Dam-Breach Flood Wave Propagation Using Dimensionless Parameters

An analytical model of flood wave propagation is used to study the sensitivity of dam-breach flood waves to breach-outflow hydrograph volume, peak discharge, and downstream-channel bed slope. Dimensionless parameters are identified for discharge and distance along the channel. A dam-breach Froude number is defined to enable analysis through a wide range of site and flow conditions. It is found that, at a certain distance downstream, the attenuated peak discharge is independent of the magnitude of the discharge at the breach site. This attenuated peak discharge is termed the “ultimate discharge,” and the distance associated with it, the “ultimate distance.” These ultimate values are a function, primarily of the downstream-channel bed slope and, secondarily, of the breach-outflow hydrograph volume.     

Disturbance Caused by Bed Sills on the Slopes of Steep Streams

Live-bed tests on steep bed slopes are run in a laboratory flume to investigate the slope of equilibrium. The tests simulate a general degradation until a new equilibrium is reached. Subsequently, different bed sill settings are tested as a countermeasure against degradation. Evidence is presented that, in some cases, the bed slope with sills is milder than the one without sills under the same flow and sediment rates. This fact implies that the efficiency of the bed sill system to stop degradation decreases as the sills are placed closer together. Tests are on the verge of sheet flow, and friction factors are heavily dependent on sediment transport. Reaches in uniform and gradually varied flow behave very differently. Bed profiles are not straight, and grain size is linked to this fact. Other detailed information on testing equipment, experimental program, time development and local scour in a total number of 111 tests is given.     

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.     

基于有限元极限平衡法的三维边坡稳定性

提出了一种基于有限元弹塑性应力场和极限平衡状态的三维边坡稳定分析方法——三维有限元极限平衡法。首先,考虑三维空间中滑动方向,提出滑动面上一点在滑动方向上的极限平衡条件,并证明滑动面上土体整体达到极限平衡状态与滑动面上土体各处在滑动方向上处于极限平衡状态等价。再通过刚体极限平衡假定计算主滑方向和滑动面上各点滑动方向。最后,定义局部安全系数为抗剪强度与滑动方向上剪应力投影的比值,基于三维边坡整体极限平衡条件将局部安全系数通过积分中值定理转变为整体安全系数。该方法计算简单,消除了剪应力比形式定义安全系数滑动面形状限制,具备合理性与有效性。算例验证结果表明,该方法滑动方向假设合理,安全系数与严格三维极限平衡法结果一致。      

Transient Hydrodynamic Dispersion in Rough Open Channels: Theoretical Analysis of Bed-Form Effects

A stochastic Lagrangian approach is proposed to describe dispersion in a two-dimensional low-regime river flow past random bed undulations characterized by the superposition of periodical and stationary exponential correlations, through a suitable time-dependent coefficient. The resulting dimensionless expression depends on the overall resistance factor, which is proportional to the average Peclet number of the process. A graphical analysis shows that the oscillatory transient originating from the periodic component of the bottom elevation pattern is enhanced by reduced global flow resistance, while relatively more intense tracer spreading is associated with wavelike profiles affected by persistent trendless random noise, which also determines the characteristic time needed by the plume to achieve the asymptotic domain. Numerical simulations, validating the first-order analytical approach in a wide range of heterogeneous bed geometries, are also discussed. A semianalytical procedure is finally suggested for the study of depth-averaged transport processes in real three-dimensional streams, based on the use of the derived dispersion coefficient.     

Effect of Floodwater Extraction on Mountain Stream Morphology

Floodwater is often extracted for consumptive purposes from western mountain streams in the United States. The long-term extraction of floodwater may alter the morphological and ecological balance of such streams. Scale model experiments based on eight mountain gravel-bed streams in Idaho were conducted to test the effects of floodwater extraction on stream morphology. The model channel transported a poorly sorted mix of model “gravel,” as well as copious amounts of model “sand.” The channel had a discontinuous floodplain, developed its own bar morphology, and contained large model colluvium as well as a bedrock platform. A mobile-bed equilibrium was first developed using a repeated hydrograph. The experiment was then repeated using a sliding cutoff discharge. The discharges in the hydrograph that were below a given cutoff discharge were reduced to 30% of bankfull discharge. By raising the cutoff discharge, it was possible to study the effect of increasing severity of floodwater extraction on stream morphology. The experiments indicated an increase in sand content on the bed surface and a decrease in the standard deviation of fluctuations in bed elevation with increasing severity of floodwater extraction.     

Three-Dimensional Asymmetrical Slope Stability Analysis Extension of Bishop’s, Janbu’s, and Morgenstern–Price’s Techniques

Most existing three-dimensional (3D) slope stability analysis methods are based on simple extensions of corresponding two-dimensional (2D) methods of analysis and a plane of symmetry or direction of slide is implicitly assumed. In this paper, 3D asymmetric slope stability models based on extensions of Bishop’s simplified, Janbu’s simplified, and Morgenstern–Price’s methods are developed. Under these new formulations, the direction of slide is unique and is determined from 3D force/moment equilibrium. Results from the new formulations are similar to the classical methods in normal cases but are numerically stable under transverse load. Further, the writers demonstrate that the present formulation is actually equivalent to the axes rotation formulation by Jiang and Yamagami but is much more convenient to be used for general problems. The writers have also discovered some inherent limitations of 3D limit equilibrium analysis which are absent in the corresponding 2D analysis.     

Downstream Hydraulic Geometry of Alluvial Channels

This study extends the earlier contribution of Julien and Wargadalam in 1995. A larger database for the downstream hydraulic geometry of alluvial channels is examined through a nonlinear regression analysis. The database consists of a total of 1,485 measurements, 1,125 of which describe field data used for model calibration. The remaining 360 field and laboratory measurements are used for validation. The data used for validation include sand-bed, gravel-bed, and cobble-bed streams with meandering to braided planform geometry. The five parameters describing downstream hydraulic geometry are: channel width W, average flow depth h, mean flow velocity V, Shields parameter τ*, and channel slope S. The three independent variables are discharge Q, median bed particle diameter ds, and either channel slope S or Shields parameter τ* for dominant discharge conditions. The regression equations were tested for channel width ranging from 0.2 to 1,100?m, flow depth from 0.01 to 16?m, flow velocity from 0.02 to 7?m/s, channel slope from 0.0001 to 0.08, and Shields parameter from 0.001 to 35. The exponents of the proposed equations are comparable to those of Julien and Wargadalam (1995), but based on R2 values of the validation analysis, the proposed regression equations perform slightly better.     

Bedload Transport in Gravel-Bed Streams with Unimodal Sediment

Bedload transport in many gravel-bed streams becomes highly complicated because of the nonuniformity of the grain size and the vertical stratification of channel bed material. A new relation for computing bedload transport rates in gravel-bed streams is proposed here. In an effort to account for the variation of the makeup of the surface material within a wide range of Shields stresses, the relation employs a two-parameter approach, one related to the material in the pavement and the other to that in the subpavement layers. The mode is used to represent the grain sizes of each layer. Available bedload transport data from gravel-bed streams with unimodal sediment are used to test the accuracy of the relation. A comparison with other bedload transport relations is also considered.     

On the Quantification of the Bed Development Time of Alluvial Meandering Streams

This paper concerns the quantitative evaluation of the time of bed development of alluvial meandering streams. In agreement with the prevailing approach, it is assumed that the stream centerline follows a sine-generated curve; the banks are rigid. The flow is turbulent and subcritical, and the flow width is much larger than the flow depth. The movable bed is flat at time t = 0; at t = Tb, the bed reaches its equilibrium or developed state. With the aid of dimensional and physical considerations, an expression is found for the duration of bed development Tb. According to this expression, Tb is proportional to the square of the flow width B and inversely proportional to the specific volumetric bed-load rate corresponding to the channel-averaged flow. The proportionality factor is found to be a function of the initial deflection angle θ0 alone. The form of this function is investigated on the basis of a series of experimental runs carried out by the writers in a sine-generated channel having an intermediate value of θ0 (i.e., an intermediate value of sinuosity), as well as data available in the literature.     

运动学和极限平衡法相结合边坡稳定性分析

本文根据非洲如瓦西矿西南边坡为案例,分别采用运动学分析方法和极限平衡法,对边坡稳定性进行分析研究。研究结果表明:西南边坡发生平面滑坡和楔形体滑坡的概率较小,但由于边坡部分岩体较破碎,结构面较发育,不排除小规模楔形体滑坡的可能。极限平衡法计算的最小安全系数均高于规范要求值,其边坡为稳定边坡。     

Effect of Coarse Surface Layer on Bed-Load Transport

Existing bed-load transport formulas may overestimate the transport rate in mountain rivers by two orders of magnitude or more. Recently published field data sets provide an opportunity to take a fresh look at the bed-load transport relationship and it is hypothesized that the overestimate is due to a failure to account for the effect of a coarse surface layer of bed material inhibiting the release of fine subsurface material. Bed-load transport is determined as gs = aρ(q?qc) where q=water discharge per unit width; qc=critical value for initiation of bed material movement; ρ=water density; and a=coefficient. The gs/q relationship is typically piecewise linear, characterized by two transport phases with, respectively, low and high rates of change. Twenty-one flume and 25 field data sets were used to quantify the relationship for Phase 2. The flume data confirm the dependence of a on S1.5, where S=channel slope, in agreement with earlier studies. The field data additionally show that a varies inversely with the degree of bed armoring, given by the ratio of surface to subsurface bed material size. The finding is consistent with the hypothesis and suggests the need to account for the bed material supply limitation in the bed-load transport formula. However, the available data are not entirely sufficient to rule out an alternative dependency, or codependency, on flow resistance. The critical conditions for initiation of Phase 2 transport are also quantified as a function of bed material size and channel slope. The resulting set of equations allows a more accurate estimation of Phase 2 bed-load transport rates. However, the equations are empirical and should be restricted for use within the range of conditions used in their development, to determine mean rather than instantaneous transport rates and to determine bulk transport rates, not transport by size fraction.     

Effects of Time and Channel Geometry on Scour at Bridge Abutments

Experiments are described to investigate local scour at bridge abutments. The experiments were performed in a two-stage channel using abutments that extended different distances onto the floodplain including right up to the edge of the main channel (Melville, Type III). To ensure the largest scour depths the conditions on the floodplain upstream of the abutment were close to critical conditions for the bed material. The time evolution of the scour and the ultimate scour depth were measured. The time development of the local scour corresponded well with the theories of Ettema and Franzetti and the theory of Whitehouse for scour at horizontal cylinders in the marine environment. Melville has suggested that scour at abutments on floodplains can be approximated by scour in rectangular channels if an imaginary boundary is assumed, separating the flow in the main channel from that on the floodplain. The experimental results confirm the validity of Melville's suggestion for the configurations tested in the experiments.     

海州矿南帮边坡三维数值分析

通过对海州矿南帮边坡呈现的变形破坏机制分析,这里的岩体质量在空间上呈现出非常明显的不均一性。基于材料均一性角度进行分析,通过三维地质力学模型来完成对南帮边坡所具有的应力场分布特征进行模拟,从而更贴近实际情况。本文研究中主要是基于有限元软件ANSYS来实现南帮边坡的模型仿真,进行三维数值模拟分析,从而可以对应力变形分布进行剖析。基于强度折减法实现对安全系数的计算。     

Probabilistic Stability Analyses of Embankments Based on Finite-Element Method

Conventional limit equilibrium methods are commonly used to assess the stability of embankments. The finite-element method, as an alternative to limit equilibrium methods, is being increasingly used in the deterministic stability analysis of slopes or embankments. In this paper, a practical procedure for integrating the finite-element method and the limit equilibrium methods into probabilistic stability analysis for embankments is presented. The response surface method is adopted to approximate the performance function for the stability problems and the first-order reliability method is used to calculate the reliability index based on an intuitive expanding ellipsoid perspective. The advantages of the response surface method as a bridge between stand-alone numerical packages and spreadsheet-based reliability analysis via automatic constrained optimization are demonstrated and discussed through a hypothetical two-layer slope and an actual case of the James Bay Dykes. The results show the ease and successful implementation of the proposed procedure for reliability analysis of embankments.     

Evaluation of Surficial Stability for Homogeneous Slopes Considering Rainfall Characteristics

Shallow slope failures in residual soil during periods of prolonged infiltration are commonly occurring in the world. This study examines an infinite slope analysis to estimate the influence of infiltration on surficial stability of slopes by the limit equilibrium method. An approximate method that accommodates the boundary condition of a uniform rainfall has been proposed to evaluate the likelihood of shallow slope failure that is induced by a particular rainfall event. The method based on the Mein and Larson model, which provides an explicit solution for preponding infiltration, has been applied to various types of soil having measured unsaturated hydraulic properties. To compare results with those obtained from the approximate method, a series of numerical analyses were carried out. According to the results, with the use of properly estimated input parameters, the approximate method was found to give results that compare reasonably well with those of more rigorous finite element analyses.     

Probabilistic Assessment of Slope Stability That Considers the Spatial Variability of Soil Properties

In this paper, a numerical procedure for probabilistic slope stability analysis is presented. This procedure extends the traditional limit equilibrium method of slices to a probabilistic approach that accounts for the uncertainties and spatial variation of the soil strength parameters. In this study, two-dimensional random fields were generated based on a Karhunen-Loève expansion in a fashion consistent with a specified marginal distribution function and an autocorrelation function. A Monte Carlo simulation was then used to determine the statistical response based on the generated random fields. This approach makes no assumption about the critical failure surface. Rather, the critical failure surface corresponding to the input random fields of soil properties is searched during the process of analysis. A series of analyses was performed to verify the application potential of the proposed method and to study the effects of uncertainty due to the spatial heterogeneity on the stability of slope. The results show that the proposed method can efficiently consider the various failure mechanisms caused by the spatial variability of soil property in the probabilistic slope stability assessment.