Experiments on the short-term development of sine-generated meandering rivers

This paper presents recent works on the simulation of short-term development of sine-generated meandering river in laboratory conditions. The influences of initial system parameters on the evolution process of rivers are investigated, including control over channel sinuousness, channel width and dominant discharge, eventually leading to different results of planforms. Measurements on the bank-line, flow field, bed topography and sediment transport rate were carried out. Braided rivers are easy to produce using non-cohesive sediments in floodplains, whereas environmental temperatures and humidities could influence the fluvial process by their effects on material cohesion. Channelized rivers were obtained in the “High Flow” conditions and the river corridor width was proven to be mainly connected with initial channel sinuousness and water discharge. Sickle-shaped and bamboo leaves-shaped sandbars were formed in the channels during the transformation process of meandering to braiding, the stability degree of sandbars reflects the adaption of channel morphology to hydrodynamic condition. Quantitative analysis confirms the formation of free steady bars, which manifests the free response as a downstream oscillation of the perturbation. Damping length is mainly affected by dominant discharge, channel width is the secondary factor, and channel sinuousness is the weakest factor. The wavelength of steady bars approximately equals to half of the initial streamwise wavelength. Sediment transport rate tends to increase with the increasing of channel sinuousness but stops growing due to the excessive increase of flow route and flow friction. The experiment results could be useful for verifying river pattern discriminant functions and offer a basis for further study on the morphological evolution of large-scale natural rivers, such as Yangtze River.     

Management of vegetation encroachment by natural and induced channel avulsions: A physical model

Flow regulation and water abstractions may change the complex relationship between river hydraulics, morphology, and riparian vegetation. As a result, rivers are likely to decrease their dynamics, increase the amount of vegetation, and modify their habitat structure. Flood events provide a natural mechanism for removal of invasive vegetation and recreation of natural floodplain habitats. This work aims at evaluating and quantifying how gravel‐bed braided rivers naturally control vegetation encroachment through morphological processes and the impact of both naturally occurring and induced avulsions. Flume experiments were conducted in a 24‐m‐long x 1.6‐m‐wide channel filled with well‐sorted sand and constant longitudinal gradient at 0.01 m/m. Once a braided network developed, the flume was seeded with Eruca sativa at a density of 1.5 seeds/cm² and grown until an approximate height of 1.1 cm. Experiments evaluated low‐, medium‐, and large‐flood events and documented morphological changes and impacts to vegetation at four intervals during the experiments. High‐resolution images captured approximately 3 m above the flume were used to produce accurate Structure‐from‐Motion‐derived topography and orthoimagery (average errors 2 mm). Vegetation dynamics were observed to be highly variable and depend on local morphological changes and bank erosion. Discharge is the first‐order control on vegetation removal, but our results show that occurrence of avulsions significantly increases vegetation removal. The experiments highlight that a relatively small amount of sediment relocation can be an effective tool to induce avulsions and reduce vegetation encroachment on regulated rivers.     

冲积河流推移质输沙最优河道形态影响因素理论分析

为从理论上确定水沙动力、阻力和边界条件对冲积河流推移质输沙最优河道形态和最小比降的影响,基于 水力半径分割方法将河床整体糙率划分为河岸糙率和河底糙率;采用河道形态自动调整变分方法,以等腰梯形为 河道过水断面,推导推移质输沙率与河道形态的关系,分析河岸与河底相对糙率、河岸坡角、流量、输沙率、中值 粒径和河底糙率对最优河道形态和最小比降的影响。结果表明：最优河道形态和最小比降随河岸与河底相对糙 率的增大而减小,随河岸坡角的增大而增大。流量或者河底糙率的增加将使最小比降减小,同时造成最优河道形 态趋向窄深。输沙率或者中值粒径的增加将使最小比降增大,但两者对最优河道形态演变的影响不同,输沙率增 大将使最优河道形态向宽浅发展,而中值粒径增加将使最优河道形态向窄深发展。     

Combination of Runoff Simulation with Sediment Modeling in Rivers and Its Application in the Lower Yellow River

Abstract

Previous research on runoff show flow and sediment movement in rivers, and river bed variations are usually separated. This paper, from the point of view of the basin as a whole system, presents an integrated approach by combining runoff simulation with numerical model of sediment transport in rivers to simulate the whole processes of hydrological variables and flow-caused bed variations from the very upper part of the basin to the river mouth. To accomplish this purpose, BTOPMC, a rainfall-runoff model, and NUSTM-1D, a well-developed numerical model for sediment transport and river bed variations, are selected to form a combined model. The application of the proposed model to the lower Yellow River shows that it can properly simulate rainfall-caused runoff, the change of suspended sediment concentration along the river, and river bed variation.     

Experimental study on the bed topography evolution in alluvial meandering rivers with various sinuousnesses

The bed morphology in alluvial meandering rivers is composed of both alternatively distributed free bars and curvature related point bars. To study the influence of channel sinuousness on the flow and sediment transport, and consequently the bed morphology in meandering rivers, experiments were carried out in a series of curved channels with erodible bed fixed side walls. Measurements on the surface flow field, bed topography evolution and sediment transport rate were performed. Experimental results show a high velocity area near convex bank at a streamwise position a little bit upstream of the apex and the increasing in sinuousness of channel tends to increase the cross-sectional variation of flow velocity at the apex of bends. Point bars and pools appear in the channel bends in all experimental runs the bedform has an obvious tendency of downstream migration as a whole. The quasi-equilibrium bedform in the channel bends are dominated by two independent factors: the influence of curvature and the influence of flow-bed instability inside the channel. The increasing of flow rate leads to a faster development of bars and pools, as well as faster downstream migration of the entire bedform. The increase of sinuousness in channel bends leads to the increase of both the spatial and the temporal fluctuation of bedforms. Bedload is dominant in the sediment transport and the sediment transport rate tends to increase with the increasing of the inlet flow rate but decrease with the increasing of the channel sinuousness. Computational results on the bed topography in river bends was also presented based on empirical models, showing that the highly sinuous channels have more uncertainty in bed morphology and more difficult to be predicted with mathematical models when compared with slightly sinuous channel bends. The coupled interaction of point bars, mid-channel bars, alternative bars and even sand ripples are thought to complicate the mechanism and make the bed morphology difficult to predict. The experiment results can be useful for further theoretical study on the dynamics of river meanders with water depth-width ratio as small as 1/10 to 1/20 and bedload dominated sediment transport.     

Development and application of 2-D mobile-bed model with bedrock river evolution mechanism

Steep slope and severe bed change are the characteristics of mountain rivers. These characteristics often cause the bed armour layer flushed away, make the bedrock exposed, and then increase the channel incision. Most mobile-bed models of past few decades aimed at the sediment transport of alluvial channel. In this paper, a bedrock river evolution mechanism is proposed, and included in a 2-D mobile-bed model, called the explicit finite analytic model (EFA). The EFA model can consider both incision and deposition over the bedrock, by combining a new stream power type of bedrock erosion rate formula with the flow and sediment transport modules. An uplifted reach of Taan River, Taiwan, caused by the Chi–Chi earthquake occurred in 1999 is chosen as the study site. Rapid bedrock incision since the uplift provides a rare chance to explore its mechanism and evolution process. The field and numerical results show that the proposed model has the capability of simulating morphological changes for bedrock rivers.     

水利枢纽下游河床冲刷与再造过程研究进展

在水利枢纽尤其是水库的拦蓄与调节作用下,下游河道的来水来沙条件显著变化,将导致下游河道不平衡输沙,引起河床冲刷与再造。水利枢纽下游河道的河床冲刷与再造过程,兼具床沙冲刷—粗化—交换—悬移质恢复等多过程耦合的微观水沙运动特性,以及泥沙冲淤—床面形态变化—纵比降改变—河型河势调整等多尺度复杂响应的宏观形态变化。国内外相关研究主要通过实测资料分析、理论研究、实体模型试验和数值模拟等手段,从微观机理和宏观规律2个主要方面开展。归纳和总结了国内外水利枢纽下游河床冲刷与再造过程研究成果,对河床冲刷与再造实体模型试验和数值模拟中的模型沙选择、泥沙恢复饱和系数等关键技术问题的研究进展进行了分析,并指出了需要进一步研究的重点和亟待解决的科技难题。     

黄河下游河道萎缩过程中输沙能力的调整

依据河床演变学原理,根据定位观测资料分析,结合河工动床模型试验,探讨了黄河下游河道萎缩过程中的输沙能力调整关系。研究表明:黄河下游河道主河槽的输沙能力调整趋势取决于河道的萎缩模式;主河槽水流挟沙力与河槽形态的关系仍然符合一般意义下的河床过程规律;河道萎缩后,河道的排沙比减小,但排沙比与河道萎缩模式无关;河道萎缩是可以逆转的。     

汶川地震对震区河流演化的影响

5.12汶川地震,不仅直接影响灾区河流的河道,并诱发了大量崩塌、滑坡、泥石流、堰塞湖等次生山地灾害,这些山地灾害的产生和发展将对震区河流的演化产生深远影响。因此,对震后山区河流河道演化规律和发展趋势进行准确的分析和预测,是震区河道修复及震后恢复重建的关键。结合野外考察资料,分别从河型、纵断面、横断面3方面分析了震区河流的演化特点。分析结果表明:河床剧烈抬升引起河型游荡分汊,两岸频发的山地灾害挤压河道;高强度山洪冲刷河床及岸滩并展宽河道,泥沙的淤积也可束窄河道;河流纵断面总体淤高,但纵断面结构渐趋稳定,大石块多且上游来水来沙丰富的河段会逐渐发育成阶梯状结构,而以淤积为主的河段,由于山洪冲刷会形成下凹型纵断面,并逐渐向直线型纵断面发展。     

三峡水库运用后武汉天兴洲分汊河段演变规律及趋势

利用武汉天兴洲分汊河段的水文与地形等实测资料,分析了水沙变化条件下本河段分流区水流动力轴线、流速与含沙量分布等变化特点,阐述了本河段水沙运动与河床演变的相互作用机制,预测了本河段的演变趋势。研究结果表明本河段分流区水流动力轴线随流量增加而逐渐偏向左侧变化,其变化规律形成了左汊的有利进流条件与较长时间维持本河段分汊的河势格局,但因上游来沙的大幅度减少,右汊冲刷幅度较左汊大,有利于右汊继续维持主汊地位。     

日本大井河河床变动的数值模拟

利用一维的泥沙数值模型研究了位于日本中部的大井河河床变动情况.在传输模块中考虑了床沙的非均质特性.1981至1996年实测的流量和泥沙数据被用于确定模型参数.为了控制下游河道的淤积,针对不同的挖沙范围和大坝的有无对大井河泥沙传输进行了30年的预测.根据计算结果提出了下游河道和河口的整治方案.     

中国冲积大河的河型分布与成因

结合遥感影像、野外调查和水沙数据,以冲积大河为研究对象,分析中国大河的河型分布及成因。中国大河的冲积河段以弯曲与辫状河型为主,局部河段为分汊与网状河型。综合考虑河型分布的一般性和特殊性,认为相对输沙率(来沙量与输沙能力之比)、相对河岸侵蚀切应力(近岸水流切应力与河岸临界抗冲切应力之比)和河谷地形控制(如节点和宽度)是决定河型成因的3个最主要的因素。     

三峡、丹江口水库运行前后坝下游不同河型稳定性对比分析

通过计算丹江口、三峡水库运用前后坝下游不同河型的河床稳定系数,分析了建库前后坝下游河床稳定性的变化情况及不同河型稳定性的差异,结果表明:坝下游不同河型的综合稳定系数表现为,分汊河型的稳定系数最大,蜿蜒河型次之,游荡河型综合稳定系数最小;同一河型在不同的河流甚至同一河流不同河段上的综合稳定系数存在差异;丹江口、三峡水库运用以后坝下游不同河型的综合稳定系数均呈增大趋势,河道总体呈现渐趋稳定的态势。     

黄河下游河道萎缩致灾机理探讨

本文基于水力学原理，结合实体模型试验及原型实测资料分析，探讨了黄河下游河道萎缩的概念、致灾效应和致灾机理。分析表明，河道萎缩是造床过程中河道排洪输沙功能衰退的一种演变现象，其致灾效应是使洪水涨率增大，同流量下洪水水位不断抬升。致灾的现象主要表现在畸型河势增加、工程险情增多、直接造成工程损失及滩区经济损失等。试验研究揭示，主河槽宽度缩窄造成洪水水位涨幅增大，河底平均高程抬升和过水断面面积减小造成洪水水位起涨的基准面抬升是河道萎缩致灾的主要原因。河道萎缩致灾效应的大小与萎缩模式有关，其中以“集中淤槽”模式所形成的致灾效应尤为明显。     

River scale model of a training dam using lightweight granulates

Replacing existing river groynes with longitudinal training dams is considered as a promising flood mitigation measure in the main Dutch rivers, which can also serve to guarantee navigability during low flows and to create conditions favourable for ecological development. Whereas the bed response in the streamwise uniform part of a river trained by a longitudinal dam can be readily predicted, the bed response at the transition zones is unclear. In the present study, we investigate the local morphological effects resulting at the intake section of a longitudinal training dam, where the flow is distributed over the main channel and a side channel in between the dam and the river shore. A sediment recirculating model with a nearly undistorted geometry with respect to the prototype was setup. Lightweight polystyrene granulates were used as a surrogate for sediment, to properly scale the Shields parameter without compromising Froude scaling, and reach dynamical similarity. A laser scanner allowed collecting high-resolution bed elevation data. Results obtained under typical low flow and high flow conditions show a general deepening of the bed in the area adjacent to the training dam, in response to narrowing of the main channel. Scour at an upstream river groyne embedded in the model showed a scour hole which was deeper than realistic. Throughout the entire domain, bedforms developed featuring geometrical properties that reproduced the prototype conditions appropriately. Based on a comparison with characteristics from the River Waal, regarded as the prototype without a longitudinal dam, lightweight sediments were considered to be a proper choice for this study, in which bedload is the main sediment transport mode. The main conclusion regards the absence of significant morphodynamic developments at the intake section, both during the high flow experiment and during the low flow experiment, which can be attributed to the alignment of the dam with the local streamlines.     

Two-Dimensional Composite Mathematical Alluvial Model for the Braided Reach in the Lower Yellow River

Abstract

The fluvial processes are considerably complicated in the braided reach of the Lower Yellow River, and the lateral or transverse channel deformation process is especially pronounced. The state-of-the-art of simulation techniques for the channel deformation in the Lower Yellow River was analyzed in this paper, and such a conclusion was drawn that existing mathematical alluvial models only can simulate the longitudinal channel deformation and cannot simulate such the lateral deformation as lateral erosion and failure of bank. Then a new two-dimensional (2D) composite model was developed herein, consisting of a 2D flow and sediment transport submodel and a riverbank-erosion submodel. This model not only can simulate the longitudinal and lateral channel deformations, but also can be used to simulate the fluvial processes of natural alluvial rivers with complex bed morphology. Through using the proposed composite model, the flood routing and channel deformation processes during the August 1982 flood were simulated. The maximum stage along the reach and the discharge hydrograph at the outlet so obtained were in good agreement with the observed data. Simulated results of the channel deformation processes showed that during the flood season, the main channel scours and the flood plain deposits; during the dry season, the main channel deposits and banks of the floodplain retreat due to lateral erosion by the flow and failure under gravity. Such results accord with the annual varying law of bed scouring and deposition in the braided reach.     

基于洪水涨冲落淤特性分析游荡性河道双岸整治对冲淤的影响

基于黄河下游及主要干支流、美国密苏里河和密西西比河的实测资料,分析了低含沙洪水和高含沙洪水的输沙与冲淤特性。研究表明,河道沿程比降虽然变缓,但河宽变窄,流速沿程增加,是造成冲积河流保持洪水输沙平衡的边界条件。底沙的运动比洪水波传播得慢,是造成洪水在河道中长距离冲刷的根本原因,而与河道的比降陡缓关系不大。河道输沙特性呈现"多来多排",是形成河床沿程冲刷的水流动力条件。通过黄河下游河道双岸整治,河道形成窄深、归顺、稳定的河槽,使洪水造床和输沙作用增强,从而对河道冲刷作用增加。     

黄河下游河槽治理方法分析

本文对已有的河槽形态与水流狭沙力关系的研究成果进行了探讨。进一步在考虑米沙组成变化、河槽综合阻力变化的条件下．计算得到了黄河下游典型断面流量分级平均水沙条件下河槽平衡输沙横剖面。平衡横断面与实际断面比较．下游无论游荡段与弯曲段，实际断面都显得过于宽浅。分析认为，尽管河槽缩窄可加大水挟沙能力．但受河型发育机制的制约．在现有水沙和边界条件下黄河下游河槽的横剖面是无法自动调整形成输沙平衡形态的。文中还利用黄河下游来水来沙和河道冲淤实测资料证明了黄河下游水流不冲狭沙量小于不淤沙量．河道从淤积转向冲刷时．水流含沙量明显降低，固此减少同流量下台沙量的变化有利于减轻下游河遭淤积。从理论分析结果看：缩窄中、小流量“下的河槽．以多级河槽方式重点整治花园口以下河段．使横断面总体形态呈上凸形．能显著减小黄柯河槽中的淤积；同时，应配合河道整治，利用水库调节水沙和进行必要的挖沙疏浚措施。具体实施方法还需要进一步研究；     

黄河下游丁坝缩窄河道泥沙冲淤特性试验研究

通过动床模型试验研究了黄河下游裴峪至官庄峪丁坝缩窄河段,在河道不同位置布设丁坝,不同情况下丁坝相对长度(丁坝长度与原河道宽度之比值)对河道泥沙冲淤变化的影响。结果表明：水沙运动要素变化受丁坝布设位置和长度等影响较大,随着丁坝相对长度的增加,汛期主河槽冲刷量和滩地淤积量逐渐增大,主槽范围相应扩大,其高程普遍呈下降趋势,特别是缩窄断面导流堤顶端部位出现明显的局部冲刷坑,束水冲沙、增大输沙能力的效果明显。当丁坝相对长度大于0.50时,主流线偏移、断面流速分布和河床冲淤的变化速率明显增大,不利于河道稳定。     

荆江关洲河段河道演变分析

关洲汊道位于葛洲坝工程下游长江荆江河段首部。其两汊年内水沙分配随来流少左右易位，临界流量为２００００ｍ＾３／ｓ，即２００００ｍ＾３／ｓ以上，左汊为主汊，２００００ｍ＾３／ｓ以下，右汊为主汊，水沙分配的这种变易性规律，与该河段特定的汊道河槽形态及相应的水力泥沙条件有关。