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.     

Numerical Analysis of River Channel Processes with Bank Erosion

This paper presents a numerical analysis of river channel processes with bank erosion. The model can be used for investigating both bed-deformation and bankline shifting in 2D plan form. The basic equations are used in a moving boundary fitted-coordinate system, and a new formulation of nonequilibrium sediment transport is introduced to reproduce the channel processes. The model was applied to examine the morphological behavior of experimental channels. Temporal changes in the plan form in a meandering channel can be classified into two patterns: meander developing and meander straightening. Comparison of the observed and calculated results indicates that the model is applicable to both channel changes under various hydraulic conditions. On the basis of the numerical findings, the paper clarifies the influence of hydraulic variables on the location of bank erosion and bed scouring. The model also was used to investigate the effect of alternate bars on bank erosion and to investigate the development of channel meandering from an initially straight channel.     

Modeling the Evolution of Incised Streams. II: Streambank Erosion

Incision and ensuing widening of alluvial stream channels is widespread in the midsouth and midwestern United States and represents an important form of channel adjustment. Streambanks have been found to contribute as much as 80% of the total suspended load. The location, timing, and magnitude of streambank erosion are difficult to predict. Results from field studies to characterize the resistance of fine-grained materials to hydraulic and geotechnical erosion, the impact of pore-water pressures on failure dimensions and shearing resistance, and the role of riparian vegetation on matric suction, streambank permeability, and shearing resistance are used to enhance the channel evolution model CONCEPTS (conservational channel evolution and pollutant transport system). This paper discusses the conceptualization of the above-mentioned physical processes, and demonstrates the ability of the derived model to simulate streambank-failure processes. The model is tested against observed streambank erosion of a bendway on Goodwin Creek, Miss. between March 1996 and March 2001, where it accurately predicts the rate of retreat of the outside bank of the bendway. The observed change in average channel width within the central section of the bendway is 2.96?m over the simulation period, whereas a retreat of 3.18?m (7.4% larger) is simulated. The observed top-bank retreat within the central section of the bendway is 3.54?m over the simulation period, whereas a retreat of 3.01?m (15% smaller) is simulated.     

Experimental Parametric Study of Suffusion and Backward Erosion

Within hydraulic earth structures (dikes, levees, or dams), internal seepage flows can generate the entrainment of the soil grains. Grain transportation affects both particle size distributions and porosity, and changes the mechanical and hydraulic characteristics of the earth’s structure. The occurrence of failures in new earth structures due to internal erosion demonstrates the urgency of improving our knowledge of these phenomena of erosion. With this intention, a new experimental device has been developed that can apply hydraulic stresses to reconstituted consolidated cohesive soils without cracks in order to characterize the erosion evolution processes that might be present. A parametric study was conducted to examine the influence of three critical parameters on clay and sand erosion mechanisms. When the hydraulic gradient was low, it was concluded that the erosion of the structure’s clay fraction was due to suffusion. When the hydraulic gradient increased, it was concluded that the sand fraction erosion initiation was due to backward erosion. The extent of the erosion was dependent on the clay content. The study underlines the complexity of confinement stress effects on both erosion phenomena.     

Cr32Ni7Mo3N特级双相不锈钢的空蚀行为

利用磁致伸缩空蚀试验机对Cr32Ni7Mo3N特级双相不锈钢在蒸馏水和人工海水中进行了空蚀实验,并采用扫描电镜跟踪观察了经不同时间段空蚀后试样的形貌.通过测量失重绘制了材料的累积失重量和失重率曲线.经电化学工作站测量了材料在静态与空蚀条件下的极化曲线和腐蚀电位变化.对比分析了Cr32Ni7Mo3N与SAF2205双相不锈钢在人工海水的抗空蚀能力.结果表明:Cr32Ni7Mo3N特级双相不锈钢空蚀破坏首先在铁素体薄弱区以及铁素体和奥氏体相界发生,并向铁素体内扩展,铁素体发生解离断裂脱落;奥氏体随着空蚀的进行,滑移线增多,显微硬度值增加,且人工海水中奥氏体显微硬度值比在蒸馏水中的高;铁素体大面积破坏后,奥氏体才失稳产生延性断裂脱落,奥氏体的存在延缓了破坏在整个材料表面上的扩展.空蚀与腐蚀交互影响导致材料在人工海水中加速破坏.Cr32Ni7Mo3N特级双相不锈钢在人工海水中的抗空蚀能力优于SAF2205双相不锈钢.      

Coupling Bank Stability and Bed Deformation Models to Predict Equilibrium Bed Topography in River Bends

Attempts to include river bank erosion predictors within morphological models are becoming increasingly common, but uncertainty surrounds the procedures used to couple bed deformation and bank erosion submodels in a way that maintains the mass continuity of eroded bank sediment. Herein we present a coupling procedure that comprises two discrete elements. First, immediately following bank failure, the slumped debris comes to rest at the bank toe as a planar surface inclined at an “angle of repose.” Second, we use a fractional transport model to simulate the subsequent erosion and deposition of the failed bank material debris. The method is demonstrated with an example in which an equilibrium bed topography model is combined with a river bank erosion model to predict the morphological response of a river bend to a large flow event.     

Development of Storm-Water Management Design Criteria to Maintain Geomorphic Stability in Kansas City Metropolitan Area Streams

Fifty-three years of historical precipitation data were applied to the U.S. Environmental Protection Agency Storm Water Management Model (SWMM) to conduct hydrologic and hydraulic simulations, generating continuous stream flow hydrographs in the receiving stream channels. On-site BMP and regional detention criteria were selected to allow postdevelopment replication of predevelopment peak flow frequency exceedance curves and the critical portions of shear stress duration curves. Instream continuous stage data generated by SWMM were used to examine erosion potential through the use of an erosion potential index. Coarse stream bed material were found to be less sensitive to changes in erosion potential due to urbanization, and extended detention ponds designed for flood control and water quality treatment were effective in reducing erosion potential. These controls were less effective in reducing erosion potential of fine loam bed material, indicating reductions of runoff volumes are required to minimize increases in channel erosion. Findings indicate that regional analysis based on local hydrologic and geomorphic characteristics are necessary to identify appropriate storm-water control requirements.     

Erosion Behaviour of WC–Co–Cr Thermal Spray Coated Grey Cast Iron under Mining Environment

Hydraulic machinery components made of grey cast iron (FG 260 grade) are preferred for engineering application because of their excellent damping properties. However when such materials are exposed to mining environments they exhibit poor erosion resistance without meeting their estimated life time. In order to enhance the service life of the material of hydraulic components, WC–Co–Cr thermal spray coating was identified. Grey cast iron samples (FG 260 grade) with and without WC–Co–Cr coating were subjected to slurry jet erosion tests by varying the impingent velocity and angle under two different pH levels at 3 and 7 which pertain to the mining environment. XRD characterization was done to identify and confirm the carbide phases present. Surface morphology studies were carried by SEM on both the substrate and coating, which revealed the erosion of grey cast iron surface, due to ploughing mechanism. In the case of WC–Co–Cr coating, at oblique angle of impact, the degradation is by micro cutting of the matrix and ploughing mechanism. At normal impingement, the fluctuating stress creates the cracks, which interlink each other and thereby causing erosion of the material.     

Reliability Analysis of Suspension Bridges against Fatigue Failure from the Gusting of Wind

A fatigue reliability analysis of suspension bridges due to the gustiness of the wind velocity is presented by combining overall concepts of bridge aerodynamics, fatigue analysis, and reliability analysis. For this purpose, the fluctuating response of the bridge deck is obtained for buffeting force using a finite-element method and a spectral analysis in frequency domain. Annual cumulative fatigue damage is calculated using Palmgren–Miner’s rule, stress-fatigue curve approach and different forms of distribution for stress range. In order to evaluate the reliability, both first-order second-moment (FOSM) method and full distribution procedure (assuming Weibull distribution for fatigue life) are used to evaluate the fatigue reliability. Probabilities of fatigue failure of the Thomas Bridge and the Golden Gate Bridge for a number of important parametric variations are obtained in order to make some general observations on the fatigue reliability of suspension bridges. The results of the study show that the FOSM method predicts a higher value of the probability of fatigue failure as compared to the full distribution method. Further, the distribution of stress range used in the analysis has a significant effect on the calculated probability of fatigue failure in suspension bridges.     

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.     

Cavitation erosion of age-hardenable aluminum alloys

The mechanisms of deformation and failure of age-hardenable aluminum alloys due to ultrasonic cavitation have been investigated. The results indicate that the mode and rate of erosion depend on both composition and heat treatment: increasing the hardness and strength by aging or alloying produces an increase in erosion resistance togetner with a change in failure mode. Alloys of low solute content erode in a manner similar to that of pure FCC metals namely by uniform ductile rupture across the surface. In the samples of 2 to 4.5 pct alloy content, however, material is lost preferentially from apparently random, isolated regions, leaving behind striated, flat-bottomed pits. The fracture surface of these pits resembles those of fatigue failures in aluminum alloys. These pits expand laterally to macroscopic proportions on continued caviation exposure without suffering any changes in their topographical features. Alloys of 9 to 10 pct solute content develop similar striated pits but on a microscopic scale. These different erosion mechanisms are discussed in terms of the microstructure of the alloys and their ability to absorb and attenuate the incident cavitation energy. Formerly Associate Professor at S.U.N.Y., Stony Brook     

Design for Stream Restoration

Stream restoration, or more properly rehabilitation, is the return of a degraded stream ecosystem to a close approximation of its remaining natural potential. Many types of practices (dam removal, levee breaching, modified flow control, vegetative methods for streambank erosion control, etc.) are useful, but this paper focuses on channel reconstruction. A tension exists between restoring natural fluvial processes and ensuring stability of the completed project. Sedimentation analyses are a key aspect of design since many projects fail due to erosion or sedimentation. Existing design approaches range from relatively simple ones based on stream classification and regional hydraulic geometry relations to more complex two- and three-dimensional numerical models. Herein an intermediate approach featuring application of hydraulic engineering tools for assessment of watershed geomorphology, channel-forming discharge analysis, and hydraulic analysis in the form of one-dimensional flow and sediment transport computations is described.     

Suction Stress Characteristic Curve for Unsaturated Soil

The concept of the suction stress characteristic curve (SSCC) for unsaturated soil is presented. Particle-scale equilibrium analyses are employed to distinguish three types of interparticle forces: (1) active forces transmitted through the soil grains; (2) active forces at or near interparticle contacts; and (3) passive, or counterbalancing, forces at or near interparticle contacts. It is proposed that the second type of force, which includes physicochemical forces, cementation forces, surface tension forces, and the force arising from negative pore-water pressure, may be conceptually combined into a macroscopic stress called suction stress. Suction stress characteristically depends on degree of saturation, water content, or matric suction through the SSCC, thus paralleling well-established concepts of the soil–water characteristic curve and hydraulic conductivity function for unsaturated soils. The existence and behavior of the SSCC are experimentally validated by considering unsaturated shear strength data for a variety of soil types in the literature. Its characteristic nature and a methodology for its determination are demonstrated. The experimental evidence shows that both Mohr–Coulomb failure and critical state failure can be well represented by the SSCC concept. The SSCC provides a potentially simple and practical way to describe the state of stress in unsaturated soil.     

Ecological and Hydraulic Studies of Step-Pool Systems

The ecological and hydraulic features of step-pool systems are studied by field investigation, measurement, sampling, and analysis. The study is done on Shengou and Jiuzhai Creeks, where step-pool systems have developed, Fork Gully, where a step-pool system is developing, and the Jinsha River and Jiangjia and Xiaobaini Ravines, where there is no step-pool system. Boulders, cobbles, and gravel tightly interlock and form the steps having an inherent stability that only extreme floods are likely to disturb. Gravel and sand deposit in the pools behind the steps. These steps and pools provide high diversity of habitat for the stream biocommunity. The density of benthic macroinvertebrates in streams with step-pool systems is several 100 times higher than neighboring streams without step-pool systems. A new habitat diversity index is proposed considering the spatial distribution of various substrates, velocity, and water depth. The study reveals that the biodiversity of benthic macroinvertebrates increases with habitat diversity. Measurements with a specially designed instrument were done to study the development of step-pool systems and its effects on resistance to the flow and stream bed stability. A step-pool system maximizes the flow resistance and protects the bed sediment from erosion. Thus, the riverbed and bank slope are stabilized. The development degree of step pools is proportional to the streambed slope. The bed resistance increases with the development degree of step pools. Riverbed inertia represents the stability of the streambed. The development of step-pool systems greatly increases the riverbed inertia, and, therefore, maximizes streambed stability.     

Hydrological Evaluation of Septic Disposal Field Design in Sloping Terrains

The most common form of onsite domestic wastewater treatment in the United States is the septic system. Although the design of these systems has been well established, no systematic evaluation of septic system performance exists for sloping hardpan soils. In this paper, we develop a simple hydrologic model for assessing the probability of failure for a set of hydrologic conditions, septic loading rates, and soil and landscape parameters that are readily available for sloping soils. To demonstrate the model capabilities, input data for a septic field of a two-person residence in the New York City drinking water basin in the Catskills was utilized. Our analysis showed that the saturated hydraulic conductivity, depth to the impermeable layer, and slope of the drain field are critical parameters to assess in the design and siting of these systems. We concluded that septic systems perform poorly in undulating landscapes where the hydraulic conductivity is low and the impermeable layer is close to the surface. Under prolonged rainfall conditions on these soils, the septic field and downslope field saturate, causing hydraulic failure of the septic system and saturation in the downslope field; as a result, effluent may be routed directly to streams via overland runoff.     

Erosion Study of New Orleans Levee Materials Subjected to Plunging Water

During Hurricane Katrina, overtopping water caused erosion and subsequent failure of several sections of I-type flood walls in New Orleans. Erosion stemmed from the kinetic energy of water falling from the top of the flood wall, unlike the typical surface erosion caused by shear flow. This study evaluated the effects of important parameters of levee soils—fines content, degree of compaction (DOC), clay mineralogy, and water content in relation to the erosion behavior of New Orleans levees subjected to the plunging water. In general, test results showed that a higher fines content contributed to greater erosion resistance. The trend became unclear when fines content exceeded 20–25%. A higher degree of compaction did not necessarily contribute to greater erosion resistance. Underwater soaked soils showed much less erosion resistance than nonsoaked soils. Soils containing expansive clay minerals showed less erosion resistance than soils containing nonexpansive clay minerals.     

Stability of I-Walls in New Orleans during Hurricane Katrina

Failures of I-walls during Hurricane Katrina were responsible for many breaches in the flood protection system in New Orleans. Six breaches were examined in detail by Task Group 7 of the Interagency Performance Evaluation Taskforce. Four of these failures and breaches, which occurred before the water levels reached the top of the wall, were not caused by overtopping erosion. The failure of the I-wall at the 17th Street Canal resulted from shear through the weak foundation clay. The south failure of the London Avenue I-wall was caused by subsurface erosion, which carried massive amounts of sand inland, and removed support for the wall, leading to catastrophic instability. At the north breach on London Avenue, the failure was caused by high pore pressures, combined with a lower friction angle in the loose sand, which resulted in gross instability of the I-wall under the water pressure load from the storm surge. Looking back, with the benefit of 20-20 hindsight, these stability and erosion failures can be explained in terms of modern soil mechanics, exploration techniques, laboratory test procedures, and analysis methods. An important factor in all of the cases investigated was development of a gap behind the wall as the water rose against the wall and caused it to deflect. Formation of the gap increased the load on the wall, because the water pressures in the gap were higher than the earth pressures that had acted on the wall before the gap formed. Where the foundation soil was clay, formation of a gap eliminated the shearing resistance of the soil on the flood side of the wall, because the slip surface stopped at the gap. Where the foundation soil was sand, formation of the gap opened a direct hydraulic connection between the water in the canal and the sand beneath the levee. This hydraulic short circuit made seepage conditions worse, and erosion due to underseepage more likely. It also increased the uplift pressures on the base of the levee and marsh layer landward of the levee, reducing stability. Because gap formation has such important effects on I-wall stability, and because gaps behind I-walls were found in many locations after the storm surge receded, the presence of the gap should always be assumed in I-wall design studies.     

新型CrMnNiN奥氏体不锈钢的抗空穴腐蚀及磨损性能

对新型CrMnNiN不锈钢JA2（0．23C－13Mn-17Cr-1.90Ni-1.5Mo-1Si-0.33N)的抗空穴腐蚀及磨损性能进行了测试，结果表明，这种奥氏体高锰不锈钢具有高的加工硬化率和好的韧性，比目前水轮机转轮及叶片常用的0Cr13Ni5Mo有更高的抗空穴腐蚀性能及耐泥沙磨损性能。