Sediment-laden Flows Research Articles

Multimode Morphodynamic Model for Sediment-Laden Flows and Geomorphic Impacts

AbstractSediment-laden flows are a complex solid-fluid interaction process. This study presents a multimode morphodynamic model system combined with shallow water theory and a nonequilibrium assumption for sediment transport. The model system aims to simulate the morphological change caused by sediment-laden flows with various sediment transport modes. It involves three modules: a hydrodynamic module, a sediment transport module, and a morphological evolution module. The hydrodynamic model is governed by modified shallow water equations considering the interaction effects of flow and sediment. A flexible sediment transport model is presented that incorporates a weight coefficient. The model can adaptively choose an appropriate transport mode according to local, real-time flow conditions. Bedload, suspended load, and total mixed sediment load are all involved. The model is solved by a second-order Godunov-type finite-volume method that is robust and accurate. Validation is demonstrated through a series of ...

Equilibrium sediment transport in lower Yellow River during later sediment-retaining period of Xiaolangdi Reservoir

The Xiaolangdi Reservoir has entered the later sediment-retaining period, and new sediment transport phenomena and channel re-establishing behaviors are appearing. A physical model test was used to forecast the scouring and silting trends of the lower Yellow River. Based on water and sediment data from the lower Yellow River during the period from 1960 to 2012, and using a statistical method, this paper analyzed the sediment transport in sediment-laden flows with different discharges and sediment concentrations in the lower Yellow River. The results show that rational water-sediment regulation is necessary to avoid silting in the later sediment-retaining period. The combination of 3 000 m3/s < Q < 4 000 m3/s and 20 kg/m3 < S < 60 kg/m3 (where Q is the discharge and S is the sediment concentration) at the Huayuankou section is considered an optimal combination for equilibrium sediment transport in the lower Yellow River over a long period of time.

Mechanism of downcutting erosion of debris flow over a movable bed

The phenomenon of debris flow is intermediate between mass movement and solid transport. Flows can be sudden, severe and destructive. Understanding debris flow erosion processes is the key to providing geomorphic explanations, but progress has been limited because the physical-mechanical properties, movement laws and erosion characteristics are different from those of sediment-laden flow. Using infinite slope theory, this research examines the process and mechanism of downcutting erosion over a moveable bed in a viscous debris flow gully. It focuses specifically on the scour depth and the critical slope for viscous debris flow, and formulas for both calculations are presented. Both scour depth and the critical conditions of downcutting erosion are related to debris flow properties (sand volume concentration and flow depth) and gully properties (longitudinal slope, viscous and internal friction angle of gully materials, and coefficient of kinetic friction). In addition, a series of flume experiments was carried out to characterize the scouring process of debris flows with different properties. The calculated values agreed well with the experimental data. These theoretical formulas are reasonable, and using infinite slope theory to analyze down cutting erosion from viscous debris flow is feasible.

Acoustic Imaging Of Experimental Subaqueous Sediment-Laden Flows And Their Deposits

Abstract This paper presents a non-invasive acoustic reflection technique to visualize sediment-laden flows over a wide range of sediment concentrations (5–75% by volume) in the laboratory. A Panametrics V301-SU Immersion transducer with 0.5 MHz center frequency was used to generate acoustic reflections that were sampled at 10 MHz. The strength of this technique is the direct characterization of both the internal structure of sediment-laden flows and their deposits, especially for high-concentration conditions. A combination of reflection continuity, amplitude, and mutual coherence were used to identify three distinct acoustic facies: short-scatter, long-scatter, and continuous-reflection. Here we present two examples in which the acoustic facies allows the interpretation of (a) the temporal evolution of internal mixing and stratification within a flow and contemporaneous deposition of its sediment and (b) the horizontal and vertical partitioning of mud and sand within a deposit generated by a sediment-laden flow.

The 1988 glacial lake outburst flood in Guangxieco Lake, Tibet, China

Abstract. The 1988 glacial lake outburst flood (GLOF) in Guangxieco Lake is studied based on geomorphological evidence, interviews with local residents, field surveys in 1990 and 2007, and satellite images from different years. The findings are as follows. (1) The outburst event was caused by two major factors, namely, intense pre-precipitation and persistent high temperatures before the outburst and the low self-stability of the terminal moraine dam as a result of perennial piping. (2) The GLOF, with the peak discharge rate of 1270 m3 s−1, evolved along Midui Valley in the following order: sediment-laden flow, viscous debris flow, non-viscous debris flow, and sediment-laden flood, which was eventually blocked by Palongzangbu River. (3) A comparison between the conditions during the outburst in 1988 and the present conditions suggests a small possibility of a future outburst unless drastic changes occur in landscape and climate. Reconstructing the outburst conditions and the GLOF processes is helpful in assessing a potential outburst in glacier lakes in Tibet.

An experimental study of fluvial processes at asymmetrical river confluences with hyperconcentrated tributary flows

This paper reports findings from experimental studies of sediment transport and bed morphology at asymmetrical confluences with hyperconcentrated tributary flows in the upper Yellow River. The results indicate that the hyperconcentrated flow confluence can be divided into four hydraulic regions, including the backwater zone above the upstream junction corner, the maximum velocity area, the separation flow zone, and the post-confluence region downstream of the junction corner. The bed morphology also consists of four basic elements, including the sandbar in the backwater zone, the bar in the separation flow zone, the thalweg for flow conveyance and sediment transport, and bars in the reach downstream of the separation zone. The sediment load of the hyperconcentrated flow from the tributary was the most important control on fluvial processes at such confluences. The increase in deposition in the backwater zone as the sediment load increased was almost linear, and the depth of sediment deposition in the backwater zone was approximately normal in distribution. The validity of a conceptual model for discriminating the status of the backwater effect, developed earlier from field data using the relationship between the sediment load and water volume of hyperconcentrated flows, was confirmed by the experiments. Deposition in the reach downstream of the junction, sandbar height in the backwater zone, and the width and length of the separation zone bar all tended to increase as the sediment load in the tributary increased. An obvious upstream-directed density current occurred in the backwater zone when the sediment concentration of the hyperconcentrated flow exceeded a critical value. The travel distance of the density current increased as the sediment load in the tributary increased. A formula was proposed, based on sediment continuity, for estimating the deposition volume in the reach downstream of the junction. Compared with ordinary sediment-laden flow confluences, hyperconcentrated flow confluences have a sandbar in the backwater zone associated with an upstream-directed density current that may sometimes block the main channel. Hyperconcentrated flow confluences have a thalweg, and so are different from debris flow confluences, which have a fan-shaped deposit.

Whole-Process Modeling of Reservoir Turbidity Currents by a Double Layer-Averaged Model

Turbidity current is formed as subaerial open-channel sediment-laden flow plunges into a reservoir. The whole process of reservoir turbidity current, i.e., formation, propagation, and recession, is generally controlled by the water and sediment inputs from upstream and also the reservoir operation scheme specifying the downstream boundary condition. Enhanced understanding of reservoir turbidity current is critical to effective sediment management in alluvial rivers. However, until now there has been a lack of physically based and practically feasible models for resolving the whole process of reservoir turbidity current. This is because the computing cost of three-dimensional modeling is excessively high. Also, single layer-averaged models cannot resolve the formation process characterized by the transition from open-channel sediment-laden flow to subaqueous turbidity current, or the upper clear-water flow as dictated by the operation scheme of the reservoir, which has significant impacts on turbidity current. Here a new two-dimensional double layer-averaged model is proposed to facilitate for the first time whole-process modeling of reservoir turbidity current. The two hyperbolic systems of the governing equations for the two layers are solved separately and synchronously. The model is well balanced because the interlayer interactions are negligible compared with inertia and gravitation, featuring a reasonable balance between the flux gradients and the bed or interface slope source terms and thus applicable to irregular topographies. The model is benchmarked against a spectrum of experimental cases, including turbidity currents attributable to lock-exchange and sustained inflow. It is revealed that an appropriate clear-water outflow is favorable for turbidity current propagation and conducive to improving sediment flushing efficiency. This is significant for optimizing reservoir operation schemes. As applied to turbidity current in the Xiaolangdi Reservoir in the Yellow River, China, the model successfully resolves the whole process from formation to recession. The present work facilitates a viable and promising framework for whole-process modeling of turbidity currents, in support of reservoir sediment management. (C) 2014 American Society of Civil Engineers.

Stochastic formulation of particle kinetics in wall-bounded two-phase flows

This paper presents a generalized framework of stochastic modeling for particle kinetics in wall-bounded flow. We modified a reflected Brownian motion process and straightforwardly obtained a Kramers equation for particle probability density function (PDF). After the wall effects were accounted for as a drift from zero in the mean displacement and suppression in the diffusivity of a particle, an analytical solution was worked out for PDF. Three distinguishable mechanisms were identified to affect the profile of particle probability distribution: external forces, turbophoresis effect, and wall-drift effect. The proposed formulation covers the Huang et al. (2009) model of a wall that produces electrostatic repulsion force and van der Waals force, as well as Monte-Carlo solutions for the Peter and Barenbrug (2002) model under a variety of relaxation times. Moreover, it successfully reproduces the two patterns of particle concentration profiles observed in experiments of sediment-laden open-channel flows. The strength of the wall-drift effect was found to be connected with the interaction frequency between particle and wall. Further exploration of the relationship among flow turbulence, particle inertia, and particle concentration is worthwhile.

Impacts of Flocculation on Sediment Basin Performance and Design

<abstract> <bold><sc>Abstract.</sc></bold> There is increasing interest in controlling turbidity in construction site runoff using chemical flocculant treatments. Since flocculated sediment is likely to behave much differently from untreated sediment, changes to current sediment basin designs may be appropriate. This study evaluated a system consisting of three fiber check dams in a lined ditch discharging to sediment basins that differed in their configuration. Three different basin configurations were tested with and without granular polyacrylamide (PAM) applied to the weir of each check dam: (1) standard basin with a 2:1 length to width (L/W) ratio, (2) horizontal basin with a 1:2 L/W ratio, and (3) standard basin with a rising floor toward the exit (spillway). All configurations included two porous baffles of jute/coir netting across the full width of the basin. For each treatment of PAM and basin configuration, sediment-laden stormwater flows (0.014 to 0.056 m³ s^-1) were introduced to the ditch for 29 min, and water samples were collected at the ditch entrance (influent), ditch exit, and basin exit. Regardless of PAM treatment, total suspended solids (TSS) concentrations were reduced by more than 55% at the ditch exit and by up to 90% at the basin exit relative to the influent (3,700 mg L^-1). The 1:2 L/W basin reduced TSS more than either of the 2:1 L/W basins without flocculation, but there was no significant difference in TSS when sediment was treated with PAM. Turbidity at the ditch exit was similar to the influent (less than 10% difference) without flocculation but was greatly reduced (>66%) with flocculation. The PAM treatment lowered turbidity further (>88%) at the basin exit and was similar among all basin configurations. The particle size distribution of flocculated sediment was shifted into coarser fractions, enhancing settling in the basin. The estimation of basin surface area requirement based on the measured particle size suggested that basins receiving flocculated sediment could be reduced in surface area and altered in configuration while improving water quality for construction site discharges.

Seismic constraints on dynamic links between geomorphic processes and routing of sediment in a steep mountain catchment

Abstract. Landscape dynamics are determined by interactions amongst geomorphic processes. These interactions allow the effects of tectonic, climatic and seismic perturbations to propagate across topographic domains, and permit the impacts of geomorphic process events to radiate from their point of origin. Visual remote sensing and in situ observations do not fully resolve the spatiotemporal patterns of surface processes in a landscape. As a result, the mechanisms and scales of geomorphic connectivity are poorly understood. Because many surface processes emit seismic signals, seismology can determine their type, location and timing with a resolution that reveals the operation of integral landscapes. Using seismic records, we show how hillslopes and channels in an Alpine catchment are interconnected to produce evolving, sediment-laden flows. This is done for a convective storm, which triggered a sequence of hillslope processes and debris flows. We observe the evolution of these process events and explore the operation of two-way links between mass wasting and channel processes, which are fundamental to the dynamics of most erosional landscapes. We also track the characteristics and propagation of flows along the debris flow channel, relating changes of observed energy to the deposition/mobilization of sediments, and using the spectral content of debris flow seismic signals to qualitatively infer sediment characteristics and channel abrasion potential. This seismological approach can help to test theoretical concepts of landscape dynamics and yield understanding of the nature and efficiency of links between individual geomorphic processes, which is required to accurately model landscape dynamics under changing tectonic or climatic conditions and to anticipate the natural hazard risk associated with specific meteorological events.

Effects of Rigid Vegetation on the Turbulence Characteristics in Sediment-Laden Flows

The effects of rigid vegetation on the turbulence characteristics were experimentally studied in the interior water flume. An ADV was used to determine the three dimensional turbulent velocities in clear water flow without vegetation, sediment-laden flow without vegetation, sediment-laden flow with submerged vegetation and sediment-laden flow with non-submerged vegetation. By experimental and theoretical analysis, the effects of rigid vegetation on the distribution of averaged velocities, turbulence intensities and Reynolds stress were summarized. In sediment-laden flow with submerged vegetation, the averaged stream wise velocities above the top of vegetation fit well with the log distribution low. The three-dimensional turbulence intensities increase from the bottom until they reach the maximum at the top of the vegetation. The method to calculate the shear velocity with the maximum of the Reynolds stress is recommended. In sediment-laden flow with non-submerged vegetation, the turbulence problems cannot be explained by theory of bed shear flow. The average velocities, turbulence intensities and Reynolds stress approximate uniformly distributed along vertical direction.

Numerical Investigation of Sediment-Laden Flow in a High-Head Francis Turbine Runner

The 3-D turbulent particulate-liquid two-phase flow equations are employed in this study. The computing domain is discretized with a full three-dimensional mesh system of unstructured tetrahedral shapes. The finite volume method is used to solve the governing equations and the pressure-velocity coupling is handled via a Pressure Implicit with Splitting of Operators (PISO) procedure. Simulation results have shown that the sand erosion rate on pressure side is more than on the suction side of the blade. The maximum value of sand volume fraction and the maximum value of sand erosion rate are at same location.

A note on acoustic measurements of turbulence, suspended sediment, and bed forms in mobile-bed experiments

Mobile-bed laboratory measurements allow observation of interactions between turbulent sediment-laden flows and irregular, non-stationary bed topography. Water in mobile-bed experiments is typically clouded by suspended particles, complicating the use of optical and laser based measurement techniques. In such conditions, acoustic techniques can be used to measure turbulence, suspended sediment transport, and bed topography; however, these procedures also have unique challenges. Here, the effect of suspended particles on acoustic Doppler turbulence measurement, the effect of particle size on acoustic measurement of suspended-sediments, and a method for statistically describing bed topography (measured acoustically or otherwise) are addressed. It is found that suspended particles have a small effect on Reynolds stresses measured with an acoustic Doppler velocimeter and that those effects could be minimized by excluding data collected with the highest and lowest acoustic amplitudes. The effect of particle size on acoustic backscatter amplitude is most strongly pronounced for particles smaller than 0.3 mm. Semivariogram analysis is shown to be a viable method for analyzing complex bed topography produced by mobile-bed experiments.

Editorial

Editorial

Inclined, collisional sediment transport

We apply the constitutive relations of kinetic theory of granular gases to the transport of cohesionless sediments driven by a gravitational liquid turbulent stream in steady uniform conditions. The sediment-laden flow forms self-equilibrated mechanisms of resistance at the bed surface, below which the sediments are at rest. This geo-physical process takes place quite often in streams at moderate slope and may be interpreted through tools common to fluid mechanics and particle physics. Taking into account the viscous dissipation of the fluctuation energy of the particles, and using approximate methods of integration of the governing differential equations, permit to obtain a set of simple formulas for predicting how depths and flow rates adjust to the angle of inclination of the bed, without requiring additional tuning parameters besides the particle and fluid properties. The agreement with laboratory experiments performed with either plastic cylinders or gravel in water is remarkable. We also provide quantitative criteria to determine the range of validity of the theory, i.e., the values of the Shields number and the angle of inclination of the bed for which the particle stresses can be mostly ascribed to collisional exchange of momentum.

Numerical prediction of turbidity currents on unsteady sediment-supply

A three-dimensional (3D) numerical model of unstable turbidity currents is developed based on the mechanism of sediment transport and turbulence theory. In this model, numerical simulation of turbidity currents without subsequent supply of muddy water was conducted using the same parameters as were used in the flume experiments. The evolution process of turbidity currents of completely losing supplies observed in the experiment was simulated by the model; validation of the numerical model and the algorithm was conducted. If momentarily interrupted process is regarded as a special case of the gradually interrupted, based on the preceding numerical simulation validity, it is feasible to simulate the motion law of turbidity currents under losing gradually supplies. By this method in this article, the characteristic of sediment-laden flow of losing gradually supplies was obtained, as well as its relationship between front velocity and sediment concentration.

A double layer-averaged model for dam-break flows over mobile bed

Dam-break flows over mobile bed are often sharply stratified, comprising a bedload sediment-laden layer and an upper clear-water layer. Double layer-averaged (DL) models are attractive for modelling such flows due to the balance between the computing cost and the ability to represent stratification. However, existing DL models are oversimplified as sediment concentration in the sediment-laden layer is presumed constant, which is not generally justified. Here a new DL model is presented, explicitly incorporating the sediment mass conservation law in lieu of the assumption of constant sediment concentration. The two hyperbolic systems of the governing equations for the two layers are solved separately and simultaneously. The new model is demonstrated to agree with the experimental measurements of instant and progressive dam-break floods better than a simplified double layer-averaged model and a single layer-averaged model. It shows promise for applications to sharply stratified sediment-laden flows over mobile bed.

Estimation of Suspended Sediment Concentrations with Adcp in Danube River

Abstract An estimation procedure for suspended sediment concentrations based on the intensity of backscattered sound of acoustic Doppler current profilers (ADCP) is introduced in this paper. Based on detailed moving and fixed boat ADCP measurements with concurrent sediment sampling, we have successfully calibrated the estimation method for a reach of River Danube in Hungary, characterized by significant suspended sediment transport. The effect of measurement uncertainty and various data filtering on sediment load determination is also analyzed and quantified. Some of the physical model parameters describing the propagation of sound in water are estimated based on known empirical formulas, while other parameters are derived from measured. Regression analysis is used to obtain a relationship between the intensity of backscattered sound and sediment concentrations. The empirical relationship has been then used to estimate the suspended sediment concentrations from the ADCP data collected in fixed and moving boat measurement operation mode, along verticals and path-lines, respectively. We show that while some measurement uncertainty is inherent to the acoustic Doppler principle, it is further enhanced by the complexity of the near-bottom sediment-laden flow. This uncertainty has then a significant effect on the local sediment load estimation. In turn, reasonable smoothing of raw velocity and backscatter intensity data shows insignificant impact on cross-sectional sediment load estimation.

Determining the viscosity and yield surface of marine sediments using modified Bingham models

The rheology of marine sediment-laden flows in a submarine environment is examined. The viscosity and yield stress of marine sediments are key parameters controlling landslide mobility: the former is related to the peak velocity and the latter is related to the final deposition of debris flow. One of the simplest rheological models is the ideal plastic Bingham model, which is often applied to diverse materials. For marine sediments, the flow behavior can be approximated by modified Bingham models, such as the bi-viscosity and Papanastasiou models. The flow of viscoplastic materials can then be described by unyielded (local shear stress τ is lower than a yield stress τ y ), yield surface (τ = τ y ), and post-yield (τ > τ y ) characteristics. It is concluded that modified Bingham models with a proper choice of flow parameters can be used to predict the unyielded, yielding and yielded zones for marine sediments. It is also demonstrated that in some cases the yield surface can be effectively obtained using the relationship between viscosity and shear rate.

Influence of Lift Force on the Settling Velocities of Rotating Particles in Two-Dimensional Shear Flow

AbstractWe derive a simple theoretical model for estimating the change in the magnitude of the particle slip velocity and the deflection in the trajectory of a particle induced by the lift force acting on a freely rotating particle settling under gravity in a shear flow. The lift force on the particle in small, intermediate, and large particle Reynolds number, is studied. We have found that the magnitude of the slip velocity is always reduced by the lift force, implying reduced particle settling velocities. The reduction in the slip velocity and the angle of deflection are the greatest when the shear is the greatest and the particle Reynolds number is the smallest. However, these effects decrease as the particle Reynolds number becomes larger. The derived equations are applied to evaluate the effect of the lift force on particle transport within turbidity currents, on the entrainment of small particles embedded in the lower boundary of a shear flow, and on sediment-laden open-channel flow.