A double layer-averaged model for stratified sediment-laden flow in open channels

Sediment-laden flows in open channels can be sharply stratified vertically, characterized by a double-layer flow structure composed of a subaqueous sediment-laden flow layer immediately over the bed and an upper clear-water flow layer. Typical examples include dam-break flows and reservoir sediment-laden flows featuring turbidity currents. In general, sharply stratified sediment-laden flows involve a number of physical factors, including sharp flow stratification, inter-layer exchange, active sediment transport, and substantial mass exchange with the bed. Double layer-averaged models are attractive in modelling such flows in connection to its vertical structure. However, existing double layer-averaged models have either partly or completely ignored the primary features of stratified open-channel sediment-laden flows and thus are not generally suitable. In the present thesis, a two-dimensional double layer-averaged model has been developed, explicitly incorporating the fundamental physical factors and therefore generally applicable for sharply stratified sediment-laden flows in open channels. First, the governing equations of the new model and the employed numerical algorithm are presented. Then, the model is applied to investigate mobile-bed dam-break flows due to instantaneous full dam break and progressive failure of a dike and landslide dams. Enhanced performance of the new model is demonstrated over the previous models. Most notably, it clearly justifies the physical necessity to incorporate sediment mass conservation. Next, the proposed model is applied to investigate reservoir sediment-laden flows featuring turbidity currents. The model is benchmarked against turbidity currents due to lock-exchange and sustained inflow. It is revealed that an appropriate clear-water outflow is favorable for turbidity current propagation, and also conducive to improving sediment flushing efficiency. As applied to prototype-scale turbidity current in the Xiaolangdi Reservoir in the Yellow River, China, the model successfully resolves the whole process from formation to recession. Following that, the hyperbolicity of the model equations is analyzed as related to dam-break flows and reservoir turbidity currents. The present model is demonstrated to preserve hyperbolicity and thus avoid Kelvin-Helmholtz instability. Computational tests for reservoir turbidity currents reveal that an excessive clear-water outflow would keep the turbidity current from being spoiled, and improves sediment flushing efficiency correspondingly

Self-Aerated Flows on Chutes and Spillways: Closure

The writer thanks the discusser (ANWAR 1994) for his interesting comments and information. The writer wishes to clarify three points : free-surface instabilities, calculations of the inception point of air entrainment and analogy between self-aerated flows and sediment-laden flows

Observations in a sediment-laden flow by use of laser-doppler velocimetry

The laser-Doppler velocimetry technique was adapted for use in sediment-laden flows. The developed instrumentation was used to make one-dimensional, instantaneous measurements of both fluid and sediment grain velocities throughout the water column in such a flow. The velocimetry results were obtained in a steady, uniform flow over a natural sediment bed in the high-transport, flat bed regime. Laser-Doppler velocimetry is particularly attractive for use in sediment-laden flows as no calibration is required and no probe is introduced into the flow field. Measurements of the fluid velocity and the occurrence and velocity of individual sediment grains are possible with the instrumentation developed in this study. The major difficulties encountered are the possible conditional sampling, hence possible biasing, of the fluid velocity data and the failure of the instrumentation to record or resolve individual sediment grains at higher sediment transport rates. The instrumentation employed in this study is still in the developmental stages and suggestions for its improvement are given. Despite the difficulties encountered, the data obtained in this study give some insights into the mechanics of suspension and entrainment of sediment during transport by water. The longitudinal turbulence intensity does not seem to be significantly affected by the presence of suspended sediment; the turbulence intensities observed in the sediment-laden flow of this study do not differ greatly from the values reported by previous investigators for clear fluid flows. The mean and standard deviation of the sediment grain velocity were observed to be less than those for the fluid velocity in the lower portion of the flow, but respectively greater near the water surface. The data demonstrate the shortcoming of the continuum approach to the mechanics of the suspension on sediment. The length (or time) scales of the fluid turbulence are smaller than the length (or time) scale of a set of sediment grains required to define suspended sediment concentration. Near the water surface, where the velocimeter acts as a grain counter, the probability density functions of the sediment grain inter-arrival times, the time between the detection of successive sediment grains, were observed to be negative exponentials. The transport of individual sediment grains might be modeled as a Poisson process. This work is the foundation of an ongoing experimental program of direct measurements of the fine-scale, time-fluctuating characteristics of sediment-laden flows. This study developed and implemented instrumentation capable of making such measurements and established a conceptual framework for the subsequent interpretation of the data obtained. Two-dimensional measurements, with improved instrumentation, will give additional insights into the mechanics of sediment transport

Multimode morphodynamic model for sediment-laden flows and geomorphic impacts

Sediment-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 test cases. The results indicate that the model can attain good agreement with measured data, thereby demonstrating the capabilities of the multimode morphodynamic model system in predicting sediment-laden flows and resulting morphological change

Evaluating the effects of sediment transport on pipe flow resistance

In this paper, the applicability of a theoretical flow resistance law to sediment-laden flow in pipes is tested. At first, the incomplete self-similarity (ISS) theory is applied to deduce the velocity profile and the corresponding flow resistance law. Then the available database of measurements carried out by clear water and sediment-laden flows with sediments having a quasi-uniform sediment size and three different values of the mean particle diameter Dm (0.88 mm, 0.41 mm and 0.30 mm) are used to calibrate the Γparameter of the power-velocity profile). The fitting of the measured local velocity to the power distribution demonstrates that (i) for clear flow the exponent δ) can be estimated by the equation of Castaing et al. and (ii) for the sediment-laden flows δ is related to the diameter Dm. A relationship for estimating the parameter Гv obtained by the power-velocity profile) and that Гf of the flow resistance law) is theoretically deduced. The relationship between the parameter Гv, the head loss per unit length and the pipe flow Froude number is also obtained by the available sediment-laden pipe flow data. Finally, the procedure to estimate the Darcy–Weisbach friction factor is tested by the available measurements

Turbulence and turbulent transport in sediment-laden open-channel flows

Some aspects of turbulence in sediment-laden open-channel flows are examined. A conceptual model based on similarity hypotheses rather than the traditional mixing-length closures is proposed. It is argued that, over a wide range of laboratory conditions, the main effect of the suspended sediment on the flow is confined to a layer near the bed. If such a distinct layer can be discerned, then this is separated from the outer flow by an inertial subregion in which the mean-velocity profile is approximately logarithmic, with an associated von Kàrman constant of ≈ 0.4, i.e., the same value as in single-phase flows. It is further shown that power-law profiles may be derived from general similarity arguments and asymptotic matching. These implications contrast with those of previous models in which changes in the mean-velocity profile are supposed to occur throughout the flow or primarily in the flow far from the bed. Length and concentration scales appropriate to sediment-laden flows are suggested. An experimental study was also undertaken. Both the saturated case, in which a sand bed was present, and the unsaturated case, in which a sand bed was absent, were investigated. The study was restricted to nominally flat beds, composed of three well sorted sands (median grain diameters ranged from 0.15 mm to 0.24 mm). A two-component laser-Doppler-velocimetry system was used for velocity measurements. Suction sampling was used to measure local mean concentrations. The major points of the conceptual model are supported by the experimental results. Higher-order statistics of the velocity field were found to exhibit little evidence of any effect on the outer flow, supporting the view that the effect of the suspended sediment is felt primarily in the inner region. This contrasts with the predictions of recent models that propose an analogy between sediment-laden flows and weakly stable density-stratified flows

Approximate Solutions for Ideal Dam-Break Sediment-Laden Flows on Uniform Slopes

Shallow water hydro-sediment-morphodynamic (SHSM) models have been applied increasingly widely in hydraulic engineering and geomorphological studies over the past few decades. Analytical and approximate solutions are usually sought to verify such models and therefore confirm their credibility. Dam-break flows are often evoked because such flows normally feature shock waves and contact discontinuities that warrant refined numerical schemes to solve. While analytical and approximate solutions to clear-water dam-break flows have been available for some time, such solutions are rare for sediment transport in dam-break flows. Here we aim to derive approximate solutions for ideal dam-break sediment-laden flows resulting from the sudden release of a finite volume of frictionless, incompressible water-sediment mixture on a uniform slope. The approximate solutions are presented for three typical sediment transport scenarios, i.e., pure advection, pure sedimentation, and concurrent entrainment and deposition. Although the cases considered in this paper are not real, the approximate solutions derived facilitate suitable benchmark tests for evaluating SHSM models, especially presently when shock waves can be numerically resolved accurately with a suite of finite volume methods, while the accuracy of the numerical solutions of contact discontinuities in sediment transport remains generally poorer

Is the von Krmn constant affected by sediment suspension?

Is the von Krmn constant affected by sediment suspension? The presence of suspended sediment in channels and fluvial streams has been known for decades to affect turbulence transfer mechanism in sediment-laden flows, and, therefore, the transport and fate of sediments that determine the bathymetry of natural water courses. This study explores the density stratification effects on the turbulent velocity profile and its impact on the transport of sediment. There is as yet no consensus in the scientific community on the effect of sediment suspension on the von Krmn parameter, . Two different theories based on the empirical log-wake velocity profile are currently under debate: One supports a universal value of =0.41 and a strength of the wake, , that is affected by suspended sediment. The other suggests that both and could vary with suspended sediment. These different theories result in a conceptual problem regarding the effect of suspended sediment on , which has divided the research area. In this study, a new mixing length theory is proposed to describe theoretically the turbulent velocity profile. The analytical approach provides added insight defining as a turbulent parameter which varies with the distance to the bed in sediment-laden flows. The theory is compared with previous experimental data and simulations using a k-turbulence closure to the Reynolds averaged Navier Stokes equations model. The mixing length model indicates that the two contradictory theories incorporate the stratified flow effect into a different component of the log-wake law. The results of this work show that the log-wake fit with a reduced is the physically coherent approximation. © 2012. American Geophysical Union. All Rights Reserved.This research was partially funded by the project P08-AGR-03925 (Junta de Andalucía).Peer Reviewe

Characterization of wood-laden flows in rivers

Inorganic sediment is not the only solid‐fraction component of river flows; flows may also carry significant amounts of large organic material (i.e., large wood), but the characteristics of these wood‐laden flows (WLF) are not well understood yet. With the aim to shed light on these relatively unexamined phenomena, we collected home videos showing natural flows with wood as the main solid component. Analyses of these videos as well as the watersheds and streams where the videos were recorded allowed us to define for the first time WLF, describe the main characteristics of these flows and broaden the definition of wood transport regimes (adding a new regime called here hypercongested wood transport). According to our results, WLF may occur repeatedly, in a large range of catchment sizes, generally in steep, highly confined single thread channels in mountain areas. WLF are typically highly unsteady and the log motion is non‐uniform, as described for other inorganic sediment‐laden flows (e.g., debris flows). The conceptual integration of wood into our understanding of flow phenomena is illustrated by a novel classification defining the transition from clear water to hypercongested, wood and sediment‐laden flows, according to the composition of the mixture (sediment, wood, and water). We define the relevant metrics for the quantification and modelling of WLF, including an exhaustive discussion of different modelling approaches (i.e., Voellmy, Bingham and Manning) and provide a first attempt to simulate WLF. We draw attention to WLF phenomena to encourage further field, theoretical, and experimental investigations that may contribute to a better understanding of flows river basins, leading to more accurate predictions, and better hazard mitigation and management strategies