Maximum Level and Time to Peak of Dam-Break Waves on Mobile Horizontal Bed

This experimental study focuses the influence of bed material mobility and initial downstream water level on maximum water level and time to peak of dam-break waves. It covers horizontal bed conditions on fixed bed, sand bed, and pumice bed. Results include water surface level time evolution, maxima wave levels and time to peak. The influence of bed material mobility and downstream water level was identified and characterized, stressing the importance of using mathematical models with appropriate sediment transport formulations instead of purely hydrodynamic models to simulate dam-break waves on mobile bed channels.     

Three-Dimensional Modeling of Nonuniform Sediment Transport in an S-Shaped Channel

A three-dimensional numerical model was applied to compute uniform and nonuniform sediment transport and bed deformation in an S-shaped laboratory channel located at the University of Innsbruck, where detailed measurements of the velocity field and bed elevation changes were made. The channel had two bends, a trapezoidal cross section, and a slope of S = 0.005. Gravel with a mean diameter of 4.2?mm was used as movable bed material and for sediment feeding. Wu’s formula for multiple grain sizes was compared with van Rijn’s formula using one grain size. Fairly good agreement was found between the computed and measured bed elevations for both approaches, whereas Wu’s formula could further improve the numerical results. Looking at the physics of the erosion pattern, the computed scour areas were located slightly more downstream than what was observed in the physical model. The current study also includes several parameter tests: grid distribution in vertical, lateral, and longitudinal direction; time step; number of inner iterations/time step; active sediment layer thickness; and the Shields coefficient. The variation of those parameters gave some differences in the results, but the overall pattern of bed elevation changes remained the same.     

Two-Phase Flow Analysis of Concentration Profiles

Two-phase flow analysis is used to analyze sediment concentration profiles in uniform open-channel flows over flat, sediment-starved beds that have high concentrations of single-sized sediment. Two-phase flow analysis can explicitly incorporate the effects of particle-particle interactions and particle inertia. Conventional convection-diffusion modeling cannot directly represent these phenomena and are thus limited. Both the two-phase flow formulation and the convection-diffusion modeling are compared against experimental data collected in sediment-starved sediment-laden flows. The two-phase flow model is shown to simulate the effect of both particle-particle interactions and particle inertia in these experimental flows. Simple criteria are given to determine when particle-particle interactions and particle inertia are important in sediment-laden open-channel flows over a flat bed. The current two-phase approach requires empirical formulas of the turbulence quantities and further experimental and analytical work is necessary to develop improved models for the velocity distribution and turbulence quantities.     

Sediment Threshold with Upward Seepage

An analytical model is presented to determine the threshold bed shear stress for noncohesive sediment motion subject to upward seepage on horizontal sedimentary bed under a stream flow. Hydrodynamic, seepage, and micromechanical forces acting on a solitary sediment particle, resting over a sedimentary bed under slip-spinning condition, are analyzed. The correlation coefficient between the results obtained using the present model and the experimental data of threshold bed shear stress with upward seepage on the horizontal bed is 0.767. It indicates that the model predicts satisfactorily the threshold bed shear stress with upward seepage.     

St. Venant–Exner Equations for Near-Critical and Transcritical Flows

The solution of the St. Venant–Exner equations as a model for bed evolution is studied under conditions when the Froude number, F, approaches unity, and the quasi-steady model becomes singular. It is confirmed that the strict criterion for critical flow, the vanishing of a water surface disturbance celerity, is not met, yet the direction of propagation of a bed wave apparently changes depending on whether F>1 or F<1. An analysis of the linearized model problem for an infinitesimal bed wave under near-uniform conditions is performed, and qualitative features of the solution are brought out. Under appropriate sediment transport conditions, when F2→1, two bed waves, one traveling upstream and the other traveling downstream, are found to develop from an initially single localized bed perturbation. Simulations of the full unsteady problem were performed with the Preissmann scheme to confirm the linear analysis and to study the effects of nonlinearity and friction. A transcritical case, in which a region where F2<1 is succeeded by a region where F2>1, is also investigated, and the solution exhibits an apparently different behavior than cases where the flow is everywhere sub- or supercritical, but can be understood as a hybrid of the latter cases.     

Sediment Threshold under Stream Flow on Horizontal and Sloping Beds

A model is presented to determine the threshold shear stress for noncohesive sediment (uniform and nonuniform) motion on horizontal and stream-wise sloping sedimentary beds, under a unidirectional steady-uniform streamflow. Hydrodynamic and particle-mechanic forces on a solitary sediment particle, resting over a sedimentary bed under the slip-spinning condition, are analyzed including the effect of turbulent fluctuations. Hydrodynamic forces such as drag, shear lift, and Magnus lift are taken into consideration. The drag coefficient is determined using an empirical formula. The inclusion of Magnus lift is significant because spherical particles spin just before dislodging downstream from their original position due to the differential hydrodynamic force along the vertical. The experimental data of sediment threshold are used to calibrate the model making the lift coefficient as a free parameter. The dependency of normalized threshold shear stress on particle parameter for various angles of repose and stream-wise bed slopes is presented graphically. The results obtained using the present model are compared with the curves proposed by different investigators and the experimental data of sediment (uniform and nonuniform) threshold for horizontal and stream-wise sloping beds.     

Numerical Model of Turbidity Currents with a Deforming Bottom Boundary

A numerical model of turbidity currents with a deforming bottom boundary has been developed. The model predicts the vertical structure of the flow velocity and concentration as well as change in the bed level due to erosion and deposition of suspended sediment. The Reynolds-averaged Navier–Stokes equations for dilute suspension have been solved using a finite volume method. The bottom boundary and the grid system are allowed to adjust in response to sediment deposition and entrainment during the computation. The model has been applied to simulate the evolution of a conservative saline density current and turbidity currents along an 11.6?m long flume that includes a slope followed by a horizontal bed. The model successfully simulates the evolution of the currents. Model results have been compared with the experimental data. Good similarity profiles of velocity and excess density or suspended sediment concentration are obtained at both the upstream supercritical and the downstream subcritical flow regions. A turbulent Schmidt number larger than one has been found to be appropriate for providing a good match with the experimental data. Changes in bed level predicted by the model have also been found to be in agreement with the experiment data.     

Exact Discontinuous Solutions of Exner’s Bed Evolution Model: Simple Theory for Sediment Bores

Determining the evolution of the bed of a river or channel due to the transport of sediment was first examined in a theoretical context by Exner in 1925. In his work, Exner presents a simplified bed evolution model derived from the conservation of fluid mass and an “erosion” equation that is commonly referred to as the sediment continuity or Exner equation. Given that Exner’s model takes the form of a nonlinear hyperbolic equation, one expects, depending on the given initial condition of the bed, the formation of discontinuities in the solution in finite time. The analytical solution provided by Exner for his model is the so-called classical or genuine solution of the initial-value problem, which is valid while the solution is continuous. In this paper, using the general theory of nonlinear hyperbolic equations, we consider generalized solutions of Exner’s classic bed evolution model thereby developing a simple theory for the formation and propagation of discontinuities in the bed or so-called sediment bores.     

Simulating Sediment Transport in a Flume with Forced Pool-Riffle Morphology: Examinations of Two One-Dimensional Numerical Models

One-dimensional numerical sediment transport models (DREAM-1 and DREAM-2) are used to simulate seven experimental runs designed to examine sediment pulse dynamics in a physical model of forced pool-riffle morphology. Comparisons with measured data indicate that DREAM-1 and -2 closely reproduce the sediment transport flux and channel bed adjustments following the introduction of fine and coarse sediment pulses, respectively. The cumulative sediment transport at the flume exit in a DREAM-1 simulation is within 10% of the measured values, and cumulative sediment transport at flume exit in a DREAM-2 simulation is within a factor of 2 of the measured values. Comparison of simulated and measured reach-averaged aggradation and degradation indicates that 84% of DREAM-1 simulation results have errors less than 3.3?mm, which is approximately 77% of the bed material geometric mean grain size or 3.7% of the average water depth. A similar reach-averaged comparison indicates that 84% of DREAM-2 simulation results have errors less than 7.0?mm, which is approximately 1.7 times the bed material geometric mean grain size or 11% of the average water depth. Simulations using measured thalweg profiles as the input for the initial model profile produced results with larger errors and unrealistic aggradation and degradation patterns, demonstrating that one-dimensional numerical sediment transport models need to be applied on a reach-averaged basis.     

Method of Solution of Nonuniform Flow with the Presence of Rectangular Side Weir

An iterative step method for solving the nonlinear ordinary differential equation, governing spatially varied flows with decreasing discharge, like the flow over side weirs, is developed. In the procedure, starting at a known flow depth and discharge in the control section, the analytical integration of the dynamic equation with bed and friction slope is carried out. The specific energy, the weir coefficient and the velocity distribution coefficient are considered as local variables, then for the explicit integration, the respective average values along the short side weir elements are assumed. The water surface profiles and the discharges for flow over side weirs, obtained with the proposed relation and valid for rectangular channels, are compared with experimental data for subcritical and supercritical flow conditions. The validation of the method is accomplished by the comparison with the solution obtained by De Marchi’s classical hypothesis, about the specific energy, which is constant along a side weir. In addition, the influence of the coefficient velocity distribution is considered.     

Finite-Volume Model for Shallow-Water Flooding of Arbitrary Topography

A model based on the finite-volume method is developed for unsteady, two-dimensional, shallow-water flow over arbitrary topography with moving lateral boundaries caused by flooding or recession. The model uses Roe’s approximate Riemann solver to compute fluxes, while the monotone upstream scheme for conservation laws and predictor-corrector time stepping are used to provide a second-order accurate solution that is free from spurious oscillations. A robust, novel procedure is presented to efficiently and accurately simulate the movement of a wet/dry boundary without diffusing it. In addition, a new technique is introduced to prevent numerical truncation errors due to the pressure and bed slope terms from artificially accelerating quiescent water over an arbitrary bed. Model predictions compare favorably with analytical solutions, experimental data, and other numerical solutions for one- and two-dimensional problems.     

Computational Dam-Break Hydraulics over Erodible Sediment Bed

This paper presents one of the first dedicated studies on mobile bed hydraulics of dam-break flow and the induced sediment transport and morphological evolution. A theoretical model is built upon the conservative laws of shallow water hydrodynamics, and a high-resolution numerical solution of the hyperbolic system is achieved using the total-variation-diminishing version of the second-order weighted average flux method in conjunction with the HLLC approximate Riemann solver and SUPERBEE limiter. It is found that a heavily concentrated and eroding wavefront first develops and then depresses gradually as it propagates downstream. In the early stage of the dam-break, a hydraulic jump is formed around the dam site due to rapid bed erosion, which attenuates progressively as it propagates upstream and eventually disappears. While the backward wave appears to migrate at the same speed as over a fixed bed, the propagation of the forward wavefront shows a complex picture compared to its fixed-bed counterpart as a result of the domination of rapid bed erosion initially, the density difference between the wavefront and the downstream ambient water in the intermediate period, and the pattern of the deformed bed profile in the long term. It is also found that the free surface profiles and hydrographs are greatly modified by bed mobility, which has considerable implications for flood prediction. The computed wave structure in the intermediate period exhibits great resemblance to available experiments qualitatively, and yet the existence of a shear wave is found in lieu of a secondary rarefaction postulated in an existing analysis. Finally, the use of the complete, rather than simplified, conservation equations is shown to be essential for correct resolution of the wave and bed structures, which suggests that previous models need reformulating.     

Predicting the Morphodynamic Response of Silt-Laden Rivers to Water and Sediment Release from Reservoirs: Lower Yellow River, China

The high sediment load of the Yellow River results in rapid infilling of its reservoirs when sediment is not regularly flushed. Simultaneously, the downstream reaches of the Yellow River experience extremely high siltation rates, which are reduced when sediment is retained in its reservoirs. To minimize siltation in the reservoirs and the downstream river bed, water and sediment are released from the reservoir in a controlled way through flushing experiments. In this paper, we analyze the effect of such a flushing event on the downstream river bed through data analysis and numerical modeling. Sedimentation may be minimized by relating the amount of sediment released from the reservoir to the sediment available for release through operational monitoring and by releasing relatively clear water after turbid water. Despite this flushing of sediment, the reservoir will eventually fill up, and more sediment released again into the lower Yellow River. The change in discharge magnitude and frequency brought about by the reservoir will then probably result in increased siltation rates in the lower Yellow River compared to the predam situation.     

Hydrodynamics of Turbid Underflows.?I: Formulation and Numerical Analysis

A mathematical model is developed for unsteady, two-dimensional, single-layer, depth-averaged turbid underflows driven by nonuniform, noncohesive sediment. The numerical solution is obtained by a high-resolution, total variation diminishing, finite-volume numerical model, which is known to capture sharp fronts accurately. The monotone upstream scheme for conservation laws is used in conjunction with predictor-corrector time-stepping to provide a second-order accurate solution. Flux-limiting is implemented to prevent the development of spurious oscillations near discontinuities. The model also possesses the capability to track the evolution and development of an erodible bed, due to sediment entrainment and deposition. This is accomplished by solving a bed-sediment conservation equation at each time step, independent of the hydrodynamic equations, with a predictor-corrector method. The model is verified by comparison to experimental data for currents driven by uniform and nonuniform sediment.     

Hydraulic Evaluation of W-Weir for River Restoration

Various structural measures have been advocated for river restoration and habitat improvement schemes. The W-weir is one such structure that can be used in mobile bed alluvial rivers to diversify habitat and provide grade control. Laboratory studies have been carried out in a large-scale meandering channel with a mobile bed to investigate their effects on flow and sediment transport processes. A W-weir placed immediately downstream of a riffle section created a strongly three-dimensional flow pattern and high-turbulence zones. Two adjacent scour holes of different depths and substrate are formed under clearwater and live bed conditions. The continuity of sediment transport along the channel was not interrupted by the structure and the upstream afflux is minimal. Overbank flow significantly influenced the action of the weir and the scour hole was shifted closer to the structure. In a relatively tight bend followed by a short crossover reach, the weir may affect bed load transport pathways in the downstream bend. Finally, the study provides insights to guide their design for restoration projects.     

A new closure methodology for 1D fully coupled models of mobile-bed alluvial hydraulics: application to silt transport in the Lower Yellow River

A new closure approach involving a common parameter has been incorporated into a 1D fully coupled model of mobile-bed alluvial hydraulics. The objective is to simplify the methodology of 1D river routing models and to improve their accuracy. The common parameter, called control factor m,introduces the concept of Rossiter modes in alluvial hydraulics and represents the interactions between the flow, the sediment transport and the bed morphology. The feasibility of the new closure approach has been established by reproducing numerically the 2002 silt flushing experiment conducted on the Lower Yellow River (LYR) downstream the Xiaolangdi reservoir. From the comparison between the experimental data and the numerical results, a time evolution of the control factor m reproducing the characteristics of the flow has been extracted. This time evolution agrees with analysis conducted previously on other datasets and with data measured during the flush. The results obtained with this time evolution for the hydraulics, the sediment transport and bed adaptation are encouraging but still need improvements and further feeding from complementary experimental data.     

Modeling Bed Changes in Meandering Rivers Using Triangular Finite Elements

A two-dimensional depth-averaged model was used for the simulation of scour and deposition in sand-bed meandering channels with fixed banks. The model employs unstructured meshes based on triangular elements and incorporates the effects of curvature-induced helical flow and transverse bed slope in the direction of bed-load sediment transport. The model was tested using experimental data from a well-known laboratory curved channel and a full scale meandering river. The numerical results agreed well with observed data, demonstrating that the model can reproduce the main features of bed profiles along meandering rivers, such as the formation of point bars and pools.     

Experimental Study of Particle Motion on a Smooth Bed under Shoaling Waves Using Particle Image Velocimetry

The motion of spherical particles (diameter 1.58 mm, specific gravity 2.5) on 2 and 3% plane slope was studied in a laboratory wave flume for shoaling wave conditions. The range of wave-height-to-water-depth ratio was 0.24相似文献   

Analytical Determination of Pollutant Wash-Off Parameters

The wash-off algorithm in the Storm Water Management Model has been widely used to simulate the wash off of sediment and pollutants from impervious areas. This algorithm indicates an exponential relationship between pollutant wash off and runoff volume. To simulate water quality, the initial mass of sediment or pollutant at the beginning of the storm and an empirical wash-off coefficient need to be determined. A simple analytical technique is developed for evaluating these parameters from measured water quality and runoff data. The proposed method is based on an expression of the exponential wash-off equation in terms of concentration rather than pollutant mass, and it can be applied to parameter evaluation without recourse to complex numerical models or optimization techniques.     

Effects of Lifting Force on Bed Topography and Bed-Surface Sediment Size in Channel Bend

In bed-load sediment transport, the lifting force plays an important role in reducing the friction between sediment particles and the bed surface, and it makes particle transportation by the shear force easier. Because the lifting force is related to vorticity, a three-dimensional (3D) numerical model incorporating large eddy simulations was applied to simulate the vorticity field in a channel bend. The results show that the distribution of vorticity is highly nonuniform, and it can lead to significant variations in lifting force and bed-load sediment transport per unit width in a channel bend. Relevant theories are modified on the basis of physical reasoning and then incorporated into numerical models to investigate the lifting-force effects on the bed topography and bed-surface sediment size gradation in a channel bend. With the lifting-force effects considered, it is shown that the errors in simulated bed topography can be reduced by approximately 40% and in bed-surface sediment size by 50%.