Numerical Modeling of Breach Erosion of River Embankments

The process of breach erosion of river embankments depends on the interaction among flow, sediment transport, and the corresponding morphological changes. Levees often consist of noncohesive material with a wide range of grain sizes. The dam material is mainly eroded due to the transport capacity of the overtopping water. Both bed load and suspended load are of importance. For breach formation, the lateral erosion due to slope instabilities has a significant impact. A depth averaged, two-dimensional numerical model was developed to account for these processes. The sensitivity of the discharge through the breach related to different processes and material parameters was investigated and compared to experimental and field data. The results show that the most sensitive parameter of an erosion-based dike-breach simulation is the breach side-slope angle which determines the lateral erosion. The application of the described Model 2dMb to different embankment failures at the Elbe River illustrates its capability in simulating overtopping breaching.     

Bank-Attached Vanes for Bank Erosion Control and Restoration of River Meanders

The effectiveness of a novel approach of using vanes, installed at a low angle and attached to the bank, for bank protection and for the restoration of river meanders has been investigated in a laboratory study. Experiments were carried out in a large-scale meandering mobile-bed channel with graded sediment. The bed topography, three-dimensional flow pattern, and turbulence characteristics in the meandering channel with or without structures are analyzed. When a single or an array of such vanes is installed, the scour hole at the base of the outer bank is infilled and the thalweg is relocated toward the center of the river. The structures induce a secondary flow cell near the outer bank which counteracts the main spiral flow in the bend. In contrast to common spurs and bendway weirs, large-scale horizontal vortices are not generated behind the structures. Vanes which grade to the bed from bankfull level at the bank show better performance than low level ones, whereas multiple structures show positive effects as far downstream as the crossover section.     

Analytical Approach to Calculate Rate of Bank Erosion

Bank erosion consists of two processes: basal erosion due to fluvial hydraulic force and bank failure under the influence of gravity. Because bank resistance force varies with the degree of saturation of bank material, the probability of bank failure is the probability of the driving force of bank failure being greater than the bank resistance force. The degree of saturation of bank material increases with river stage; therefore, the frequency of bank failure is correlated to the frequency of flooding. Consequently, the rate of bank erosion is due to both basal erosion and bank failure, and bank failure is a probabilistic phenomenon. In this paper, for cohesive bank material experiencing planar bank failure, a deterministic approach was adopted for basal erosion analysis, whereas the probability of bank failure was included in the analysis of bank failure. A method for calculating the rate of bank erosion was derived that integrates both basal erosion and bank failure processes, and accounts for the effects of hydraulic force, bank geometry, bank material properties, and probability of bank failure.     

Case Study: Two-Dimensional Model Simulation of Channel Migration Processes in West Jordan River, Utah

The paper presents the application of a two-dimensional depth-averaged numerical model to simulate the lateral migration processes of a meandering reach in the West Jordan River in the state of Utah. A new bank erosion model was developed and then integrated with a depth-averaged two-dimensional hydrodynamic model. The rate of bank erosion is determined by bed degradation, lateral erosion, and bank failure. Because bank material in the West Jordan River is stratified with layers of cohesive and noncohesive materials, a specific bank erosion model was developed to consider stratified layers in the bank surface. This bank erosion model distinguishes itself from other models by relating bank erosion rate with not only flow but also sediment transport near the bank. Additionally, bank height, slope, and thickness of two layers in the bank surface were considered when calculating the rate of bank erosion. The developed model was then applied to simulate the processes of meandering migration in the study reach from 1981 to 1992. Historical real-time hydrographic data, as well as field survey data of channel geometry and bed and bank materials, were used as the input data. Simulated cross-sectional geometries after this 12-year period agreed with field measurements, and the R2 value for predicting thalweg elevation and bank shift are 0.881 and 0.706, respectively.     

基于API579准则对弯管腐蚀数值模拟的分析

为定量描述高压回注管道弯管处由于流体速度与方向的改变而引起的物理冲刷腐蚀,本文以川西北气矿的高压回注水管道为例,利用Fluent软件建立了垂直弯管冲刷腐蚀模型,并利用API579准则对相应弯管处的剩余强度进行评价和研究。结果表明,腐蚀速率最大区域发生在弯管段外壁。同时,根据数值模拟计算对各控制参数根据遗传算法进行优化处理。针对管道内壁结垢无法测量准确数据的情况,建立了一套相对完整的评价优化分析系统。     

Stream Bank Erosion: In Situ Flume Tests

A new technique for testing the erodibility of cohesive stream banks using an in situ flume is presented. The erosion rate is estimated from direct measurements of bed surface elevations by acoustic sensors. The sediment resuspension rate is obtained using data on sediment concentrations measured by optical backscatter sensors and from water samples. The bed-load contribution to the total erosion rate is evaluated from the conservation equation for sediments. Temporal patterns of erosion and resuspension rates are studied employing stepwise increments of bed-shear stress. The data show that bed load plays a significant role in cohesive bank erosion. The data analysis suggests that erosion and resuspension thresholds observed in experiments were very low or equal to zero. The data support the power type equation for the erosion and resuspension rates with bed-shear stress as the key factor. The data also highlights the potential importance of mud content and water content on erosion.     

River Bifurcation Analysis by Physical and Numerical Modeling

In the framework of a river regulation design of the Po River Delta (Northern Italy), a study on a large physical model of the bifurcation Po di Goro-Po di Venezia was conducted with the main objective of determining the discharge subdivision rate at the river node, in order to assess the inflow conditions in the Po di Goro River for flood risk analysis. In this context, a two-dimensional depth averaged numerical model was tested against measured values, with reference to the prototype. In this paper a comprehensive analysis and discussion of the results is reported in order to highlight the applicability of numerical models in comparison with physical ones in river engineering applications.     

Monitoring River Channel and Flume Surfaces with Digital Photogrammetry

This paper describes and illustrates a technique for high resolution monitoring of the surface morphology of water-worked sediments. The monitoring uses close-range digital photogrammetry. While photogrammetry is a long-established technique, more recent developments in digital photogrammetry allow application in fluvial research to be highly cost effective in both flume and natural river channel studies. Results are presented that involve two scales of laboratory flume: a smaller-scale application associated with sediment sorting processes in a straight channel; and a larger-scale application involving sediment transport and bed material feedbacks in a meandering channel subject to overbank flows. A preliminary assessment of data quality is undertaken with encouraging results. The precision of elevation estimates corresponds to the scale of the imagery acquired and hence may be controlled by design of the image acquisition process.     

数控电火花线切割机床加工编程浅析

八钢数控电火花线切割机床加工,是用计算机进行自动编程.现在比较常用的CAD/CAM软件有CAXA、Autop、Mastercam、Pro/e、UG、YH等,八钢股份公司维护中心线切割机床自动编程软件, 使用北京某公司自主开发的CAXA线切割软件.切割机床利用工具电极(钼丝)和工件两极之间脉冲放电时产生的电腐蚀现象对工件进行尺寸加工.     

Sediment Erosion Characteristics in the Anacostia River

Four in situ experiments on sediment erosion characteristics were conducted at the Anacostia River that runs through Washington, D.C. Supplemental erosion rate data were also obtained by carrying out five laboratory experiments using sediment samples collected at the field. In laboratory experiments, the sediment samples were mixed with tap water and placed in the flume to form beds for finding the difference in terms of erosion characteristics caused by different sediment composition among the five samples. This approach enables the finding of erosion characteristics for the entire tidal Anacostia River with limited resources. The in situ measured critical bed-shear stresses τcr for erosion at the water-sediment interface z = 0 varies from 0.03 to 0.08?Pa. Field results indicated that τcr(z) increases with the depth z and becomes more than 0.6 to 0.7?Pa with an erosion thickness of less than 1?cm. Sediment beds prepared at a laboratory appear having an upper limit on how much τcr(z) can be developed.     

Numerical Simulation of Longitudinal and Lateral Channel Deformations in the Braided Reach of the Lower Yellow River

Bank erosion frequently occurs in the Lower Yellow River (LYR), playing an important role in the evolution of this braided river. A two-dimensional (2D) composite model is developed herein that consists of a depth-averaged 2D flow and sediment transport submodel and a bank-erosion submodel. The model incorporates a new technique for updating bank geometry during either degradational or aggradational bed evolution, allowing the two submodels to be closely combined. Using the model, the fluvial processes in the braided reach of the LYR between Huayuankou and Laitongzhai are simulated, and the calculated results generally agree with the field measurements, including the water-surface elevation, variation of water-surface width, and variations of cross-sectional profiles. The calculated average water-surface elevation in the study reach was 0.09?m greater than the observed initial value, and the calculated mean bed elevation for six cross sections was 0.11?m lower than the observed value after 24 days. These errors are attributed to the large variability of flow and sediment transport processes. Sensitivity tests of three groups of parameters are conducted, and these groups of parameters are related to flow and sediment transport, bank erosion, and model application, respectively. Analysis results of parameter sensitivity tests indicate that bank erodibility coefficient and critical shear stress for bank material are sensitive to the simulated bank erosion process. The lateral erosion distance at Huayuankou will increase by 19% as the value of bank erodibility coefficient changes from 0.1 to 0.3, and it will decrease by 57% as the value of critical shear stress for bank increases from 0.6 to 1.2?N/m2. Limited changes of other parameters have relatively small effects on the simulated results for this reach, and the maximum change extent of calculated results is less than 5%. Because the process of sediment transport and bank erosion in the braided reach of the LYR is very complicated, further study is needed to verify the model.     

Direct Numerical Simulation of Microparticle Motion in Channel Flow with Thermophoresis

The motion of microparticles in a vertical flow channel driven by drag and thermophoretic forces was simulated using the direct numerical simulation method with the particles tracked using the Lagrangian method. The particle motions were analyzed for five sizes of particles (dp = 1, 2.5, 10, 20, and 100?μm) at three temperature differences (Δt = 0, 130, and 180°C). The results showed that the effect of thermophoresis on the deposition near the wall decreases with increasing particle diameter and can be neglected for particles with dp = 100?μm. Since the deposition rates of inhalable particles (PM10) increase dramatically as the thermophoresis force increases, especially for particles with dp = 1?μm, thermophoresis is an effective method for collecting inhalable particles.     

Three-Dimensional Numerical Simulation of Compound Channel Flows

A three-dimensional numerical study is presented for the calculation of turbulent flow in compound channels. The flow simulations are performed by solving the three-dimensional Reynolds-averaged continuity and Navier–Stokes equations with the k?ε turbulence model for steady-state flow. The flow equations are solved numerically with a general-purpose finite-volume code. The results are compared with the experimental data obtained from the UK Flood Channel Facility. The simulated distributions of primary velocity, bed shear stress, turbulent kinetic energy, and Reynolds stresses are used to investigate the accuracy of the model prediction. The results show that, using an estimated roughness height, the primary velocity distributions and the bed shear stress are predicted reasonably well for inbank flows in channels of high aspect ratio (width/depth ≥ 10) and for high overbank flows with values of the relative flow depth greater than 0.25.     

Numerical Model for Sediment Transport and Bed Degradation in the Yangtze River Channel Downstream of Three Gorges Reservoir

This paper presents a two-dimensional morphological model for unsteady flow and both suspended-load and bed-load transport of multiple grain size to simulate transport of graded sediments downstream from the Three Gorges Reservoir. The model system includes a hydrodynamic module and a sediment module. The hydrodynamic module is based on the depth-averaged shallow water equations in orthogonal curvilinear coordinates. The sediment module describing nonuniform sediment transport is developed to include nonequilibrium transport processes, bed deformation, and bed material sorting. The model was calibrated using field observations through application to a 63-km-long alluvial river channel on the middle Yangtze River in China. A total of 16 size groups and a loose layer method of three sublayers were considered for the transport of the nonuniform bed materials in a long-term simulation. Predictions are compared with preliminary results of field observations and factors affecting the reliability of the simulated results are discussed. The results may be helpful to the development of more accurate simulation models in the future.     

New Facility to Study River Abrasion Processes

This technical note presents a new facility which was constructed in order to study sediment and bedrock abrasion processes during fluvial transport. These processes exert an important control on long-term landscape evolution but they are still poorly understood and inadequately quantified. The proposed experimental device is an annular flume with four fluid injections coupled to a close water circuit and a powerful pump, in order to reach hydrodynamic conditions up to whose prevailing in mountain streams. Fluid surface geometry and visualization experiments with a high speed camera allow us to monitor hydrodynamic variables and sediment motion during the experiments. Despite the circular geometry of the flume, pebble trajectories are found to closely mimic the bedload behavior in straight flume. Based also on a direct comparison with pebble abrasion rates along a large Himalayan River, we hypothesize that our device simulates in a realistic way transport processes and consequently abrasion processes in mountain rivers.     

高炉炉衬侵蚀边界识别的数值模拟

利用非稳态热传导反问题(IHCP)数学模型研究了高炉监测热电偶数据与高炉炉衬侵蚀厚度的关系.求解此反问题采用了整个时间域上的空间距离步进迭代算法和离散软化过滤法.根据上海梅山钢铁厂3号高炉采集的热电偶数据,模拟计算得出高炉炉衬剩余厚度随时间的变化关系.模拟结果证明了边界识别算法的可行性及其广阔的应用前景.     

Numerical Modeling of Bed Evolution in Channel Bends

A two-dimensional numerical model is developed to predict the time variation of bed deformation in alluvial channel bends. In this model, the depth-averaged unsteady water flow equations along with the sediment continuity equation are solved by using the Beam and Warming alternating-direction implicit scheme. Unlike the present models based on Cartesian or cylindrical coordinate systems and steady flow equations, a body-fitted coordinate system and unsteady flow equations are used so that unsteady effects and natural channels may be modeled accurately. The effective stresses associated with the flow equations are modeled by using a constant eddy-viscosity approach. This study is restricted to beds of uniform particles, i.e., armoring and grain-sorting effects are neglected. To verify the model, the computed results are compared with the data measured in 140° and 180° curved laboratory flumes with straight reaches up- and downstream of the bend. The model predictions agree better with the measured data than those obtained by previous numerical models. The model is used to investigate the process of evolution and stability of bed deformation in circular bends.