小型水坝拆除后河貌演变模拟分析——以西河水坝为例

小型水坝随着服役时间的增长,其面临的安全、经济和生态方面问题日益突出,针对一些修复价值较低的病险水坝,实施降等或报废拆除已成为一种综合最佳的管理措施。拆坝后,原库区大量淤沙无控释放将改变原有河道形态,对水生生物栖息地造成影响。为预测拆坝后河道在不同时间尺度上的变化特征,以长江流域乌江水系内的西河水坝为研究对象,建立二维水沙数学模型,分别研究了拆坝后水沙输运造成的短期和长期河床形态变化。结果表明:拆坝后短期内,坝址上游主河道发生了强烈的冲刷下切,且水库淤沙前缘部分出现了显著的淤积抬高,相比而言,坝址下游河床变化并不明显,只有坝下河段及河口附近出现较显著的泥沙淤积;在拆坝后长期的河床演变过程中,坝址上下游河道均发生了不同程度的冲刷下切,拆坝2年后下游河床逐渐趋于稳定,而上游主河道由冲刷下切转化为冲淤交替的演变趋势,河床形态不断调整变化。本研究可为病险坝和小水电报废拆坝后的河道治理、水生生物栖息地修复提供参考依据。     

Mechanism of the giant Seimareh Landslide,Iran, and the longevity of its landslide dams

Seimareh Landslide (SL) is globally recognized as one of the largest rock mass movements in the world. It is located along the border of Ilam and Lorestan provinces in southwest Iran, in the heart of the Zagros Mountain Range. There are controversial findings about the mechanism of the landslide formation. This field work study reviewed the possible mechanisms of failure and analyzed post-failure geomorphic features. Drainage pattern disturbance in the depositional region and consequent dammed lake formation are among the most significant characteristics of these features. Seimareh, Jaidar and Balmak are three large landslide-dammed lakes. The present study analyzed the processes responsible for the formation and erosion of the Jaidar and Seimareh Landslide dams using the available annual sedimentation and field measurements of the sediment deposited in these lakes. The results showed that the SL dam has been formed about 935 years after the landslide event. Detailed field investigations indicated a specific hydro-morphological condition in the landslide area. The results implied that the main causes of the failure were probably the particular hydro-morphological characteristic of the landslide source area together with the enormous eroding energy resulted from merging of two high-flow rivers which eroded the base of the southern flank of Kabir-kuh Mountain. However, the unusual size of the landslide suggests that an external factor, e.g., a huge earthquake, might have triggered the failure.     

Initiation process of debris flows on different slopes due to surface flow and trigger-specific strategies for mitigating post-earthquake in old Beichuan County, China

The 12 May 2008 Wenchuan earthquake (Ms 8.0) in China, produced an estimated volume of 28 × 10⁸ m³ loosened material, which led to debris flows after the earthquake. Debris flows are the dominant mountain hazards, and serious threat to lives, properties, buildings, traffic, and post-earthquake reconstruction in the earthquake-hit areas. It is very important to understand the debris flow initiation processes and characteristics, for designing debris flow mitigation. The main objective of this article is to examine the different debris flow initiation processes in order to identify suitable mitigation strategies. Three types of debris flow initiation processes were identified (designated as Types A, B, and C) by field survey and experiments. In “A” type initiation, the debris flow forms as a result of dam failure in the process of rill erosion, slope failure, landslide dam, or dam failure. This type of debris flow occurs at the slope of 10 ± 2°, with a high bulk density, and several surges following dam failure. “B” type initiation is the result of a gradual increase in headward down cutting, bank and lateral erosion, and then large amount of loose material interfusion into water flow, which increases the bulk density, and forms the debris flow. This type of debris flow occurs mainly on slopes of 15 ± 3° without surges. “C” type debris flow results from slope failures by surface flow, infiltration, loose material crack, slope failure, and fluidization. This type of debris flow occurs mainly on slopes of 21 ± 4°, and has several surges of debris flow following slope failure, and a high bulk density. To minimize the hazards from debris flows in areas affected by the Wenchuan earthquake, the erosion control measures, such as the construction of grid dams, slope failure control measures, the construction of storage sediment dams, and the drainage measures, such as construction of drainage ditches are proposed. Based on our results, it is recommend that the control measures should be chosen based on the debris flow initiation type, which affects the peak discharge, bulk density and the discharge process. The mitigation strategies discussed in this paper are based on experimental simulations of the debris flows in the Weijia, Huashiban, and Xijia gullies of old Beichuan city. The results are useful for post-disaster reconstruction and recovery, as well as for preventing similar geohazards in the future.     

The Effect of Impervious Clay Core Shape on the Stability of Embankment Dams

One of the most important dangers that treat earth dams which can lead to interior failure over a prolonged period is the hydraulic fracturing factor. In the case of zoned dams, due to differences in stiffness of the core and its abutment zone, differential settlements occur between them. This factor is responsible for the arching phenomenon. Differential settlements between core and shell cause cracks within the core initially sub-surface, Those cracks may develop the first impounding causing internal erosion on the dam core. In this research, using a computer modeling of Ghavoshan rockfill dam (located the west part of Iran) as a case study computed by SIGMA/W program, the role of the dam core shape on those factors is demonstrated. It is found that an inclined core shape is preferred in a condition that is especially important settlements of construction during for dam body. The result of finite element analysis indicates desired conditions from the point of view of stress, deformation and resistance against hydraulic fracturing for the same width of dam designs. Moreover, this can be higher priority for embankment dam designs.     

Impact and analysis of geotechnical processes on earthfill dam breaching

For over three decades, emergency planners have used numerical models to predict breaching in earthfill dams due to extreme events such as overtopping. However, current models neglect the role of the unsaturated zone present within the downstream face of an earthfill dam. This leads to an incorrect estimation of the time and space evolution of the breaching process, as such models often oversimplify governing geotechnical aspects such as the presence of the unsaturated soil medium in the vicinity of the breach channel. The stress state in the soil due to matric suction acts as a stabilizing force for the breaching mechanism and influences the erosion of the breach channel, especially during the initial phases of the breaching. The side-slope failure mechanism observed along the breach channel is also influenced by the negative pore-water-pressures in the soil. Based on a comprehensive experimental research program carried out in the Hydraulics Laboratory at the University of Ottawa, Canada, several new concepts are proposed to incorporate geotechnical factors and techniques which must be considered during the construction of earthfill dam models for laboratory testing. Two main findings emerged from this experimental work. First, the installation of a drainage mattress at the downstream toe of the dam depressed the phreatic surface through the earthfill dam body, which caused a lag in the breaching process due to the infiltration and reduced erosion occurring in the breach channel. Second, it is essential to control compaction during the construction of the earthfill dam model, since this significantly influences the erosion, as well as the side-slope failures which occur in the breach channel. Future studies are under way by the authors with the purpose of scaling of parameters such as the matric suction and soil erodibility.     

Measurement of debris mass changes and assessment of the dam-break flood potential of earthquake-triggered Hattian landslide dam

The Hattian landslide, which was triggered by the 2005 Kashmir earthquake, formed one of largest landslide dams in the world and it has posed a serious threat of flooding to people living in the lower reach of the Jhelum River. In order to understand deformation occurring in the body of the dam, physical measurements using a Differential Global Positioning System (DGPS) were conducted. Gradual deformation and slowly developing backward erosion initially were observed, leading eventually to a sudden creation of a deep hollow on the downstream slope of the landslide dam. The dimensions of this eroded gully were determined by laser scanning, and the results showed a significant loss of soil volume and a large change in the body of the dam. A breach formation model was used to predict the outflow hydrograph generated by constant downcutting of dam during a breaching event. A run-off analysis of the outflow hydrograph was conducted to evaluate inundation levels of flood waves in case the dam is breached. Hazardous downstream locations were identified near the junction of the Karli and Jhelum Rivers, suggesting a need for early warning system in order to avoid loss of lives.     

Evaluation of sediment yield and soil loss by the MPSIAC model using GIS at Golestan watershed, northeast of Iran

Watershed degradation due to soil erosion and sedimentation is considered to be one of the major environmental problems in Iran. In order to address the critical conditions of watershed degradation in arid and semiarid regions, a study based on the Modified Pacific Southwest Inter-Agency Committee (MPSIAC) model was carried out at Golestan watershed, northeast of Iran. The model information layers comprising nine effective factors in erosion and sedimentation at the watershed site were obtained by digitalization and spatial interpolation of the basic information data in a GIS program. These factors are geology, soil, climate, runoff, topography, land cover, land use, channel, and upland erosion. The source data for the model were obtained from available records on rainfall and river discharge and sediment, topography, land use, geology, and soil maps as well as field surveys and laboratory analysis. The results of the MPSIAC model indicated that 60.75 % (194.4 km²) and 54.97 % (175.9 km²) of the total watershed area were classified in the heavy sedimentation and erosion classes, and the total basin sediment yield and erosion were calculated as 4,171.1 and 17,813.4 m³ km^?2 year^?1, respectively. In the sensitivity analysis, it was found that the most sensitive parameters of the model in order of importance were topography (slope), land cover and use, runoff, and channel erosion (R ²?=?0.92–0.94), while geology, climate (rainfall), soil, and upland erosion factors were found to have moderate effect to the model output (R ²?=?0.74–0.59).     

Evaluating sedimentation effect on reservoirs water quality: a case study,Shahriyar reservoir

In this research, total dissolved solids (TDS) was simulated by CE-QUAL-W2 software in two scenarios of with and without sedimentation in normal, dry, and wet periods of inflow for Shahriyar reservoir, an under-construction dam located in Iran. Shahriyar reservoir is a dam with a total capacity of 700 million cubic meters (MCM) and a dead volume of 360 MCM. In case of with sedimentation compared with without sedimentation, results showed an annual average increase of 4, 4.3, and 8.7% in TDS of outflow for normal, wet, and dry periods, respectively. There is no significant change in TDS profile of the reservoir in both conditions. According to the average outflow TDS of 1448, 1301, and 1555 mg/L compared with the average inflow TDS of 1714, 1520, and 1835 in normal, wet, and dry conditions, respectively, the effect of dam on inflow TDS adjustment can be also observed. With the mentioned outflow TDS, there is no limitation to use of reservoir water for agricultural purposes based on FAO irrigation guidelines for both scenarios.     

A study on geotechnical characterization and stability of downstream slope of a tailings dam to improve its storage capacity (Turkey)

Tailings, waste products of the ore-dressing processes, are usually stored in a slurry form and pumped into sedimentation pond which is surrounded by natural heights and/or artificial walls. The database of International Commission of Large Dams and United Nations Environment Programme showed that several hundreds of tailings dam have failed since 1910. One of the reasons of these failures is slope instability. Construction of mine tailings dams in Turkey has started to increase, particularly in the recent decade. This paper focuses the study undertaken to assess the geotechnical characteristics and stability of downstream slope of a tailings dam at a copper-zinc mine in the Black Sea Region of Turkey. At the mine site there are two tailings dams located at two different elevations. The upper dam was completely filled and is out of service. Increasing amount of tailings filling the pond of the lower dam resulted in a limited storage, and therefore, it has been decided that heightening of the dam about 7 m to improve its storage facility would be necessary. For the purpose, a series of investigations in the vicinity of the tailings dam and some geomechanical laboratory tests were carried in conjunction with the use of empirical methods to determine mechanical properties of the current dam material, tailings and the rockfill material that will be used for heightening of the dam. The results of the static and pseudo-static stability analyses based on the methods of limit equilibrium and numerical modelling indicated that circular and non-circular failures are the most critical modes of failure for the dam and safety factor gradually decreases when the tailings rise up to a specific elevation in the pond. However, the ranges of the calculated safety factors from both methods are considerably similar and suitable for heightening of the dam by 7 m in terms of stability. The results suggest that any instability is not expected under both static and dynamic conditions after the current height of the tailings dam is increased by 7 m, if the slope geometry considered in the analyses is not changed and the tailings in the pond do not rise up to an elevation of about 475 m. In addition, magnitude of permanent settlement of the dam due to a probable earthquake was also assessed using two empirical methods and the results from both methods showed similar amount of settlements which were in tolerable limits.     

低坝影响下的流场与淤积形态特性试验研究

为研究径流式低坝影响下的水流流动与泥沙淤积特性,开展水槽试验,基于图像测量技术,获取并解析坝附近区域流场信息及典型淤积形态。结果表明：坝前附近流段纵向流速在垂线上出现衰减区,减幅随水流强度增大而减小;坝顶断面纵向和垂向流速沿垂线的分布均呈现显著的分区特性,分区界限几乎不受水流强度的影响;随坝顶水深增加,坝下游漩涡涡心向下游及河底移动,面积和强度皆增大;坝上游淤积形态特性对水流强度的变化非常敏感,在较低强度来流下,呈接近坝体的稳定曲面斜坡,而在高强度来流下,不形成稳定淤积体;坝下游形成动态稳定的淤积斜坡,纵剖表面线呈抛物线规律,随来流强度变异程度小。     

低坝影响下的流场与淤积形态特性试验研究

为研究径流式低坝影响下的水流流动与泥沙淤积特性,开展水槽试验,基于图像测量技术,获取并解析坝附近区域流场信息及典型淤积形态。结果表明:坝前附近流段纵向流速在垂线上出现衰减区,减幅随水流强度增大而减小;坝顶断面纵向和垂向流速沿垂线的分布均呈现显著的分区特性,分区界限几乎不受水流强度的影响;随坝顶水深增加,坝下游漩涡涡心向下游及河底移动,面积和强度皆增大;坝上游淤积形态特性对水流强度的变化非常敏感,在较低强度来流下,呈接近坝体的稳定曲面斜坡,而在高强度来流下,不形成稳定淤积体;坝下游形成动态稳定的淤积斜坡,纵剖表面线呈抛物线规律,随来流强度变异程度小。     

粘土心墙坝漫顶溃坝过程离心模型试验与数值模拟

利用作者研制成功的溃坝离心模型试验系统,对粘土心墙坝漫顶溃决过程进行了试验研究,结果发现粘土心墙坝与均质坝溃决机理与溃口发展规律明显不同,随着漫坝水流对下游坝壳冲蚀程度的增加,粘土心墙发生剪断破坏,溃口洪水流量迅速增大.基于上述试验结果,提出了一个描述粘土心墙坝漫顶溃坝过程的数学模型,并建议了相应的数值计算方法.该模型...     

Impact of river damming on sediment texture and trace metals distribution along the watershed and the coastal zone of Nestos River (NE Greece)

This study examines the sediment particle size distribution and the trace metal concentrations from a dammed-river watershed (Nestos River) to its deltaic zone in NE Greece. The study area is relatively unpolluted. The distribution of trace metals (Cu, Cr, Cd, Ni, Pb, Hg) in sediments throughout the catchment area showed selective “trapping” of certain elements behind the two artificial dams (Thissavros and Platanovrisi dams) in the watershed and a sudden reduction downstream (83% for Cd, 81% for Cr, 94% for Cu, 90% for Ni, 86% for Hg and 33% for Pb). Marked sediment particle separation is observed at the upstream dam (Thissavros), where coarse material including sand is trapped (coarse fraction 12.9–49.3%). Fine-grained material (<63 μm) is trapped behind the Platanovrisi dam (68.1%), and the reservoir showed elevated metal concentrations, especially for Cu and Cd (16.3 and 0.5 μg/g, respectively). Lead exhibited a homogenous distribution throughout the watershed (20.1–32.3 μg/g). All other trace metals (Cu, Cr, Cd, Ni and Hg) decline sharply downstream of the dam complex. In the delta system, nearshore sediments consist of shallow deposits in the vicinity of river mouth and are enriched in Cr (4.4–53.0 μg/g) and Ni (2.6–44.3 μg/g), while the further offshore and slightly deeper (20–40 m) sediments illustrate elevated Hg (0–0.07 μg/g), Cd (0.09–0.18 μg/g), Cu (11.5–18.3 μg/g) and Ni (38–54.5 μg/g).     

喜马拉雅中段冰湖终碛坝体溃决过程实验研究

终碛坝广泛分布于世界各地的高山和极高山区。为了探究终碛坝的溃决过程,了解溃口的演变特征,文章以嘉龙错终碛坝的原位实验,模拟了终碛湖漫顶溃决过程。通过分析实验结果发现:（1）根据观察,将终碛坝溃决过程划分为坝体下游坡面冲刷、“溯源侵蚀”、出水口下切和溃口拓宽四个阶段。（2）上游湖区崩塌体激发的涌浪会造成溃口内的瞬时流量增加数倍,从而使得在有涌浪和无涌浪的条件下,“溯源侵蚀”过程出现陡坎和斜坡两种下切型。（3）通过分析溃口下切侵蚀过程,发现溃口的下切侵蚀发展过程主要受到坝体孔隙比和细粒含量的影响,并且溃口中点侵蚀率与水流剪应力存在一定的线性关系,符合线性侵蚀模型。通过分析发现,嘉龙错终碛坝的侵蚀系数为0.051,临界启动应力为237.64 Pa。与堰塞坝相比,可侵蚀系数比更小,而临界启动应力更大。     

考虑不同泄流槽方案的堰塞坝溃决机理分析——以唐家山堰塞坝为例

近年来,频发的地质构造活动和极端气候灾害诱发了大量堰塞坝,严重威胁上下游群众的生命财产安全。开挖泄流槽是最常用降低堰塞坝溃决风险的措施,由于时间非常急迫、交通极度瘫痪,其开挖量非常有限,因此如何利用有限的开挖量将溃坝风险降低至最小是亟待解决的问题。本文基于水土耦合冲刷机理,提出了考虑不同泄流槽方案的堰塞坝溃决机理分析方法,并应用于唐家山堰塞坝。该方法根据水力学参数和坝体抗冲刷性参数动态计算瞬时坝体冲刷率,进而分析泄流槽对溃决全过程的影响,从而自动获取最优的泄流槽设计方案。将此方法应用于唐家山堰塞坝案例发现:唐家山堰塞坝泄流槽最优设计时溃坝洪峰流量为1700m3·s-1,小于实际峰值流量6500m3·s-1,主要是因为增大泄流槽的纵坡率,显著增强溃坝前的冲刷并形成双洪峰,从而有效降低了溃决峰值流量。由于复合槽相对较小的水力半径限制了溃坝前的冲刷,使得临溃时水位较高,因此溃坝峰值流量比单槽大,溃坝风险降低效果不如单槽。     

Analysis of fundamental physical factors influencing channel bank erosion: results for contrasting catchments in England and Wales

Channel bank erosion processes are controlled by numerous factors and as such are both temporally and spatially variable. The significance of channel bank erosion to the sediment budget is difficult to quantify without extensive fieldwork/data analysis. In this study, the importance of key physical factors controlling channel bank erosion, including channel slope, upstream catchment area, channel confinement, and sinuosity, was explored using regression analysis. The resulting analysis can be used in practical studies to provide a first approximation of bank erosion rates (in catchments similar to those investigated). A data set of channel bank erosion rates covering eight contrasting river catchments across England and Wales, over a time period of up to 150 years, was created using a modified GIS methodology. The best predictors were found to be upstream area, channel confinement, and sinuosity with respect to dimensionless width-averaged retreat rates (m m^?1 yr^?1). Notwithstanding these relationships, the results highlight the variability of the magnitude of sediment production by channel bank erosion both within and between catchments.     

Sedimentation in bottomland hardwoods downstream of an east Texas dam

Dams and reservoirs are often efficient sediment traps, and conventional wisdom holds that fluvial sediment supplies are reduced well downstream. However, there are reasons to question the extent to which fluvial and alluvial sediment supplies are reduced more than a few kilometers downstream of dams. Sedimentation in bottomlands of Loco Bayou, east Texas, was investigated at a site less than 16 km downstream of Loco Dam and Lake Nacogdoches, which controls 86% of the 265-km² drainage area. Turbidity levels are generally as high or higher than those on Loco Bayou upstream of the lake. Sedimentation rates on the lower floodplain since the dam was completed are 11 mm year^-1 or more. This rate is high enough to suggest that the dam has no effect on sediment supplies 16 km downstream. The spatial pattern of sedimentation and the vegetation distribution suggest that the elevation and frequency of flooding, not fluvial sediment availability, are the critical factors in determining sediment supplies to these floodplains.     

河流堰塞的地貌响应

堰塞作为一种极端地表过程,深刻影响着河流地貌的变化,特别是河流纵剖面的变化。其对河流纵剖面的影响主要体现在两方面:一方面,堰塞坝将抬高局地的侵蚀基准面,阻碍了上游河道侵蚀,形成河流裂点;另一方面,堰塞坝溃决往往形成大型/巨型洪水,造成下游河道和岸坡的剧烈侵蚀。稳定的堰塞坝形成后,在1~105 a的时间尺度上对河流裂点的发育以及河流纵剖面变化上甚至会超过构造、气候和岩性作用,占据主导。本文在简要概述堰塞地貌相关概念的基础上,介绍了部分河流堰塞的研究方法和案例,以及河流堰塞的发育过程和研究意义。目前多仅从堰塞坝与河流纵剖面的空间关系的相关性来论证其地貌响应,并且发现一些堰塞坝与河流纵剖面的相关性,但是也有一些古堰塞坝对现代河流纵剖面的影响并不显著,原因可能与堰塞坝规模、溃决洪水次数、堵江的持续时间和距今年代的不同有关,目前还缺乏深入研究。     

Bridge construction and river channel morphology—A comprehensive study of flow behavior and sediment size alteration of the River Chel,India

Human has always modified its surroundings for better adaptation which include the building of urban landscapes, dams, and engineering constructions like bridges and other infrastructures. It has been unveiled from the study of River Chel that when the rail bridge was constructed before 1913, the natural width of the river was nevertheless maintained but the problem was initiated and after the road bridge construction in 1970s when the river width was minimized,the morphological changes started in a large scale. Such modifications have affected nature as well as human communities both positively and negatively. This paper will try to portray the evolution of channel shifting and changes of sediment size regime that the River Chel is experiencing due to the construction of bridges across the middle part of its course. So, this study is concerned about the changes in channel pathway since last 100 years and field-based hydrological parameters and sediment size analysis reveals some distinct changes in the channel planform with alteration of sediment size regime. Simulation of the hydraulic modeling in HEC-RAS specifies the probable affected area with bridges and without bridges in both upstream and downstream of the river course. Hence, the study unveils the alteration of the river hydrology and sediment size caused due to anthropogenic effects and impact of such flow analysis has been evaluated through the bridge scour calculation by CUS method. The discussion concludes that the Odlabari road bridge construction (after 1970) has modified the normal hydrological behavior of the river with a higher probability of bank erosion in downstream and floods in upstream resulting river bottleneck condition.     

Monsoonal changes inferred from the Middle to Late Holocene sediments off Landfall Island,North Andaman

In this paper, sediment texture, clay mineral composition, and δ ¹⁸O data on Globigerinoides ruber of a sediment core collected from a water depth of 250 m near Landfall Island, Bay of Bengal, is presented to understand paleoenvironmental shifts during the Mid–Late Holocene period. From the sediment core, five organic carbon-rich samples were radiocarbon dated and the reservoir-corrected ages range from 6,078 to 1,658 years BP. The marine sediment core is represented dominantly by clayey silt with incursions of coarser components that occur around 6,000, 5,400, and 3,400 years BP. The sedimentation of the coarser detritus is due to strengthened southwest monsoon (SWM) since 6,500 to 5,400 years BP. Clay minerals are represented by smectite, illite, kaolinite, and chlorite in varying amounts. High kaolinite content and K/C ratio indicate intense SWM and strong bedrock weathering from the hinterland (~6,500–5,400 years BP). Incidence of smectite (48.82 to 25.09 %) and chlorite/illite (C/I) ratio (0.56 to 0.28) indicate an overall weakened southwest monsoon since 6,000 to 2,000 years BP with a brief incursion of extremely reduced SWM around 4,400 to 4,200 years BP. This is corroborated with the oxygen isotope on G. ruber that reveals a significant shift in the isotopic values ~4,300 years BP (?3.39?‰), indicating weakening in SWM. Subsequently, fluctuations in the intensity of SWM are observed since 2,000 years to present.