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.

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.

Coupled Model of Two-phase Debris Flow,Sediment Transport and Morphological Evolution

The volume fraction of the solid and liquid phase of debris flows, which evolves simultaneously across terrains, largely determines the dynamic property of debris flows. The entrainment process significantly influences the amplitude of the volume fraction. In this paper, we present a depth-averaged two-phase debris-flow model describing the simultaneous evolution of the phase velocity and depth, the solid and fluid volume fractions and the bed morphological evolution. The model employs the Mohr–Coulomb plasticity for the solid stress, and the fluid stress is modeled as a Newtonian viscous stress. The interfacial momentum transfer includes viscous drag and buoyancy. A new extended entrainment rate formula that satisfies the boundary momentum jump condition （Iverson and Ouyang, 2015） is presented. In this formula, the basal traction stress is a function of the solid volume fraction and can take advantage of both the Coulomb and velocity-dependent friction models. A finite volume method using Roe’s Riemann approximation is suggested to solve the equations. Three computational cases are conducted and compared with experiments or previous results. The results show that the current computational model and framework are robust and suitable for capturing the characteristics of debris flows.

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.

Sediment Transport in Rivers and Coastal Waters

Following Bagnold's approach, a relationship between sediment transport and energy dissipation is developed. The major assumption made in the study is that the near bed velocity plays a dominant role in the process of sediment transport. A general relationship between energy dissipation and sediment transport is first proposed. Then the equations for total sediment transport are derived by introducing the appropriate expression of energy dissipation rate under different conditions, such as open channel flows, combination of wave and current, as well as longshore sediment transport. Within the flows investigated, the derived relationships are fairly consistent with the available data over a wide range of conditions.

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.

Time-Dependent Sediment Transport Subjected to Downward Seepage

Experiments were conducted using cohesionless sand particles with median diameter of 0.48 mm to investigate the time variation of sediment transport rate under the influence of local downward seepage.The experimental results show that the bedload transport rate in terms of volumetric sediment transport rate per unit width increased rapidly with time in the presence of suction,eventually reaching a peak beyond which it started to decrease.The trend of reduction was significantly reduced beyond 8 400 s after the test started.The analytical expression was derived in terms of dimensionless sediment transport rate and dimensionless time.The hypothesized relationships were compared with the experimental data,indicating a good agreement with each other.

黄土高原径流侵蚀功率输沙模型的改进

水土流失对流域生态危害严重,输沙量模拟和预测可以为流域水土流失防治提供依据,因此精确的输沙模型是流域水土流失治理的重要工具。为了精确模拟变化环境下黄土高原年输沙量,该研究基于黄土高原19个水文站的径流和输沙数据,通过随机森林变量重要性度量方法评估年径流侵蚀功率、淤地坝指数、淤地坝相对指数、归一化植被指数、不透水地面积等因子对流域年输沙量的影响,使用非线性最小二乘法估算年输沙模型参数,对比分析不同因子组合的年输沙模型精度,提出适用性较强的黄土高原年输沙模型,据此开展年输沙量变化贡献率分析。结果表明:1)以幂函数形式构建的仅含径流侵蚀功率单因子输沙模型精度与流域面积有显著的负相关关系,相关系数为-0.505(P<0.05),模型精度随着流域面积增大而下降,在面积大于7000 km^(2)的流域适用性较差;2)年径流侵蚀功率、淤地坝指数及不透水地面积因子组合建立的多因子年输沙模型在黄土高原适用性最佳,模型在率定期纳什效率系数平均值为0.84,均方根误差平均值为0.21亿t,在验证期纳什系数平均值为0.79,均方根误差平均值为0.27亿t。3)影响研究流域年输沙量变化的因素依次是:年径流侵蚀功率、不透水地面积和淤地坝指数。研究可以为黄土高原不同区域水土流失防治和生态治理工作提供理论支撑。

不同沿岸输沙率计算公式对比分析

经历近100 a的发展,沿岸输沙率计算公式研究领域已形成许多代表性成果,系统研判各计算公式的预测精度对于快速准确判断研究区域沿岸输沙能力具有重要意义。依据使用率高、公式结构简洁、考虑参数相对全面等标准,选择了国内外7个沿岸输沙率计算公式,采用现场原型沙、室内原型沙和室内轻质沙等多种实测资料,对各公式的计算准确程度和适用范围进行了定量检验。结果表明,CERC公式和Kamphuis公式适用于现场原型沙预测,但对室内轻质沙预测的偏离程度较大;MH-Kamphuis公式和van Rijn公式对各种实测资料的预测均总体偏小;赵今声公式和Bayram公式的预测精度最低,预测偏小程度较大。经比较,孙林云公式对各项实测资料的符合程度总体较高,预测精度提高了36%~73%,具有更广泛的适用范围,推荐用于推导沙质海岸沿岸输沙模型相似律比尺关系。

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。

渗流输沙过程的颗粒运动状态和影响因素分析

基于力学平衡,提出了渗流作用下土体内部颗粒运动的控制方程,计算了颗粒运动状态。同时综合考虑颗粒受力及输运容许度的随机分布,提出了颗粒运动概率的计算方法。通过与试验观测的临界水力梯度、输沙量和渗透系数的对比验证,证明颗粒运动方程和运动概率计算的可靠性。研究结果表明:土颗粒在由静起动的过程中做加速运动,加速历程很短,其运动特征与土颗粒粒径及孔隙率有关;渗流输沙过程中颗粒运动概率不断增加,主要是因为土体孔隙变大,可动颗粒受力增加,输运的几何容许度增加引起的。在渗流输沙的过程中,输沙率随时间是先增大后减小的,且与颗粒运动概率密切有关。在输沙的前期孔隙率是主要因素,而在后期可动颗粒的数量是主要影响因素。

吐鲁番市高昌区不同固沙植物的防风固沙效益

为了解不同防风固沙林对风速、输沙通量和土壤蚀积程度的影响,以期为荒漠化地区植物选择和配置提供技术支持,以吐鲁番市高昌区恰特喀勒乡公相村为实验地,利用气象站、风速仪、集沙仪等监测不同高度下梭梭林、柽柳林以及梭梭-柽柳混交林的风速、地表粗糙度、输沙通量等指标,分析比较不同固沙林的风速廓线、风速增加率、地表蚀积量及风沙堆积物变化规律。结果表明:随着风速的增加,输沙通量逐渐增加,地表粗糙度和蚀积深度呈现降低趋势;不同固沙林不同高度(h)的风速增加率均表现为中层(50 cm<h≤100 cm)最大,低层(10 cm<h≤50 cm)最小,说明固沙植物对低层风速有显著降低作用,增强了中层风速;不同固沙林输沙通量随着高度增加逐渐减小;柽柳林、梭梭-柽柳混交林后出现堆积现象,梭梭林前后为侵蚀现象。梭梭林、柽柳林以及梭梭-柽柳混交林均降低了近地表风速,有效拦截了风沙流,改变了表层风沙堆积物物质组成,梭梭-柽柳混交林防风固沙效果较好。

黄河河南段水沙变化规律与影响机制研究

黄河河南段为典型游荡型河段,其水沙变化剧烈、主槽摆动频繁、河床变形严重,对河道生境产生了严重影响。以1960—2019年小浪底、花园口、高村水文站水沙数据为研究对象,结合Mann-Kendall检验、滑动T检验、小波分析等统计方法探究了黄河河南段水沙变异周期特征,并应用双累积曲线法定量评估了降雨和人类活动对河南段径流量和输沙量减少的贡献率。结果表明:河南段年输沙减少趋势大于年径流减少趋势,小浪底到花园口间基本冲淤平衡,花园口到高村间以淤积为主,河南段汛期以泥沙淤积为主、非汛期冲刷;小浪底站输沙量在1997年发生突变,而花园口、高村站径流量和输沙量均在1985年发生突变;调水调沙对于径流量增加效果要大于输沙量增加,人类活动对河南段径流输沙减少起主导作用,对河南段径流量平均贡献率为80.94%,对输沙量平均贡献率为94.95%。

Sedimentation Processes at the Navigation Channel of the Liquefied Natural Gas (LNG) Port, Nile Delta, Egypt

Liquefied Natural Gas (LNG) port is located at Abu Qir Bay on the northwestern coast of the Nile delta, Egypt. The port was constructed in 2004 to export liquefied natural gas worldwide. The offshore basins of this port including the turning and berthing areas (15-m depth) are connected to the deep water by a 15-m depth dredged channel that extends 4 km offshore. However, the navigation channel and its contiguous basins have experienced problematic shoaling that might affect the navigation activities of gas tankers. Sedimentation processes have been investigated by analyses of waves, currents, bathymetry, grain size of seabed and channel dimensions. Sedimentation rates are estimated using a developed numerical model. Sedimentation rate fluctuates between 0.048 × 106 m3/month and 0.388 × 106 m3/month, with an annual sedimentation rate of 1.977 × 106 m3/yr. The variance in the sedimentation rates between winter and summer resulted in increasing of current speed and direction flowing towards offshore. The sedimentation process is influenced by the temporal variability in the direction and intensity of the predominant waves, currents, orientation of navigation channel, basin breakwaters, seafloor morphology and sediment sources. Due to the geographic location of LNG port it lays within a sediment sink for sediments supplied from different alternating directions by several pathways, flowing towards the N-W, S-W, N-E, and S-E quadrants. Most of these currents components are substantially effective in transporting fine-grained sediment towards the navigation channel axis and contiguous basins. Together with these currents, the predominant NW and SE waves acting to agitate and stirrup sediments in the vicinity of the port, and thereby accelerating sedimentation rates.

Unit stream power,minimum energy dissipation rate,and river engineering

Unit stream power is the most important and dominant parameter for the determination of transport rate of sand,gravel,and hyper-concentrated sediment with wash load.Minimum energy dissipation rate theory,or its simplified minimum unit stream power and minimum stream power theories,can provide engineers the needed theoretical basis for river morphology and river engineering studies.The Generalized Sediment Transport model for Alluvial River Simulation computer mode series have been developed based on the above theories.The computer model series have been successfully applied in many countries.Examples will be used to illustrate the applications of the computer models to solving a wide range of river morphology and river engineering problems.

Sediment generation and soil mound denudation in areas of high-density tree throw along a river valley in the Jura Mountains, Switzerland

A preliminary field-based investigation was undertaken in a small(<10 km^(2))river valley located in the mountainous Jura region of northwest Switzerland.The aims of the work were to assess sediment generation and annual sediment transport rates by tree throw on forested hillslopes,and to document surface hydrology characteristics on four fresh soil mounds associated with recent tree throws over a 24-day monitoring period.For the soil mounds,average sediment recovery ranged from 7.7-28.2 g(dry weight),equivalent to a suspended sediment concentration of 145.2-327.8 g L^(-1),and runoff coefficients ranged from 1.0%-4.2%.Based on a soil bulk density value of 1,044 kg m^(-3),upslope runoff generation areas were denuded by an average 0.14 mm by the end of the 24-day monitoring period,representing an erosion rate equivalent to 2.1 mm yr^(-1).A ca.50 cm high soil mound could therefore feasibly persist for around 200-250 years.For tree throw work,the dimensions of 215 individual tree throws were measured and their locations mapped in 12 separate locations along the river valley representing a cumulative area equivalent to 5.3 ha(av.density,43 per ha).Tree throws generated a total of 20.1 m^(3) of fine-sediment(<2 mm diameter),or the equivalent of 3.8×10^(-4) m^(3) m^(-2).The process of tree throw was originally attributed to two extreme weather events that occurred in west and central Europe in late December 1999.Taking the 18-year period since both storms,this represents an annual sediment transport rate of 2.7×10^(-5) m^(3) m^(-1) yr^(-1).Exploring the relationship with wind on fall direction,65.5%of tree throws(143)generally fell in a downslope direction irrespective of hillslope aspect on which they were located.This infers that individual storms may not have been responsible for the majority of tree throws,but instead,could be associated with root failure.Given the high density of tree throws and their relative maturity(average age 41 years),we hypothesise that once trees attain a certain age in this river valley,their physiognomy(i.e.height,mass and centre of gravity)compromises their ability to remain securely anchored.We tentatively attribute this possibility to the presence of bedrock close to the surface,and to the shallow soil profile overlaying steep hillslopes.

Sediment control function of river notches

The purpose of this study is to investigate the control function and mechanisms of natural river notches. Physical and numerical experiments are analyzed in this study for two representative types of sediment events: high intensity and short duration Type A sediment disaster events, and low intensity and long duration Type B moderate non-disaster events. Two dimensionless parameters, sediment trapping rate and reduction rate of peak sediment transport, are defined to evaluate the sediment control function of river notches. Study results indicate that the contraction ratio of the notch has a significant influence on sediment control function, with high contraction ratios resulting in both high sediment-trapping and high reduction rates. River notches provide better sediment control during Type A events than Type B events. The sediment control mechanism of river notches is the result of multiple interactions among river flow, sediment transport, and riverbed variation. Analysis of these interactions supports the significant protection role of river notches on sediment control for disaster

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。

Incipient Velocity of Non-uniform Sediment in Sloping River Bends

A model for incipient movement of sediment in rolling pattern was established. In this model, the starting of sediment particles under low transport rate, the exposure degree of sediment, the lateral slope of water surface and the effect of transverse circulating current induced by the hydraulic structure of bend flow were fully considered. A theoretical formula for the incipient velocity of non-cohesive and non-uniform sediment in sloping river bends was developed. The results from the theoretical formula compared well with the experimental data.

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。