Modeling on Flash Flood Disaster Induced by Bed Load

Flash floods result from a complex interaction among hydro-meteorological, hydrological, and hydraulic processes across various spatial and temporal scales. Sichuan Province suffers flash floods frequently owing to mountain weather and topography. A flash flood and gravel bed load transport are two key relative problems in mountain river engineering. Bed materials are often encountered in alternate scouring and deposition in mountain fluvial processes during a flash flood. In this circumstance, CRS-1 bed load numerical model jointly with scale physical model is employed to predict water level and gravel bed scour and deposition for design of flood control dykes and flash flood disaster mitigation. A case study on the mechanism of a flash flood disaster induced by bed load transport for a hydropower station in Sichuan Province is conducted. Finally, suggestions to protect the hydropower station are proposed.

Non-capacity transport of non-uniform bed load sediment in alluvial rivers

Rivers often witness non-uniform bed load sedim ent transport. For a long tim e, non-uniform bed load transport has been assum ed to be at capacity regime determined exclusively by local flow. Yet whether the capacity assumption for non-uniform bed load transport is justified remains poorly understood. Here, the relative time scale of non-uniform bed load transport is evaluated and non-capacity and capacity models are compared for both aggradation and degradation cases with observed data. As characterized by its relative time scale, the adaptation of non-uniform bed load to capacity regime should be fulfilled quickly. However, changes in the flow and sedim ent inputs from upstream or tributaries hinder the adaptation. Also, the adaptation to capacity regime is size dependent, the finer the sediment size the slower the adaptation is, and vice versa. It is shown that the capacity model may entail considerable errors compared to the non-capacity model. For modelling of non-uniform bed load, non-capacity modelling is recommended, in which the temporal and spatial scales required for adaptation are explicitly appreciated.

Incipient Motion of Bed Load Sediment and Its Application in the Three Gorges Hydro Project

A formula for the calculation of critical shear stress of non-uniform sediment was developed by on river bed has been established while considering the relative degree expose of grains.The values of the Coeffi- cients in the formula have been obtained.The formula has been verified with prototype observation data, flumes data of both nature and lightweight sand.The computed results fully reflect the characteristics of thresh- old motion of non-uniform sediment.The result have been used in some physical mo...

An Investigation of Stochastic Nature of Bed Load Motion in Chuanjiang River

Bed Load Motion and its transport rate is one of the basic issues in river dynamics.In this paper, the authors discussed the stochastic nature of bed load motion in Chuanjiang River in details.Chungjiang lies in the upstream reach of Yangtze River.Its stochastic nature is shown in the following four aspects.Firstly, even though all the conditions are the same,due to the fluctuation of the flow,the bed load discharge and the location and width of sediment transport belts are different.Secondly,during the ...

Numerical Simulation of Bed Load and Suspended Load Sediment Transport Using Well-Balanced Numerical Schemes

Sediment transport can be modelled using hydrodynamic models based on shallow water equations coupled with the sediment concentration conservation equation and the bed con-servation equation.The complete system of equations is made up of the energy balance law and the Exner equations.The numerical solution for this complete system is done in a seg-regated manner.First,the hyperbolic part of the system of balance laws is solved using a finite volume scheme.Three ways to compute the numerical flux have been considered,the Q-scheme of van Leer,the HLLCS approximate Riemann solver,and the last one takes into account the presence of non-conservative products in the model.The discretisation of the source terms is carried out according to the numerical flux chosen.In the second stage,the bed conservation equation is solved by using the approximation computed for the system of balance laws.The numerical schemes have been validated making comparisons between the obtained numerical results and the experimental data for some physical experiments.The numerical results show a good agreement with the experimental data.

Development of All-Weather and Real-Time Bottom-Mounted Monitor of Bed Load Quantity

Quantity of bed load is an important physical parameter in sediment transport research. Aiming at the difficulties in the bed load measurement, this paper develops a bottom-mounted monitor to measure the bed load transport rate by adopting the sedimentation pit method and resolving such key problems as weighing and desilting, which can achieve long-time, all-weather and real-time telemeasurement of the bed load transport rate of plain rivers, estuaries and coasts. Both laboratory and field tests show that this monitor is reasonable in design, stable in properties and convenient in measurement, and it can be used to monitor the bed load transport rate in practical projects.

Concept of Bed Roughness Boundary Layer and Its Application to Bed Load Transport in Flow with Non-Submerged Vegetation

Ecosystem conservation has become one of the purposes in river management as well as flood mitigation and water resources management, and understanding of river flow and morphology in a stream with vegetation becomes important. Recently 2D depth averaged analysis is familiar even in a stream with vegetation by taking account of form drag due to vegetation. However, the shear stress in vegetated area is not properly described because the resistance law due to bed roughness is not reasonably modified in vegetated area. In this study, we discussed the bed roughness boundary layer in flow with non-submerged vegetation to deduce a reasonable relation between U and u* in vegetated area toward improving the analysis of sediment transport. The results show that the modification of resistance law using by thickness, velocity distribution in that layer was found to bring significant improvement of accurate estimation of shear velocity and subsequently the sediment transport. The proposed modification is improved by 2D depth averaged analysis based on this concept, and its application is certificated through flume experiment.

Comparison of depth-averaged concentration and bed load flux sediment transport models of dam-break flow

This paper presents numerical simulations of dam-break flow over a movable bed. Two different mathematical models were compared: a fully coupled formulation of shallow water equations with erosion and deposition terms(a depth-averaged concentration flux model), and shallow water equations with a fully coupled Exner equation(a bed load flux model). Both models were discretized using the cell-centered finite volume method, and a second-order Godunov-type scheme was used to solve the equations. The numerical flux was calculated using a Harten, Lax, and van Leer approximate Riemann solver with the contact wave restored(HLLC). A novel slope source term treatment that considers the density change was introduced to the depth-averaged concentration flux model to obtain higher-order accuracy. A source term that accounts for the sediment flux was added to the bed load flux model to reflect the influence of sediment movement on the momentum of the water. In a onedimensional test case, a sensitivity study on different model parameters was carried out. For the depth-averaged concentration flux model,Manning's coefficient and sediment porosity values showed an almost linear relationship with the bottom change, and for the bed load flux model, the sediment porosity was identified as the most sensitive parameter. The capabilities and limitations of both model concepts are demonstrated in a benchmark experimental test case dealing with dam-break flow over variable bed topography.

Bed Load Transport Rate in Scouring and Armoring Process of Non-uniform Sediment River Bed

Flume experiments were carried out to study bed load transport rate during rive bed scouring and ar- moring.A theoretical differential equation linking the transport rate to the probability of incipient motion of non-uniform sediment is solved.The transport rate is shown to decrease exponentially with time,according to the theory,which is in good agreement with the experiment data.

Evaluation and Improvement of Bed Load Formula Using Tapi River Data, India

The effect of non uniformity of bed material on the sediment transport has been studied by various investi-gators in the past. In the present paper the bed load transport rate has been estimated for non uniform bed material considering the various variables like discharge, hydraulic mean depth, flow velocity, bed slope, average diameter of particle etc. by collecting field data of Tapi River. The majority of the bed load formulae represent a functional relationship between bed load discharge and shear stress. This study focuses on evalu-ating the bed load using Einstein’s formulae. The bed load of pre monsoon season is estimated using various field parameters. The mathematical model has been developed using effective shear stress and bed load dis-charge. The statistical analysis, multiple regression and curve fitting (by nonlinear square fitter) is carried out using allometric function of Micro cal Origin 7.5. The proposed model has been tested using five years field data of Tapi River other than that used for the development of model. The value of rmse is close to zero in-dicates a perfect fit between measured and predicted values. The inequality coefficient is close to 0.50 sug-gest moderate relationship between estimated and computed bed load.

The Effect of Grain Properties for Artificial Bedload Supply

The German Armed Forces University in Munich has conducted experiments in a laboratory flume to determine the influence of roundness on bed load transport.The investigations were assigned by the German Federal Institute of Hydrology (BIG),with a focus on incipient motion,transport velocity and the depth of mixing into the riverbed. The results of the experiments show that the transport velocity of angular graim is lower and the critical shear stress for incipient motion is slightly higher than of well-ro...

Assessment of the fraction of bed load concentration towards the sediment transport of a monsoon-dominated river basin of Eastern India

Given the challenges of re-creating complex bed load(BL)transport processes in rivers,models are preferred over gathering and examining field data.The highlight of the present research is to develop an approach to determine the ungauged bed load concentration(BLC_(u))utilizing the measured suspended sediment concentration(SSC)and hydraulic variables of the last four decades for the Mahanadi River Basin.This technique employs shear stress and SSC equations for turbulent open channel flow.Besides,the predicted BLC_(u)is correlated with SSC using a power relation to estimate BLC_(u)on the river and tributaries.Eventually,different BL functions(BLF)efficiency is assessed across stations.The model predicted BLC_(u)is comparable with the published data for sandy rivers and falls within±20%.Outliers in hydraulic and sedimentological statistics significantly influence estimating the BL fraction apart from higher relative ratios and catchment geology.The constants of power functions are physically linked to sediment transport configuration,mechanism,and inflow to the stream.The stream power-based BLF best predicts the BL transport,followed by shear stress and unit discharge approaches.The disparity in the estimation of BLC_(u)results from station-specific physical factors,sampling data dispersion,and associated uncertainties.

The Bedload Movement in the Changjiang Estuary

Sandwaves in the Changjiang estuary were measured with a shallow sediment profiler and an echosounder from 1978 to 1988. The data, together with grain size and bedform of sediment indicates that the bedload movement by rolling and saltation is of great significance to sediment transport and is the principal factor responsible for sandwave and sandbody development in the estuary. The sandwaves were found well-developed, which is related to the tidal range and the velocity of ebb current. However, the further growth is restricted by strong flood current prevailing in the estuary. Because of the significant bedload, the sandbodies shift obviously and frequently, and sometimes the exchange of position occurs between the sandbodies and tidal channels. As a result, ships are regularly forced to change their navigation course.

CONCENTRATION DISTRIBUTION OF SEDIMENT IN BED LOAD LAYER

In this paper the concentration profile in bed load layer is derived based onkinetic theory. According to observations, particles moving in near wall region behave differentlyduring ejection and sweeping of turbulence burst, as indicates that they are subject to differentinfluences from turbulence, and therefore, the forces acting on particles are not the same.Consequently, particles moving in bed load layer are classified into two groups, one lifted upwardby ejections, the other carried back to bed by sweepings, and the forces corresponding to upward anddownward motions are proposed. By solving the basic transport equation of kinetic theory, thevelocity distribution functions, upward and downward fluxes of particles in bed load layer arederived. Upon assumption of e-quilibrium sediment transport, concentration profile in bed load layeris obtained. Verification is also presented in this paper, which shows that the concentrationprofile produced by the relation proposed in this paper agrees with observations well.

Prediction of bed load sediments using different artificial neural network models

Modeling and prediction of bed loads is an important but difficult issue in river engineering.The introduced empirical equations due to restricted applicability even in similar conditions provide different accuracies with each other and measured data.In this paper,three different artificial neural networks(ANNs)including multilayer percepterons,radial based function(RBF),and generalized feed forward neural network using five dominant parameters of bed load transport formulas for the Main Fork Red River in Idaho-USA were developed.The optimum models were found through 102 data sets of flow discharge,flow velocity,water surface slopes,flow depth,and mean grain size.The deficiency of empirical equations for this river by conducted comparison between measured and predicted values was approved where the ANN models presented more consistence and closer estimation to observed data.The coefficient of determination between measured and predicted values for empirical equations varied from 0.10 to 0.21 against the 0.93 to 0.98 in ANN models.The accuracy performance of all models was evaluated and interpreted using different statistical error criteria,analytical graphs and confusion matrixes.Although the ANN models predicted compatible outputs but the RBF with 79%correct classification rate corresponding to 0.191 nctwork error was outperform than others.

Study on bed load transport for uniform sediment in laminar flow

Previous studies focused on the bed load transport rate for the condition of turbulent flow,while the knowledge of sediment transport in laminar flow is very limited.As an extreme case to reflect the viscous effect on sediment transport,sediment transport in laminar flow is considered in this paper.There are at least two factors affecting the transport rate of sediment under laminar flow conditions: (1) fluid forces;(2) particle to particle interactions.Together,these two factors represent the physical transport system.First,an exposure degree Probability Density Function (PDF) is developed to explore how the transport rate can be associated with characteristics of laminar flow and this factor reflects the particle to particle interactions,and the pickup probability equation in the absence of turbulence is developed based on the stochastic approach which reflects the exposure degree influence.Then,the formulas to calculate the critical shear stress of incipient motion and the bed load transport rate of fine uniform sediment are established.The derivation is made mainly based on Einstein’s bed load theory;we choose Einstein’s equation to model this system because we believe that the probabilistic approach taken is an appropriate way to account for the spatial and temporal variations in the forces causing sediment transport.These formulas have been tested against a wide range of existing laboratory data and compared with other existing empirical or semiempirical methods.The predictions by these newly proposed formulas are very good.

Nonlinear dynamical characteristics of bed load motion

Bed forms of various kinds that evolve naturally on the bottom of sandy coasts and rivers are a result of the kinematics of bed load transport. Based on the group motion of particles in the bed load within the bottom layer, a study on the nonlinear dy- namics of bed load transport is presented in this paper. It is found that some development stages, such as the initiation, the equilibrium sediment transport, and the transition from a smooth bed to sand dunes, can be accounted for by different states in the nonlinear sys- tem of the bed load transport. It is verified by comparison with experimental data reported by Laboratoire Nationae D’Hydraulique, Chatou, France, that the evolution from a smooth bed to sand dunes is determined by mutation in the bed load transport. This paper pre- sents results that may offer theoretical explanations to the experimental observations. It is also an attempt to apply the state-of-the-art nonlinear science to the classical sediment transport mechanics.

Experimental study on bed-load sediment transport under irregular wave and current combined flow

Using an irregularly oscillating tray and flume, a series of experiments are completed to evaluate bed-load sediment transport rate under irregular wave -current coexistent field. Testing conditions include three interaction angles 0', 45', 90' and two kinds of median sizes (0.38 and 1.10 mm). The results of transport rate show that the net sediment transport rate can be expressed approximately as the function of the maximum bottom shear stress of waves, mean shear stress of current and the grain size.

Experiment and simulation on the pyrite removal from the recirculating load of pulverizer with a dilute phase gas-solid fluidized bed

In order to reduce the energy consumption and subsequent air pollution of coal-fired power station, based on the analysis to size and density distribution of particles from the recirculating load of the classifier of pulverizer, the separation experiment on sampling material from power plant with a dilute phase fluidized bed to remove pyrite and other minerals and numerical simulation on the separation process were done. The results show that the minimum fluidization velocity is 1.62 cm/s. Pyrite and other minerals in the material are separated. Ash of the upper and bottom layer material account for 33.34% and 73.42% respectively and sulfur content occupy 1.12% and 8.96% respectively. Scanning electron microscopy and spectroscopy tests show that sulfur in the bottom material exist in the form of pyrite. Numerical simulation on the flow field form of the dilute phase separation bed with gas-solid two phase and particle motion verifies the experimental results.

A numerical simulation study on mechanical behaviour of coal with bedding planes under coupled static and dynamic load

To investigate the bedding influence on coal mechanical behaviour in underground environments such as coal or rock burst, simulations of dynamic SHPB tests of pre-stressed coal specimens with different bedding angles were carried out using a particle flow code 2-dimensional(PFC2D). Three impact velocities of 4, 8 and 12 m/s were selected to study dynamic behaviours of coal containing bedding planes under different dynamic loads. The simulation results showed that the existence of bedding planes leads to the degradation of the mechanical properties and their weakening effect significantly depends on the angle h between the bedding planes and load direction. With h increaseing from 0° to 90°, the strength first decreased and subsequently increased and specimens became most vulnerable when h was 30° or 45°.Five failure modes were observed in the specimens in the context of macro-cracks. Furthermore, energy characteristics combined with ultimate failure patterns revealed that maximum accumulated energy and failure intensity have a positive relation with the strength of specimen. When bedding planes were parallel or perpendicular to loading direction, specimens absorbed more energy and experienced more violent failure with increased number of cracks. In contrast, bedding planes with h of 30° or 45° reduced the specimens' ability of storing strain energy to the lowest with fewer cracks observed after failure.