塔里木河植被根系对河岸冲刷特性影响的现场试验

为研究现场条件下植被根系对河岸冲刷特性的定量影响,设计制作了冲刷试验装置,在塔里木河干流上游选取了4个典型位置,开展了胡杨、芦苇、红柳和骆驼刺4种典型根系的18组土体起动切应力和冲刷系数现场试验。试验结果表明：(1)起动切应力随根系含量增大基本呈幂函数关系增大,且相同根系含量对应起动切应力红柳最大,胡杨和骆驼刺次之,芦苇最小;分析了单位根系含量变化对起动切应力的影响效果,其顺序为骆驼刺>胡杨>红柳>芦苇。(2)建立了起动切应力与根系体积密度的定量关系式;根系的存在可以减小土体冲刷系数,其值随根系含量增大基本呈幂函数关系减小,且单位根系含量增加对冲刷系数的减小效果次序为骆驼刺>胡杨>红柳>芦苇。(3)建立了有、无根系影响的冲刷系数与起动切应力幂函数关系式,并与已有试验结果进行了对比分析;本文试验结果与现有研究结果规律一致,但对应的系数和指数均有所差异。研究成果能为塔里木河植被护坡和河岸冲刷过程计算提供参考依据。     

下荆江二元结构河岸土体特性及崩岸机理

三峡水库蓄水后下荆江河段河床冲刷下切, 局部河段崩岸险情频繁发生。为研究下荆江二元结构河岸的土体特性及崩岸机理,结合近期该河段崩岸情况,现场查勘了6个崩岸点,并对河岸土体进行了室内土工试验。试验结果表明下荆江河岸土体的垂向组成具有典型的二元结构特征:下部非粘性土（沙土）层较厚,上部粘性土（低液限粘土）层较薄且松散。以河岸崩塌过程分析为基础,提出了二元结构河岸发生绕轴崩塌时上部土层稳定性的计算方法。结合近岸水动力条件计算及土工试验结果,定量分析了二元结构河岸的崩塌机理及其影响因素:① 下部沙土层的起动流速比近岸流速小得多,故该土层容易受水流冲刷;②上部粘性土层崩塌前抗冲强度很大,但多为低液限粘土且相对松散,崩塌后堆积在岸边容易分解并被水流带走;③ 河岸稳定安全系数在一个水文年内呈周期性变化,落水期安全系数最小,故容易引发崩岸,该计算结果与近期崩岸实际统计结果一致。     

黄河三角洲粉质土海床临界起动切应力影响因素研究

在黄河口潮滩和室内环形土槽模拟新沉积的海床土体,观测固结过程中沉积物临界起动切应力的变化及对应的土体重度、含水率、贯入阻力、剪切强度指标值的变化,分析黄河三角洲粉质类土临界起动切应力的影响因素与变化规律。研究发现：黄河三角洲粉土的临界起动切应力与重度、贯入阻力和抗剪强度之间呈正相关关系,而与含水率呈负相关关系,并且与干密度、抗剪强度之间具有非常好的定量关系。将黄河三角洲粉土临界侵蚀切应力的试验结果与其他学者建立的计算公式进行了比较,发现在固结开始试验与计算结果间的吻合性较好,随着固结时间差异性变大,表现出黄河口粉质土的独特性。     

考虑起动概率的无粘性均匀沙起动条件

根据无粘性均匀沙起动的力学条件,在已有研究成果基础上,推导出泥沙起动概率与水流条件之间的关系式,得到考虑起动概率的起动切应力及起动流速公式,并将起动条件与起动标准联系分析.通过对现有几个典型无粘性均匀沙起动条件公式分析表明,本文公式较为合理.     

土体渗透稳定性判定准则

土体的渗透稳定性是指在渗流条件下宽级配土体内粗颗粒阻止细颗粒流失的能力,土体的渗透稳定性受几何条件、水力条件和物理条件的影响。从几何条件出发,通过对收集的167种土的室内渗透侵蚀试验结果的分析,基于对土体渗透稳定性控制变量地研究,将土分成良好级配土和间断级配土两大类;基于细粒（小于0.063 mm）含量的不同,将每类土又细分为3类,针对不同细类土提出了不同的渗透稳定性几何判定准则。从水力条件出发,研究了应力状态对土体渗透侵蚀起动及破坏水力梯度的影响。试验结果表明,起动水力梯度和破坏水力梯度都随着围压的增大而增大,是由于增大围压使得颗粒间的摩擦力增大的结果。     

细颗粒流失对砂砾石土本构关系的影响研究

采用悬挂式防渗墙的坝基砂砾石土中可能发生潜蚀(suffusion)侵蚀而流失细颗粒,导致土体的强度和变形模量降低,从而对坝基防渗墙及上部坝体造成不利影响。定量描述细颗粒流失量对砂砾石土应力-应变关系影响的模型是定量评估这种影响的基础之一。提出了研究这种本构模型的试验方案,进行了三轴和侧限压缩试验研究了颗粒流失量对应力-应变关系的影响。颗粒流失未改变土体的应力-应变特性,但降低了土体的强度和变形模量。通过建立模型参数与流失量的关系,可以用同一个本构模型描述流失量时空变化的砂砾石土本构关系。基于邓肯E-B模型,建立了模型参数与颗粒流失量之间的定量关系表达式,从而实现了颗粒流失对应力-应变关系影响的定量描述方法。     

黄土陡坡降雨冲刷试验及其三维颗粒流流-固耦合模拟

针对坡角为70°的黄土边坡,进行2.7 mm/min降雨强度下的室内坡面冲刷试验。根据边坡冲刷破坏的过程特征,将边坡侵蚀方式演变过程归纳为试验初期的片蚀、中期的细沟侵蚀、到后期的切沟侵蚀和坍塌。试验中坡顶处侵蚀强度大于其他部位,当坡面形成上下贯通的切沟之后,水流开始掏蚀沟槽底部土体,随着冲刷的持续,切沟两侧土体强度降低,坡顶土体发生坍塌。以此为基础利用三维颗粒流软件PFC3D对边坡降雨冲刷过程进行流-固耦合模拟,在模拟颗粒大变形的同时得到颗粒运动轨迹、孔隙率、流体单元内流速等重要参数,这些参数的定量变化过程反映了降雨过程中边坡遭受侵蚀程度及水流侵蚀能力的分布规律:坡顶处侵蚀最为强烈、水流侵蚀能力最强,且两者随高度降低呈减小趋势,与室内试验结果一致。     

淤泥质浅滩泥沙临界起动切应力剖面确定

为了确定淤泥质浅滩泥沙的临界起动切应力垂线剖面,采用音叉密度计在淤泥质连云港徐圩浅滩进行了定点密度垂线分布测量,并针对该海域的泥沙利用长水槽和环形槽开展了泥沙起动室内试验。淤泥密度现场结果表明,浅滩泥沙密度与深度满足对数型关系;室内试验得出密度1 050~1 400 kg/m³的泥沙临界起动切应力值为0.1~1.0 Pa,泥沙临界切应力与密度呈指数关系;进而确定了临界起动切应力与深度的关系即临界起动切应力剖面,该剖面关系式可供数学模型模拟淤泥质浅滩的泥沙起动过程参考。     

等应力增量比路径下接触面应变软化模型

等应力增量比路径单剪试验条件下,部分土体与结构接触面表现出明显的应变软化和剪胀特性。基于广义位势理论,将土与结构的接触面问题看作应力空间上的二维问题,势函数取法向应力和切向应力,用塑性状态方程取代传统的屈服面,建立等应力增量比路径条件下的土与结构接触面应力-应变软化模型。采用指数函数,拟合单向压缩试验中法向应力与法向应变的关系,采用复合指数函数,拟合应力比与切向应变的关系,采用另一复合指数函数,拟合法向剪胀分量与切向应变的关系。通过对拟合函数进行微分,确定模型中待定系数的求解方法。结合试验结果对模型进行验证,模型拟合效果良好,具有一定的合理性。     

应力状态对粗颗粒土静止侧压力系数影响试验研究

土体静止侧压力系数K0随应力状态的变化规律对深入把握其力学性质非常重要。然而,由于适用于粗粒土K0试验仪器及方法较少,对粗粒土K0随应力状态变化规律的研究几乎是空白的。为研究加载和卸载过程中粗颗粒土静止侧压力系数K0随应力状态的变化规律,利用新近研制的大型K0试验仪对某砂卵砾石料和堆石料进行了不同应力状态下的K0试验。K0试验结果表明：颗粒形态、应力状态、级配都对粗粒土K0有一定影响,其中应力状态影响最为明显;加、卸载时粗粒土K0随竖向应力增大呈减小的趋势。通过对K0试验结果进行深入分析,提出了粗颗粒土在加载过程中K0与竖向应力的关系式,并进行了验证。在此式基础上,基于前人总结所得K0与OCR的关系式,总结了一个反映任意固结状态下粗颗粒土K0随应力状态变化的关系式。     

Prediction of river bank erosion and protection works in a reach of Chenab River,Pakistan

The impacts of floods on river bank erosion are generally significant in the alluvial river reaches. This paper presents the prediction of the river bank erosion along the right bank in the reach of Chenab River (starting from downstream of Marala Barrage) where excessive erosion had been reported. The bank erosion is predicted due to flow/flood events of 2010 by coupling the output from the two-dimensional numerical model to the excess shear stress approach. The predicted bank erosion was compared with the one estimated from Landsat images. The Landsat ETM+ images were processed in the ArcGIS software to assess the external bank erosion. The results show that the excess shear stress approach underpredicts the bank erosion. Therefore, the erodibility coefficient was modified by forcing the best agreement between predicted and estimated (i.e., from Landsat images) bank erosion which was used for further analysis. The results reveal that coupling the output from the numerical model to the excess shear stress approach (by modifying the erodibility coefficient) predicts the river bank erosion with a reasonable level of accuracy, thus helpful to identify the locations for the protection works. The predicted river bank erosion presents good coefficient of determination (R²) of 0.82 when compared with the estimated bank erosion from Landsat images. The findings of the present study will help to implement the river protection works at the identified locations in the selected reach of River Chenab and will also act as a guideline for similar river reaches.     

弯道凹岸不同部位崩塌体对近岸水流结构影响的试验研究

二元结构河岸失稳后,上部河岸发生崩塌并堆积在近岸,从而改变近岸水流结构并影响河岸二次侵蚀过程。为了探索弯道凹岸不同部位的崩塌体对近岸水流结构的影响,以长江中游石首河段为背景,开展概化模型试验,采用多普勒流速仪（ADV）精细测量崩塌体附近三维水流流速,并对比分析有无崩塌体时水流及壁面剪切力的变化规律。结果表明:位于顶冲点上游的崩塌体,使纵向流速最大区域远离凹岸,从而使崩塌体与河岸坡脚之间区域内的平均壁面剪切力减小;而位于顶冲点下游的崩塌体,使纵向流速最大区域靠近凹岸,从而使崩塌体与河岸坡脚之间区域内的平均壁面剪切力增大。研究成果丰富了对二元结构河岸侵蚀过程的认识,有利于河岸侵蚀模型的发展与完善。     

植被作用下土壤抗剪强度和径流侵蚀力的耦合效应

利用野外径流小区动态监测和人工模拟降雨试验,阐明了草被和灌木的减流减沙效应,从力学层面揭示了坡面侵蚀产沙的过程机理。结果表明,与裸地相比,野外坡面草地和灌木地径流量分别减少28.1%~56.5%和85.7%~100%、产沙量分别减少84.9%~90.7%和98.5%~100%;在人工模拟降雨强度下,草地和灌木地径流量分别减少51.9%~90.9%和61.7%~80.6%、产沙量分别减少93.6%~99.2%和95.5%~99.2%;植被具有明显的增强土壤抗剪强度的作用,不同植被坡面抗剪强度与剪切面上的法向压力成正比,且符合库仑定律;不同下垫面条件下土壤黏聚力与坡面径流量和侵蚀产沙量呈显著的负相关关系,随着黏聚力的增大,径流量和侵蚀产沙量呈下降趋势;草地和灌木地坡面侵蚀临界径流切应力分别为裸地的2.64~3.16倍和2.44~3.18倍,建立了不同被覆坡面临界径流切应力与土壤抗剪强度和黏聚力的关系。研究结果对定量评价植被减蚀作用和深化土壤侵蚀力学过程有一定的参考意义。     

A comparative analysis of interface erosion tests

Interface erosion is one of the main phenomena in dams, dikes and their foundations which may increase their failure risk. In laboratories, the jet erosion test (JET) and the hole erosion test (HET) are commonly used for the evaluation of the sensibility of interface erosion of fine soils. The results are interpreted by two distinct methods that are valid for one test only. A new energy analysis of the tests is developed, relating the total eroded mass to the dissipated fluid energy, and a new erosion resistance index is proposed. Seven naturally occurring fine-grained soils, covering a large range of erodibility, are compacted with the Proctor protocol, and they are tested with the two devices. It was shown that by using the commonly used methods, the values of the erosion coefficient are systematically higher with the JET than with the HET and the HET critical shear stress is about fifty times higher than the JET critical shear stress. Thus, the relative soil classifications yielded by the two erodimeters are not exactly the same. Based on energy analysis, values of erosion resistance index are roughly the same for each tested soil with the two apparatuses and a single classification of soil erodibility is obtained.     

凹岸崩塌体对急弯河道水力特性的影响

二元结构河岸下部非黏性土层持续冲刷侵蚀,上部黏性土体崩塌并堆积于凹岸坡脚,改变弯道水流结构并影响河岸二次侵蚀过程。为探究崩塌体对急弯河道水力特性的影响,以荆江石首河段为背景,建立弯道三维水流数学模型,模拟不同崩塌体尺寸下的弯道水流结构,并对比分析壁面剪切力的变化规律。结果表明：(1)崩塌体堆积于凹岸坡脚驱离水流动力轴线,减小凹岸次环流强度并改变环流方向。(2)崩塌体对弯道不同区域壁面剪切力的影响不同,其中,床面及凸岸坡面的平均壁面剪切力增大;凹岸坡面平均壁面剪切力在崩塌体头部及上游区域减小,在崩塌体尾部区域增大。(3)崩塌体越大,剪切力变化幅度越大,剪切力相对变化幅度为崩塌体截面积变化幅度的2%～10%。研究成果可为河道整治及岸线规划利用提供技术支撑。     

长江荆江河段滩槽演变与航道水深资源提升关系

长江是中国的“黄金水道”,通过系统性航道整治和疏浚维护,长江荆江河段航道水深已由2002年的2.9 m提升至2020年的3.5～3.8 m,但仍低于上游三峡大坝库区(4.5 m)及下游河段(4.5～6.0 m),航道水深与上下游不衔接且制约长江航道综合效益发挥。为了适应上下游航道水深,需提升荆江河段航道尺度,亟需明确航道水深资源、碍航特征与河道演变等关系。以长江荆江河段为对象,分析1960—2020年水沙及地形等资料,开展长江中游荆江河段滩槽演变与航道水深资源提升关系研究。研究表明：三峡工程运行后荆江河段以枯水河槽冲刷为主,冲刷量占全部冲刷量的90.97%,江心洲和边滩面积减少18.3%,其中江心洲、边滩面积减幅分别为9.4%和24.9%;在河床冲刷与航道整治工程实施条件下,以4.5 m×200 m(水深×宽度)进行航道尺度核查,荆江河段碍航总长度占全河段5.3%;4.5 m水深碍航特征包括砂卵石河段枯水位下降幅度高于河槽下切深度引起航道水深不足,沙质河床内弯曲河段“凸岸侧边滩冲刷、凹岸侧深槽淤积”引起滩槽形态及航道边界不稳定,以及分汊河段内洲滩萎缩与汊道间不均衡冲刷引起枯水航路不稳定...     

Field investigation of erosion resistance of common grass species for soil bioengineering in Hong Kong

Grass cover is considered as a sustainable means of controlling soil erosion and enhancing durability of soil slopes. A number of grass species are commonly available for soil bioengineering in Hong Kong, but their capacities to control soil erosion have not been characterized quantitatively. The main objectives of this paper are to study the influence of soil density on characteristics of grass roots, to measure the erodibility parameters of the root-permeated soils at two growth stages, and to select the proper Hong Kong grass species that effectively control soil erosion. Three types of Hong Kong turf grass including Cynodon Dactylon, Paspalum Notatum, and Zoysia Matrella were planted on three soil grounds with degrees of compaction of 80, 90, and 100 %, respectively. The featural parameters of grass roots on each compacted ground, including root mass density, root volume density, and root depth, were measured in two growth stages. A jet index apparatus was applied to measure two erodibility properties (i.e., coefficient of erodibility and critical shear stress) of these vegetated soils in the two test stages. Cynodon Dactylon and Zoysia Matrella have higher root mass density values than Paspalum Notatum does, and reduce the susceptibility of soil erosion more effectively. Therefore, the two grass species are suggested for soil bioengineering in Hong Kong.     

Quantification of riverbank erosion and application in risk analysis

The estimation of river bank erosion requires the knowledge of both local hydrodynamic and erodibility characteristics. Models exist in literature that allow the estimation of the river bank shear stress, the fundamental parameter in evaluating the retreat given the discharge flow and the geometry of the river channel. In this study, two hydrodynamic models (1-D and 2-D) were combined with three shear stress models in order to obtain an estimation of the retreat on a study case on the river Cecina in Tuscany, Central Italy. A calibration of the models was performed based on observations from aerial photos of the region over a period of 10 years (1994–2004), and the results of the different combinations of the models are discussed and compared. A framework was developed for the risk analysis of land loss due to bank erosion based on the analyses of discharge flow time series and an excess shear stress erosion model. An application to the study case is provided by using the results of fluvial erosion modelling.     

粘性土及粘性土夹沙的起动规律研究

从水流脉动的机理出发,以粘性土的宾汉应力作参数,建立了粘性土的临界起动剪切力公式.根据一系列实验结果,分析了不同情况下粘性土夹沙的起动条件,归纳出统一的表达式.此外,还对上部水流为浑水的情况下,粘性土及粘性土夹沙的起动条件作了定性分析.