河道抗冲刷土工加筋草皮生态护坡技术应用研究

项目开展了加筋草皮护坡抗冲刷性能研究,探讨了不同条件下土工加筋草皮破坏机制及破坏过程,分析了土工材料与加筋技术对抗冲刷性能的影响;通过数值模拟与典型区域现场试验,揭示了不同流速条件下两种典型土工加筋草皮的侵蚀程度和侵蚀速率的变化规律,验证了其对边坡稳定性的提升效果.     

海堤背水坡加筋草皮抗冲蚀能力试验研究

在台风和寒潮等极端事件条件下海堤背水坡易受到越浪和溢流的侵蚀作用,海堤背水坡的保护越来越受到重视;同时人们对海岸工程也提出了更多的生态和景观要求。加筋草皮作为一种新兴的护坡方式,发挥了土工织物防冲固草和草根系固土的作用,既提高了植被护坡的抗水力侵蚀能力,也满足生态功能。为研究背水坡加筋草皮的护坡性能,基于室内水槽试验,探究了不同加筋草皮方式在背水坡遭受高速持续水流冲刷和越浪水体斜向冲刷两种作用下抗侵蚀性能。试验结果表明:不同方式的加筋草皮抗侵蚀有差异,抗侵蚀性能由强到弱为三维土工网垫与土工格栅联合加筋、三维土工网垫＞土工格栅＞天然草皮。相同流量下卷破波斜向冲刷作用下较持续水流正向冲刷破坏更为强烈。     

无芒雀麦草皮护坡抗水流侵蚀试验研究

为了分析无芒雀麦草皮护坡的抗水流侵蚀特性,基于床面切应力对植被曳引作用是造成草皮护坡破坏的直接原因的原理,设计了一套流速范围在1.5～6.0m/s的装配式恒定有压流冲刷装置。采用此试验装置,得出了无芒雀麦草皮护坡在连续稳定水流冲刷作用下发生侵蚀破坏的关键水力学参数,无芒雀麦草皮的曼宁糙率系数为0.019,最大抗冲流速可达5.0m/s,其破坏等级为Ⅱ级。     

植被护坡水流冲刷稳定性试验研究

从植被护坡水流冲刷稳定性设计的角度出发,对紫羊茅草皮及早熟禾草皮分别从坡面流与有压流两个方面进行了水流冲刷稳定性试验。试验结果表明:坡面流间歇冲刷8次,瞬时流速达6.0 m/s,累积冲刷时间120 s,紫羊茅草皮可发生IV级的冲蚀破坏。早熟禾草皮在5.0 m/s的流速下,冲刷1 h后,其草皮层出现最高级别的破坏,测定其发生破坏时的床面剪应力为103.13 Pa,摩阻流速为0.32 m/s。     

坡面沟蚀及其分形特性试验研究

随着降雨击溅及坡面径流的持续冲刷,坡面侵蚀将由面蚀向沟蚀转化,并形成复杂的坡面微地形,坡面微地形又将改变坡面侵蚀方式。通过降雨及直接径流冲刷条件下的坡面侵蚀试验,研究了坡面侵蚀过程和冲刷流速(量)、侵蚀率与坡面微地形分形特征之间的关系,结果表明:侵蚀坡面地形分维值与冲刷流量和坡度存在明显的相关关系,把流速和坡面侵蚀率作为坡面地形演变的直接指标,均存在分维值随其先增大后减小的规律。     

土工垫加筋生态护坡草皮抗剪性能试验研究

土工垫加筋草皮具有减小浅层滑坡和表面侵蚀的作用,为探究土工垫加筋草皮抗剪强度特性,在室内培育土工垫加筋生态草皮的基础上,对土工垫加筋草皮原状样进行了室内直剪试验,获得了土工垫加筋草皮剪切应力与位移关系曲线、抗剪强度参数等结果,并通过对比试验分析了土工垫和植被根系加筋量、土体含水量等对其抗剪性能的影响。试验表明:土工垫和根系共同加筋作用不会改变应力与位移关系曲线变化特征,但可显著提高加筋草皮的抗剪强度,在本次试验中与素土相比加筋后强度最大增幅为164.7%,强度增幅受法向应力水平影响显著,随着法向应力的增大,强度增幅逐渐减小;基于摩尔-库伦强度准则,获得了土工垫加筋草皮的黏聚力和摩擦角,结果显示加筋增加了草皮的黏聚力,但对内摩擦角几乎没有影响;随着加筋量的增大,土工垫加筋草皮的抗剪强度均近似按指数形式增大,加筋量越高土体抗剪强度增量也越大;低含水率试样对应的抗剪强度较大,含水率增大时,由于基质吸力降低,抗剪强度也会降低。研究成果对加筋草皮的推广应用具有参考价值。     

部分植被明渠水流流动特性三维数值模拟研究

利用在sigma坐标下含有植被明渠水流三维数学模型,湍流模型采用鲁俊修正的Smagorinsky模型,用分步算法和有限差分离散求解控制方程,数值模拟研究了部分植被化明渠水流流动特性。分析了植被区域内流速和切应力分布特征以及明渠水流的流量和植被密度的关系,数值计算表明植被密度增加,明渠水流的流量减小,但随着植被密度增加超过一定定值,明渠水流的流量不变。     

水流作用下沙漠风积沙冲刷速率二维模型探析

根据集中水流下沙漠风积沙的冲刷破坏过程和机理,以表面水流单宽流量来模拟水流条件,下游端部的初始落差来模拟沙漠土的跌坎地形,下游端部跌坎位置随时间的变化率,来确定溯源冲刷速度和表面剥蚀速率。对沙漠风积沙冲刷速率的影响因素进行分析,建立了集中水流作用下沙漠风积沙冲刷速率二维模型。     

植草型生态护岸水力特性试验研究

为研究不同水流条件下植草型生态护岸上的植被表面曼宁糙率系数和Darcy-Weisbach阻力系数的变化规律,在室内大流速水槽内模拟河道径流对草皮护坡进行了冲刷模型试验。试验结果显示:(1)试验所选用的狗芽根(Cynodon dactylon)及高羊茅(Festuca arundinacea)的曼宁糙率值在0.025～0.090范围内;(2)植被表面曼宁糙率系数随流速的增大而减小,当流速超过4 m/s时,糙率值稳定在0.025～0.035之间,在某一稳定流量下,植被表面曼宁糙率值首先随冲刷历时的增加而递减,而后逐渐趋于稳定,并在护坡破坏时发生突增;(3)Darcy-Weisbach阻力系数不仅与植被的种类及生长状况有关,而且和植被的平均瞬时弯曲高度与水力半径的比值有关,随着水流流速及历时的增加阻力系数也随之减小。     

一种装配式有压流冲刷装置的研制

基于床面切应力对植被的曳引作用是造成草皮护坡破坏的直接原因的原理,研制了一套水流流速范围在1.5~6 m/s的装配式恒定有压流冲刷装置。试验测得有机玻璃水槽的糙率及人工培育早熟禾草皮的曼宁糙率在合理的范围内。采用此试验装置,得出了紫羊茅草皮护坡发生水流冲刷破坏时的关键水力学参数,床面切应力、摩阻流速,及冲刷历时等。     

Comparison of Direct and Indirect Boundary Shear Stress Measurements along Vegetated Streambanks

Estimates of boundary shear stress along vegetated streambanks are needed to predict streambank fluvial erosion. Because fluvial shear stress cannot be directly measured in the field, reliable estimation techniques using field instrumentation are needed. This study evaluated local bank shear stress estimation methods applicable to sloping, vegetated streambanks. Two reaches of a second order stream were modelled in a flume using a fixed‐bed Froude‐scale modelling technique. One reach was dominated by dense shrubs while the other reach was located in a mature forest. Direct measurements of local bank shear stress using a hot‐film anemometer were compared to estimates based on velocity measurements (logarithmic method, Reynolds stresses, and turbulent kinetic energy). For channels with no or widely spaced vegetation, the velocity‐based estimates underestimated the bank shear stress due to secondary flow contributions. For banks with dense vegetation, Reynolds stresses and turbulent kinetic energy estimates were statistically similar to direct measurements on average, but substantial error occurred when making point comparisons. Velocity‐based estimates generally over predicted bank stress in areas of high shear at the vegetation edge and underpredicted stress within dense vegetation. Ultimately, results suggest that none of tested techniques can be broadly applied to streambanks, and flow structure is critical in selecting the appropriate estimation technique. Copyright © 2016 John Wiley & Sons, Ltd.     

不同植被类型对堆积体坡面径流特性的影响

针对植被防护堆积体侵蚀动力机制的问题,通过野外模拟降雨试验,分析直根系和须根系植被对堆积体径流流速、水力和水动力参数的影响。结果表明:植被削减堆积体平均侵蚀速率达88.34%~92.88%,直根系消减平均流速效益为50.51%,须根系为21.32%~35.61%;裸坡和植被堆积体径流流型均属于层流(雷诺数<40),裸坡径流在降雨强度≤1.2 mm/min时处于急流态,直根系堆积体均处于缓流态,须根系堆积体在急流态和缓流态间变化。直根系对坡面径流的阻滞作用大于须根系;植被防护下堆积体侵蚀速率、水力和水动力参数与裸坡呈显著性差异(P<0.05),径流剪切力和径流功率可用于较好地刻画坡面侵蚀动态过程,呈显著线性关系(R²为0.63~0.96)。研究成果可为生产建设项目工程堆积体水土流失量预测模型的植被因子修订提供科学依据。     

FLOW STRUCTURE OF PARTLY VEGETATED OPEN-CHANNEL FLOWS WITH EELGRASS

Aquatic vegetation can influence the transport of sediment and contaminants by changing the mean velocity and turbulent flow structure in channels. It is important to understand the hydraulics of the flows over vegetation in order to manage fluvial processes. Experiments in an open-channel flume with natural vegetation were carried out to study the influence of vegetation on the flows. In a half channel with two different densities of vegetation, the flow velocity, Reynolds stresses, and turbulence intensities were measured using an Acoustic Doppler Velocimeter (ADV). We obtained velocity profiles in the lateral direction, Reynolds stresses in the vertical direction, and the flow transition between the vegetated and non-vegetated zones in different flow regimes. The results show that the streamwise velocity in the vegetated zone with higher density is almost entirely blocked. Reynolds stress distribution distinguishes with two different regions: inside and above the vegetation canopies. The turbulence intensities increase with increasing Reynolds number. The coherent vortices dominate the vertical transport of momentum and are advected clockwise between the vegetated zone and non-vegetated zone by secondary currents (a relatively minor flow superimposed on the primary flow, with significantly different speed and direction), generated by the anisotropy of the turbulence.     

HIGH-SPEED FLOW EROSION ON A NEW ROLLER COMPACTED CONCRETE DAM DURING CONSTRUCTION

A new roller compacted concrete dam of Fengman Hydropower Station is to be built in the toe of the old dam,which was identified as a dangerous dam.The new dam during construction would be influenced by the high-speed flow discharged from the old dam,which is an important problem to be considered for the first time in China,and which would affect the construction of the whole project.Therefore,a series of erosion experiments were conducted in this article.A high-speed flow erosion test apparatus was developed for the erosion experiments of the new dam materials.The maximum jet velocity goes up to 40 m/s and the section area of the nozzle is 0.0025 m2.In the process of experiments,the equipment shows a good performance.Erosive wear tests for two types of materials used in the new dam,a roller compacted concrete and a distorted concrete with four kinds of ages were carried out with the flow velocity in the range of 30 m/s-35 m/s.Erosion parameters and erosion laws for the two types of concretes with different ages were determined,and a general relationship between the erosion rate and the flow velocity is obtained as:,with the velocity exponent between 3.33 and 3.93.It is concluded that the erosion resistance of the distorted concrete is better than that of the roller compacted concrete and the mechanical properties of the concretes of over 14 d age are influenced slightly by the water impact.The test results might serve as a practical technique guide for the safety of this project during its construction in the flood season.     

Effects of Rigid Unsubmerged Vegetation on Flow Field Structure and Turbulent Kinetic Energy of Gradually Varied Flow

The influence of rigid unsubmerged vegetation on flow structure and turbulent kinetic energy of gradually varied flow are experimentally investigated in this research. Natural reed stems of different densities are employed to examine the effects of the rigid unsubmerged vegetation on the flow in rivers. The results reveal that the vegetation existence significantly changes the gradually varied flow state from type M1 to type M2 in the vegetation section. The traditional power law describing the vertical flow velocity profile is evidently invalid when the vegetation density becomes high. With the irregularity index proposed in the research, the irregularity of vertical flow velocity profile in vegetated reach can be exponentially described in relation to the vegetation density. Furthermore, the turbulent kinetic energy is found to increase and reach a maximum value near the end of the vegetation section that is a potential localized erosion area. The results of the research have significances in river ecological restoration applications utilizing aquatic vegetation. Copyright © 2014 John Wiley & Sons, Ltd.     

Submerged flexible vegetation impact on open channel flow velocity distribution: An analytical modelling study on drag and friction

In this paper,an analytical model that represents the stream wise velocity distribution for open channel flow with submerged flexible vegetation is studied.In the present vegetated flow modelling,the whole flow field has been separated into two layers vertically:a vegetated layer and a non-vegetated free-water layer.Within the vegetated layer,an analysis of the mechanisms affecting water flow through flexible vegetation has been conducted.In the non-vegetated layer,a modified log-law equation that represents the velocity profile varying with vegetation height has been investigated.Based on the studied analytical model,a sensitivity analysis has been conducted to assess the influences of the drag(C_D)and friction(C_f)coefficients on the flow velocity.The investigated ranges of C_D and Cf have also been compared to published values.The findings suggest that the C_D and Cf values are non-constant at different depths and vegetation densities,unlike the constant values commonly suggested in literature.This phenomenon is particularly clear for flows with flexible vegetation,which is characterised by large deflection.     

Hydrodynamics of longitudinally discontinuous,vertically double layered and partially covered rigid vegetation patches in open channel flow

The aim of this paper is to numerically investigate how the flow structures are affected through a longitudinally discontinuous and vertically two‐layered vegetation occupying half width of the channel, with steady flow rate and subcritical conditions. A three‐dimensional (3‐D) Reynolds stress turbulence model (RSM), incorporated by Computational Fluid Dynamics (CFD) code FLUENT, was first validated with the experimental data, and then used for simulation purpose. The results showed that the flow stream‐wise velocities within the gap regions are visibly slower than that in the vegetation patch regions. Along the cross section, the velocity in the vegetation region (VR) reduced significantly due to resistance offered by the vegetation, which affected the channel conveyance; as compared to the free (non‐vegetated) region. The flow instability in the lateral direction was triggered by the flow shear due to the presence of partly distributed vegetation, resulting in the formation of coherent vortices and exchange of momentum at the interface. The discharge percentage passing through the free region (FR) was found to be 144–525% larger than that passing through the VR. The flow resistance increased significantly with higher vegetation density, whereas it decreased when both the vegetation layers were submerged. Moreover, the flow characteristics profiles in large gaps were more stable than in small gaps. The turbulent kinetic energy (TKE) and turbulence intensity also increased significantly through the patch regions compared to that of the gap regions. The results indicated that the flow structures and the flow resistance are strongly influenced by partial and discontinuous vegetation.     

淹没柔性植物对明渠推移质运动影响的实验研究

由于水流中植物的存在,使推移质泥沙运动规律变得更为复杂,目前,含植物水流中推移质泥沙运动的研究成果较少。实验研究了含淹没柔性植物水流中,不同植物密度及水深对时均流速、脉动强度、泥沙起动流速以及推移质输沙率的影响。结果表明,在植物顶部以下,有植物时的时均流速明显小于无植物时的流速,而脉动强度大于无植物时的脉动强度;当水深相同时,随着植物密度的增大,泥沙起动流速呈逐渐减小的趋势,并与植物密度呈较好的三次方多项式相关关系;植物密度对推移质输沙率的影响并非呈单一变化规律,随着植物密度的增大,推移质输沙率呈先增大然后逐渐减小的趋势,当植物密度相同时,水深越小,泥沙起动流速越小,推移质输沙率越大。     

新疆夏特水电站水轮机抗磨措施研究

新疆夏特水电站所在克孜河中游含沙量高,是典型的多泥沙河流电站,高含沙量对电站水轮机过流部件的泥沙磨损影响很大。详细分析了电站泥沙的形成及输沙能力以及电站泥沙颗粒级配情况,对夏特电站可采用的水轮机进行了沙水流动的数值模拟和关键过流部件泥沙磨损试验,预估了水轮机的泥沙磨损情况,并提出了抗泥沙磨损的措施。研究结果为夏特水电站水轮机的抗磨设计和运行提供了重要参考依据。     

FLOW STRUCTURE AND SEDIMENT TRANSPORT WITH IMPACTS OF AQUATIC VEGETATION

Aquatic vegetation plays an important role in the flow structure of open channels and thus changes the fate and the transport of sediment. This article proposes a three-dimensional turbulence model by introducing vegetation density and drag force into the control equations of water flow in the presence of vegetation. The model was used to calculate the impacts of submerged vegetation on the vertical profiles of longitudinal flow velocities, the changes of the depth-averaged flow velocities in a compound channel with emergent vegetation in the floodplain, the removal of suspended sediment from the channels by emergent vegetation, and the bed changes around and in a vegetated island. Numerical investigations show that aquatic vegetation retards flow in the vegetation zone, reduces the sediment transport capacity, and contributes to erosion on both sides of the vegetated island. Calculated results agree well with experimental results.