous studies have reported on past geomorphic change, it is a common difficulty to identify the relative role of driving human or climatic forces. To that end, the possibilities of rare events logistic regression are explored in a 1300-hectare large, ancient forest in central Belgium. Here, 43 permanent gullies and 71 closed depressions were observed that were probably caused by an anthropogenically induced land use change in the past. This human origin was confirmed for a limited number of case-study areas by recent geomorphic research, with dating and soil profile analysis. The objective of this study is to analyze the factors controlling the position of all gullies and closed depressions in the study area and investigate their mutual relation. This will indicate whether or not the results obtained for the small case-study areas can be extrapolated to the rest of the geomorphic features found in the whole study area.The obtained logistic regression models show that both anthropogenic and physical factors are important in the spatial distribution of both geomorphic features. For the prediction of the occurrence of closed depressions, anthropogenic factors are most important however. The probability of finding a depression is larger close to reported prehistoric and Roman archaeological sites, on well-developed, undisturbed Luvisols and on northeast- to southeast-facing slopes. The local slope gradient of the soil surface is the most important factor in the multivariate model for predicting the presence of the permanent gullies. The occurrence probability of permanent gullies is also larger on northeast- to southeast-facing slopes. The anthropogenic factor expressed by the distance to the closed depressions is significant as a univariate model. This suggests that a critical slope gradient is a prerequisite for gully incision, but gully formation still needs to be triggered by a human-induced land use change. It can therefore be concluded that both the gullies and the closed depressions in the Meerdaal Forest are related to past human activity, either directly, through mining of loess or indirectly, through runoff production on cropland. This study also showed the potential of rare events logistic regression for analyzing the spatial pattern of geomorphological features in relation to environmental factors.