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.

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.

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.

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.

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.

ANALYSIS OF SALTATION CHARACTERISTICS OF BED-LOAD TRANSPORT IN FLOWING WATER

The study of bed-load transport is of great significance both in theory and in practice.This paper discusses the saltation of bed-load solid grains in flowing water.Experiments and theoretic analysis have been made by means of high-speed photographing and advanced data processing technique with a combined method based on mechanical and statistical theories.It indicates that the saltation is the main form of the bed-load transport under ordinary flowing conditions.In the meantime,taking suecessive saltation as the mod- el of bed-load transport,systematic analysis has been made with regard to the kinematic properties and mechanism of saltation.The statistical analysis shows that the probability density functions of the relative height and length of saltation are in conformity with Γ-type distribution,while the probability density func- tions of the relative velocities of saltation are in conformity with the Gaussian distribution.

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.

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...

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.

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.

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 ...

金沙江下游三堆子河段卵石推移质输沙率公式研究

金沙江上中游梯级水库运用后,进入下游的悬移质输沙量大幅减少,水沙条件显著变化;同时,卵石推移质输沙量在总输沙量中的比重明显增加,对水库淤积进程影响显著,开展变化水沙条件下金沙江下游卵石推移质输移规律研究具有重要意义。基于下游入口三堆子水文站2008—2021年卵石推移质实测资料,对经典起动流速公式和推移质输沙率公式进行了检验与修正。结果表明:与经典起动流速公式计算结果相比,通过引入以卵石粒径为变量的系数,所得到的不同粒径范围的卵石推移质起动流速修正公式,计算精度显著提高;经典推移质输沙公式的计算结果在输沙强度较低时与实测值偏差很大,通过建立输沙强度与水流强度的关系,并引入隐暴参数对公式进行修正,明显提升了其在低输沙率情况下的计算精度。修正后的起动流速公式和推移质输沙率公式能更有效地反映三堆子河段卵石推移质的输移规律。

典型非均匀推移质输沙率公式适用性比较

基于等中值粒径的“单、双峰型级配”推移质输沙试验以及“驼峰”输沙率试验数据,选择Einstein、吴伟明、Wilcock和吴保生等4种典型非均匀推移质输沙率公式计算“单、双峰型级配”推移质输沙率,将两种级配的输沙率比值及曲线形态与试验成果对比分析。结果表明:吴伟明公式在反映“单、双峰型级配”推移质输沙差异上表现最好,这4种公式均无法反映“驼峰”输沙率现象。基于多参数考虑的Patel公式较这4种公式能更好地反映上述两种现象。充分考虑床沙级配对分组沙起动切应力和输沙有效Shields数的影响,是提高非均匀推移质输沙率公式精度和适用性的关键。

Numerical simulation of sediment transport in coastal waves and wave-induced currents

Prediction of coastal sediment transport is of particularly importance for analyzing coast erosion accurately and solving the corresponding coast protection engineering problems.The present study provided a numerical scheme for sediment transport in coastal waves and wave-induced currents.In the scheme,the sand transport model was implemented with wave refraction-diffraction model and near-shore current model.Coastal water wave was simulated by using the parabolic mild-slope equation in which wave refraction,diffraction and breaking effects are considered.Wave-induced current was simulated by using the nonlinear shallow water equations in which wave provides radiation stresses for driving current.Then,sediment transport in waves and wave-induced currents was simulated by using the two-dimensional suspended sediment transport equations for suspended sediment and the bed-load transport equation for bed load.The numerical scheme was validated by experiment results from the Large-scale Sediment Transport Facility at the US Army Corps of Engineer Research and Development Center in Vicksburg.The numerical results showed that the present scheme is an effective tool for modeling coastal sediment transport in waves and near-shore currents.

粗化床面发展过程中非均匀沙起动试验研究

非均匀沙在水流作用下会形成表层粒径大于底层的床面粗化现象,同时表层粗颗粒还会形成聚集结构体。已有非均匀沙起动研究仅考虑粒径差异,对多颗粒组成的粗化结构体及其影响问题考虑不足。在水槽中开展了非均匀沙床面粗化发展过程试验,分析了粗化床面条件下的起动条件及起动判别标准。结果表明,粗化床面发展可分为中粗颗粒出露、中颗粒结构体形成、粗颗粒结构体形成等3个阶段,粗化结构体对泥沙颗粒的起动和输移造成较大影响。粗化床面条件下非均匀沙起动临界希尔兹参数为0.015 5~0.023 5,小于均匀沙床面。USACE输沙率标准不适用于非均匀沙粗化床面,而Parker标准与韩其为标准均可在一定程度上反映非均匀沙粗化对泥沙起动的影响,其中韩其为标准与实测值更接近。

河床表层冻融作用下推移质输移特征研究

高纬度河流系统常会经历季节性冻结与融化。为揭示冻融作用对床面推移质运动的影响,本研究以未经历冻融过程的常规室温(20℃)推移质输沙实验结果为基准,保证实验条件不变,开展了均匀沙与非均匀沙床表层冻融后的室温(20℃)推移质输沙实验,综合比较了冻融作用影响下推移质输沙率与级配组成特征。结果表明,河床表层冻融后的推移质输沙率均高于无冻融作用的推移质输沙率,据此提出了有冻融影响与无冻融影响下的推移质输沙率比值ζ。研究显示,冻融作用下的推移质输沙率呈有规律变化:剪切力较小时ζ较大,剪切力较大时ζ减小并逐渐趋于1.0;非均匀沙中,随着泥沙粒径增大,各粒径组的ζ逐渐减小;低流量驱动时,冻融作用对非均匀输沙级配组成的影响显著,并随着流量增大而逐渐减弱。本研究剖析了冻融作用对推移质输沙率与级配组成的影响,有助于认识冰冻区河流的河床稳定性与河床演变情势。

恒定流作用下珊瑚砂输运特性试验研究

【目的】针对现有对珊瑚砂起动和输运研究的不足,基于室内水槽试验,研究了恒定流作用下珊瑚砂的起动规律和输运特性。【方法】利用电子显微镜对珊瑚砂颗粒进行观测,确定了珊瑚砂中轴长度与筛分粒径之间的关系;利用现代化测量技术测得了珊瑚砂起动流速和输沙率。【结果】与经典的石英砂起动流速及输沙率计算公式进行了比较分析,并修正了经验公式,使其适用于珊瑚砂起动流速的计算。【结论】由于珊瑚砂的形状极不规则,相比筛分粒径相同的石英砂,珊瑚砂的起动流速更小,珊瑚砂单宽推移质输沙率随水深的增加而显著减小。研究成果丰富了泥沙输运理论。

河道推移质最大输沙能力影响因素敏感性分析

河道推移质最大输沙能力受到水动力和河道边界多因素的共同影响。采用水力半径分割方法,以河岸坡角为θ的等腰梯形为过水断面,λ为河岸与河底的相对糙率,d50为床沙中值粒径,nb为河底糙率系数,Q为流量,S为比降,利用变分方法解析各因子对河道的推移质最大输沙能力和相应最优河道形态的影响程度。结果表明:河道推移质最大输沙率随着λ的增大而增大,随着θ的增大而减小,即河岸糙率的相对增大或河岸坡度的减小都会提升最大输沙能力;流量Q、比降S的增大,均可提升最大输沙能力;河底糙率系数nb减小、流量维持不变将会降低最大输沙能力,床沙中值粒径d50减小则会提升最大输沙能力;既不特别宽浅又不特别窄深的河道具有最大的推移质输沙能力。

基于随机森林算法的推移质输沙率计算研究

为探寻随机森林算法在预测推移质输沙率方面的效果,本文筛选了3060组天然河道输沙数据,将其划分为三种方案,得到了算法在单一流域、综合流域情况下的预测数据,并利用沙莫夫公式的计算结果作为对比。结果表明:水力参数与输沙率之间的相关性和参数在算法中的重要性并不完全一致;同一参数以不同形式出现在算法运算中时,会呈现不同的重要性;在足够的数据量支持下,随机森林算法相比于传统公式明显有更好的预测效果。

A STUDY ON NONEQUILIBRIUM TRANSPORT OF NONUNIFORM BEDLOAD IN STEADY FLOW

In the case of steady flow, an equilibrium state for an alluvial reach referring to the input of sediment over a sufficiently long time (time interval in the order of months) is equal to the output of sediment. A nonequilibrium state exists under steady flow when the sediment transport rate changes with time and space, so that there is no balance between input and output of sediment. The experiments were performed in an open circuit tilting flume with the bed slope of 0. 001-0. 009, the flume length of 30m and flume width of 0. 5m. The boundary condition at upstream and in all experiments, the rate of sedimant supply was zero. The coal or sand was se- lected as the Wed material. The grain diameter of the bed material varied in the range of 0.05- 20. 00ram. The equation of nonequilibrium transport of nonuniform bedload is derived. The grain distribution of the size distribution of bedload is calculated by modified Gessler's formula. The grain size distribution of bed material is calculated by CARICHAR mixed layer model. These equations are applied to numerical simulation of armoring of riverbeds. The results of the bedload transport rate and the grain size distribution of the bedload at the end of the down- stream in the process of time, as well as the grain size distribution of the bed material in the pro- cess of time and distance are all in close agreement with the measurements.