Influence of Stratification and Shoreline Erosion on Reservoir Sedimentation Patterns

Sedimentation in the main pool of a deep (maximum depth: 50?m), 227?km2 hydropower reservoir was modeled using a three-dimensional numerical model of hydrodynamics and sedimentation for different wind, inflow, and outflow conditions. Short-term velocity measurements made in the reservoir were used to validate some aspects of the hydrodynamic model. The effects of thermal stratification on sedimentation patterns were investigated, since the reservoir is periodically strongly stratified. Stratification alters velocity profiles and thus affects sedimentation in the reservoir. Sedimentation of reservoirs is often modeled considering only the deposition of sediments delivered by tributaries. However, the sediments eroding from the shorelines can contribute significantly to sedimentation if the shorelines of the reservoir erode at sufficiently high rates or if sediment delivery via tributary inflow is small. Thus, shoreline erosion rates for a reservoir were quantified based on measured fetch, parameterized beach profile shape, and measured wind vectors, and the eroded sediments treated as a source within the sedimentation modeling scheme. The methodology for the prediction of shoreline erosion was calibrated and validated using digital aerial photos of the reservoir taken in different years and indicated approximately 1?m/year of shoreline retreat for several locations. This study revealed likely zones of sediment deposition in a thermally stratified reservoir and presented a methodology for integration of shoreline erosion into sedimentation studies that can be used in any reservoir.     

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.     

Cellular-Automata Model for Dense-Snow Avalanches

This paper introduces a three-dimensional model for simulating dense-snow avalanches, based on the numerical method of cellular automata. This method allows one to study the complex behavior of the avalanche by dividing it into small elements, whose interaction is described by simple laws, obtaining a reduction of the computational power needed to perform a three-dimensional simulation. Similar models by several authors have been used to model rock avalanches, mud and lava flows, and debris avalanches. A peculiar aspect of avalanche dynamics, i.e., the mechanisms of erosion of the snowpack and deposition of material from the avalanche is taken into account in the model. The capability of the proposed approach has been illustrated by modeling three documented avalanches that occurred in Susa Valley (Western Italian Alps). Despite the qualitative observations used for calibration, the proposed method is able to reproduce the correct three-dimensional avalanche path, using a digital terrain model, and the order of magnitude of the avalanche deposit volume.     

1D Modeling of Mud∕Debris Unsteady Flows

The National Weather Service 1D dynamic flood routing model FLDWAV is enhanced to include capability of modeling mud∕debris unsteady flows by including an additional friction slope term in the momentum equation of the Saint-Venant equations. Three techniques are incorporated into the model to determine the mud∕debris related friction slope term due to the internal viscous dissipation of non-Newtonian fluids and granular sliding friction of coarse-grained debris surges. These techniques are tested and some computational results are compared with observed field data and experimental data.     

Runout Analysis of Slurry Flows with Bingham Model

In this paper, the Lagrangian finite element method is formulated along with the Bingham model for simulating natural transient slurry flows. The numerical model thus developed is validated with available experimental results. As well, a back-analysis of the dynamic behavior of the 1997 Lai Ping Road landslide in Hong Kong is conducted. Correlating well with field observations, the modeling results are satisfactory in terms of predicting the hazard areas of the debris trail during runout and the debris distribution on deposition fan. The present method is also able to demonstrate the multidirectional sliding features of natural slurry flows. It is noteworthy that the availability of high quality data on site topography is crucial for modeling such a gravity-driven flow. A knowledge of dynamic runout information is useful in estimating landslide mobility, potential hazard areas, and debris impact loads, and in evaluating the effectiveness of counter measures in the debris released area, in relation to downslope property development.     

Erosion of Cohesive Sediments: Resuspension, Bed Load, and Erosion Patterns from Field Experiments

New field data on cohesive sediment erosion is presented and discussed, with particular focus on partitioning the total erosion into resuspension and bed load. The data were obtained using a recently developed in situ flume of the National Institute of Water and Atmospheric Research, New Zealand. The erosion rate is estimated from direct measurements of bed surface elevations by acoustic sensors, whereas resuspension rate is obtained using data on sediment concentrations measured by optical backscatter sensors. The bed- load contribution to the total erosion rate is evaluated from the conservation equation for sediments. To test repeatability, the data from the in situ flume are compared with those from a previous version of the flume. The results show that comparative studies of in situ flumes and standardized deployment procedures enable direct comparison of experimental data on cohesive sediment erosion. Overall, the data show that a commonly used assumption that the erosion rate is equal to the resuspension rate is not always valid as bed load plays a significant role in cohesive sediment erosion. The data also highlight the importance of clay content and other sediment physical characteristics in the sediment mixture.     

Modeling Sediment Transport Using Depth-Averaged and Moment Equations

A 1D mathematical model to calculate bed variations in alluvial channels is presented. The model is based on the depth-averaged and moment equations for unsteady flow and sediment transport in open channels. Particularly, the moment equation for suspended sediment transport is originally derived by the assumption of a simple vertical distribution for suspended sediment concentration. By introducing sediment-carrying capacity, suspended sediment concentration can be solved directly from sediment transport and its moment equations. Differential equations are then solved by using the control-volume formulation, which has been proven to have good convergence. Numerical experiments are performed to test the sensitivity of the calibrated coefficients α and k in the modeling of the bed deposition and erosion. Finally, the computed results are compared with available experimental data obtained in laboratory flumes. Comparisons of this model with HEC-6 and other numerical models are also presented. Good agreement is found in the comparisons.     

Modeling Unsteady Flow Characteristics of Hydropeaking Operations and Their Implications on Fish Habitat

Reservoir releases associated with energy production and flood mitigation need to be reconciled with efforts to maintain healthy ecosystems in regulated rivers. Unsteady flow phenomena caused by hydropeaking operations typically affect riverbed erosion and fish displacement. A three-dimensional hydrodynamic model is used to simulate the flow characteristics during the passage of the rising limb of an observed hydropeaking event in a gravel-bed reach of Smith River, Virginia. The calculated time-dependent water surface elevations, velocities, and shear stresses are compared with field measurements. Further, comparison based on numerical simulations of this historical and a hypothetical “staggering” hydropeaking event reveals that the latter has the capability of reducing the area subject to erosion and prolonging refugia availability for juvenile brown trout. Issues related to the adoption of either a truly dynamic modeling approach or a quasi-steady methodology for simulating unsteady flows are examined through a proposed unsteadiness flow parameter. The insights obtained from this study can assist in properly accounting for the impact of hydropeaking operations on fish habitat and instream flow management.     

Desktop Method for Estimating Vessel-Induced Sediment Suspension

Riverbank erosion involves engineering and environmental concerns. Among several dominant mechanisms of riverbank erosion, navigation effects caused by the passage of vessels are quite important. Field data indicate that large vessels generate large drawdown and small wave heights, whereas small vessels such as pleasure craft generate small drawdown and large wave heights. Passage of both types of vessels may result in bank erosion and a substantial increase in suspended-sediment concentration. Since comprehensive numerical modeling is time-consuming, a desktop computational approach has been offered that helps in providing preliminary answers to several questions related to vessel-induced sediment resuspension. Due to the uncertainties inherent in sediment calculations, order-of-magnitude values are often adequate for decision making. A preliminary assessment of possible impact on vegetation or benthic organisms can then be made based on this information. PC-based FORTRAN programs were developed for (1) computation of time series of vessel-induced waves; (2) erosion and deposition of cohesive sediment under waves and nearshore currents; and (3) computation of noncohesive suspended-sediment concentration caused by river current alone. An application to the Upper Mississippi River Navigation Study is presented. The riverbank sediments were labeled as soft, medium, and hard using their erodibility characteristics. Three water depths (0.5, 1.0, and 1.5 m) were considered. Vessel-induced wave heights ranged from 10 to 60 cm. The maximum concentrations were obtained in 0.5 m water depth with a wave height of 30 cm and in 1.0 m depth with a wave height of 60 cm. The estimated maximum concentrations were 1,463, 56, and 4 mg∕L for soft, medium, and hard beds, respectively. Such results will be useful for preliminary assessment of relative impact of increased barge traffic in the river and identification of potential areas along the riverbanks that are likely to be sensitive from the point of view of environmental considerations.     

Experimental Investigation of the Hydraulic Erosion of Noncohesive Compacted Soils

This paper presents the results of a laboratory investigation whose purpose was to evaluate the effects of compaction on the erodibility of cohesionless soils. By means of a recently developed flume experiment, sediment erosion rates and incipient motion, as a function of shear stress, average velocity, and dry density, have been determined for three compacted sand and gravel mixtures. A preliminary comparison of the incipient motion values shows that granular soils compacted at the Proctor optimum have a higher resistance to free surface flow erosion than those compacted at lower and higher densities. This leads one to infer that the Proctor optimum, generally used as a standard for construction, might also be an optimum for hydraulic resistance and stability. Additional comparison of the experimental data with two commonly used incipient motion criteria also suggests that Yang’s criterion is a better predictor of soil detachment than the Shields-Yalin criterion.     

Variation of Flow Pattern with Sinuosity in Sine-Generated Meandering Streams

The effect of channel sinuosity on flow pattern in meandering streams is investigated. The centerlines of the idealized meandering streams under consideration follow sine-generated curves, and the banks are rigid; the flow is turbulent and subcritical. This study focuses on the vertically averaged flow over a flat (horizontal at any cross section) bed formed by a granular material. The “flat bed” is viewed as the initial surface of a moveable bed at the beginning of an experiment (at time t = 0). A series of laboratory flow measurements involving the systematic variation of the deflection angle θ0 from 30 to 110°, is used. It is found that every different sinuosity (every different θ0) has its own convective flow pattern, i.e., its own distribution in plan of (the L/2 long) convergence–divergence zones of flow. As θ0 increases, a gradual change in flow pattern is observed. Two expressions defining the observed θ0 variation of the convective flow pattern are introduced. It is shown, with the aid of the sediment transport continuity equation, that the geometry of the developed bed at the end of an experiment is strongly related to the convective behavior of the vertically averaged (initial) flow over the flat bed at t = 0. In particular, information on the θ0 variation of the convective pattern of the initial flow can be used to estimate the location of erosion–deposition zones and the location(s) of the most intense erosion–deposition corresponding to any θ0.     

Assessing the Potential of Internal Erosion and Suffusion of Granular Soils

This study presents a new empirical criterion for assessing the potential of internal erosion and suffusion of granular soils. This method considers the bimodal structure of a soil having a primary coarse fabric and loose finer particles based on the porosities influenced by the particle size distribution and the degree of compaction. By comparing the representative particle size of a loose finer fraction with the controlling constriction size of a primary coarse fabric, a distinct boundary between internally stable and unstable soils with respect to internal erosion may be found.     

精矿库安全滑触线改进实践

贵溪冶炼厂备料车间二系统整个精矿库滑触线因起重机长期满负荷工作,现场环境腐蚀性大等外力影响,滑触线水平和上下都存在一定位移,部分位置轻微变形,从而使部分滑触线护套变形、损坏,滑触线辅助导体发热,造成极大的安全隐患。因滑触线在长期使用中其部分接触面腐蚀,集电器接触过程中产生打火、拉弧等现象,严重影响现场相关电气设备正常运行。针对滑触线问题和相关的检修过程提出具体改进方案并予以实施。     

Numerical Simulation of Relatively Wide, Shallow Channels with Erodible Banks

A two-dimensional numerical model was developed to simulate relatively wide, shallow rivers with an erodible bed and banks composed of well-sorted, sandy materials. A moving boundary-fitted coordinate system was used to calculate water flow, bed change, and bank erosion. The cubic interpolated pseudoparticle method was used to calculate flow, which introduced little numerical diffusion. The sediment-transport equation for the streamline and transverse transport was used to estimate bed and bank evolution over time, while considering the secondary flow. Bank erosion was simulated when the gradient in the cross-sectional direction of the banks was steeper than the submerged angle of repose because of bed erosion near the banks. The numerical model reproduced the features of central bars well, such as bar growth, channel widening due to divergence of the flow around the bars, scour holes at the lee of the bars, and the increase of bar size with time. These features were in accordance with the observations for laboratory experiments. It also reproduced the features of braided rivers, such as the generation of new channels and abandonment of old channels, the bifurcation and confluence of channels, and the lateral migration of the channels. The model showed that the sediment discharge rate fluctuated with time, one of the dynamic features observed in braided channels.     

Management of Fluid Mud in Estuaries, Bays, and Lakes. I: Present State of Understanding on Character and Behavior

Fluid mud is a high concentration aqueous suspension of fine-grained sediment in which settling is substantially hindered. It constitutes a significant management problem in rivers, lakes, estuaries, and shelves by impeding navigation, reducing water quality and damaging equipment. Fluid mud accumulations have been observed in numerous locations worldwide, including Savannah Harbor, U.S., the Severn Estuary, U.K., and the Amazon River Delta, Brazil. This paper describes the present state of knowledge on fluid mud characteristics, processes, and modeling. Fluid mud consists of water, clay-sized particles, and organic material and displays a variety of rheological behaviors ranging from elastic to pseudo-plastic. It forms by three principle mechanisms: (1) the rate of sediment aggregation and settling into the near-bottom layer exceeds the dewatering rate of the suspension; (2) soft sediment beds fluidized by wave agitation; and (3) convergence of horizontally advected suspensions. Once formed, fluid mud is transported vertically by entrainment and horizontally by shear flows, gravity, and streaming. If not resuspended, it slowly consolidates to form bed material. Quantitative relationships have been formulated for key fluid mud formation and movement mechanisms, but they rely on empirical coefficients that are often site- or situation-specific and are not generally transferable. Research to define general relationships is needed.     

DEM Simulation of Particle Damage in Granular Media — Structure Interfaces

Using the discrete element method (DEM) with clustering, a novel means of numerically modeling damage of particles is presented. Damage, such as grain crushing, is treated by allowing clusters to break apart according to a failure criterion based upon sliding work. If the accumulated work done on an individual DEM particle of a cluster exceeds a threshold, that particle is allowed to break from the cluster. A value for the critical energy density is determined by comparing the degree of particle breakage from numerical simulations to data from laboratory tests. Numerical simulations were also conducted to determine the impact of particle damage on interface behavior. It was found that a very distinct shear zone was evident when particle damage was considered and that this occurred without significant reduction of the maximum shear strength of the medium. Also, the degree of damage was shown to be related to the angularity of the clusters.     

Effect of Progressive Failure on Measured Shear Strength of Geomembrane/GCL Interface

This paper presents experimental data and numerical modeling results that illustrate the effects of progressive failure on the measured shear strength of a textured geomembrane/geosynthetic clay liner (GMX/GCL) interface. Large direct shear tests were conducted using different specimen gripping/clamping systems to isolate the effects of progressive failure. These tests indicate that progressive failure causes a reduction in measured peak shear strength, an increase in the displacement at peak, an increase in large displacement shear strength, and significant distortion of the shear stress–displacement relationship. A numerical model was developed to simulate progressive failure of a GMX/GCL interface. Measured and simulated shear stress–displacement relationships are in good-to-excellent agreement at four normal stress levels. The model was then used to investigate mechanisms of progressive interface failure and factors that control its significance. The results indicate that accurate measurements of shear stress–displacement behavior and strength are obtained when gripping surfaces prevent slippage of the test specimen and the intended failure surface has the lowest shear resistance of all possible sliding surfaces. The use of proper gripping surfaces is expected to reduce difficulties in test data interpretation and to increase the accuracy and reproducibility of test results.     

Management of Fluid Mud in Estuaries, Bays, and Lakes. II: Measurement, Modeling, and Management

Techniques for measurement, modeling, and management of fluid mud are available, but research is needed to improve them. Fluid mud can be difficult to detect, measure, or sample, which has led to new instruments and new ways of using existing instruments. Multifrequency acoustic fathometers sense neither density nor viscosity and are, therefore, unreliable in measuring fluid mud. Nuclear density probes, towed sleds, seismic, and drop probes equipped with density meters offer the potential for accurate measurements. Numerical modeling of fluid mud requires solving governing equations for flow velocity, density, pressure, salinity, water surface, plus sediment submodels. A number of such models exist in one-, two-, and three-dimensional form, but they rely on empirical relationships that require substantial site-specific validation to observations. Management of fluid mud techniques can be classified as those that accomplish: Source control, formation control, and removal. Nautical depth, a fourth category, defines the channel bottom as a specific fluid mud density or alternative parameter as safe for navigation. Source control includes watershed management measures to keep fine sediment out of waterways and in-water measures such as structures and traps. Formation control methods include streamlined channels and structures plus other measures to reduce flocculation and structures that train currents. Removal methods include the traditional dredging and transport of dredged material plus agitation that contributes to formation control and/or nautical depth. Conditioning of fluid mud by dredging and aerating offers the possibility of improved navigability. Two examples—the Atchafalaya Bar Channel and Savannah Harbor—illustrate the use of measurements and management of fluid mud.     

Compatibility of Reservoir Sediment Flushing and River Protection

In this paper, we propose a system of numerical models for the compatibility assessment of reservoir sediment flushing and protection of downstream river environments. The model system is made up of two simulation models. The first model simulates soil erosion in watershed slopes and sediment transport in the tributary of the reservoir by means of a weighted essentially nonoscillatory (WENO) method, which is conservative and fourth-order accurate in space and time. The second model simulates velocity and suspended solid concentration fields in the reservoirs. This model is based on the three-dimensional (3D) numerical integration of motion and concentration equations, expressed in contravariant form on a generalized boundary-conforming curvilinear coordinate system by using a conservative and higher-order accurate numerical scheme. The proposed system of models is applied to the Pieve di Cadore (Veneto, Italy) reservoir and to its catchment area. By comparing suspended solid concentrations that are discharged through the bottom outlets during flushing operations with suspended solid concentrations in the main river during natural flooding, we perform an assessment of the compatibility between sediment flushing and the protection of the river ecosystem downstream.     

载流条件下Cu-Ag-Fe合金的摩擦磨损行为

研究了电流强度、滑动速度和载荷对Cu-Ag-Fe合金的磨损性能的影响.结果表明：由于Fe元素的时效析出强化作用,Cu-Ag-Fe合金的电磨损性能比Cu-Ag合金高出2倍多,大大提高了该合金的使用寿命.随电流强度、滑动速度和载荷的增加,合金的质量磨损率明显增加.在受电滑动条件下,磨损形式以粘着磨损,磨粒磨损和电侵蚀磨损为主,并且随着电流强度的增加,粘着磨损现象加剧.