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1.
Considerable progress has been made recently in characterising the patterns displayed by the anabranches of braided rivers. However, the physical processes of sediment scour, transfer and deposition that govern the generation and evolution of anabranch channels remain largely unexplained. Direct measurement of three-dimensional flow fields and morphological evolution of the anabranches in the braided Brahmaputra–Jamuna River, Bangladesh, were undertaken to investigate the interactions between fluvial processes and anabranch morphology. These data were used to elucidate the circumstances leading to the bifurcation of a single channel, which is a topic of fundamental importance to understanding the physical processes responsible for braiding. Results indicate that division of the velocity field into multiple threads within a single channel precedes a division in the cross-sectional morphology of the channel and appears to be a necessary prerequisite for development of a bifurcation. An empirical relationship was established to discriminate between channels with single and multi-thread velocity fields, based on the depth-to-width ratio and specific energy of the flow at a representative channel cross-section. This function requires further validation, but could be used to predict the conditions under which a single channel is likely to bifurcate to produce two anabranches. 相似文献
2.
Small meandering channels of about 1 m wide on an intertidal mudflat in the Westerschelde estuary the Netherlands) were studied with the aim to improve understanding of the effect of highly cohesive bed and bank sediment on channel inception and meander geometry and dynamics. The study is supported by experiments and modelling. The estuarine meandering channels are less dynamical than alluvial meandering rivers, and the dynamics are more localised. Moreover, the high thresholds for bed sediment erosion and for bank failure lead to two processes, uncommon in larger rivers, that cause most of the morphological change. First, the beds of the channels are eroded by backward migrating steps under hydraulic jumps, while the remainder of the bed surface along the channel is hardly eroded. Second, channel banks erode i) where eroding steps locally cause undercutting of otherwise stable channel banks and ii) in very sharp bends where the flow separates from the inner-bend channel boundary and impinges directly on the bank on the opposite side of the channel. Further morphological change is probably induced by rainfall splash erosion and by storm waves that weaken the mud, and by large mud fluxes from the estuary. The steps were successfully reproduced in laboratory flume experiments. An existing model for step migration predicted celerities consistent with field and laboratory observations and demonstrated a strong dependence on the threshold for erosion. Bank stability models confirm that banks and steps only fail when undercut and weakened by waves, rain or excess pore pressure in agreement with observations. The effects of a high threshold for bank erosion was implemented in an existing meander simulation model that reproduced the observed locations of bank erosion somewhat better than without the threshold, but flow separation and its effect on meander bends remains poorly understood. 相似文献
3.
北洛河下游河槽形成与输沙特性 总被引：8，自引：0，他引：8
北洛河发湖泊于黄河粗沙来源区，年均含沙量达１２８ｋｇ／ｍ＾３年均流量仅２５ｍ＾３．ｓ，是典型的多沙河流，但由于泥沙主要由高含沙洪水输送，平水流量小，含沙量低，经常保持窄深稳定河槽，使高含沙洪水挟带的泥沙能顺利输送而不淤，并形成弯曲性河流。 相似文献
4.
长江河口悬浮泥沙的混合过程 总被引：2，自引：0，他引：2
根据准同步观测的悬浮泥沙及表层沉积物粒度、流速、含沙量资料, 分析了长江口及临近海域悬浮泥沙在河口的混合过程。长江河口-陆架系统悬浮泥沙中值粒径呈现“细-粗-细”的变化规律, 河口上段悬浮泥沙中值粒径为8.9 μm, 拦门沙海域为10.5 μm, 陆架区为4.5 μm, 北支为9.9 μm, 杭州湾口为5.6 μm, 泥沙类型为粘土质粉砂。河口上段和陆架区悬浮泥沙与表层沉积物的垂向混合作用较弱, 拦门沙区域二者发生强烈的混合和交换, 悬浮泥沙在由长江河口向陆架系统输移过程中仅有表层泥沙保留了流域输入的泥沙粒度特征。长江口悬浮泥沙中值粒径与含沙量呈良好的正相关关系, 水流的剪切作用是引起拦门沙海域泥沙再悬浮、近底高含沙量和悬浮泥沙粒径增加的主要原因, 悬浮泥沙粒径和含沙量的增加主要由粉砂组分的增加引起。2007 年长江河口区范围内悬浮泥沙中值粒径比2003 年普遍减小11％, 含沙量比2003 年减小22%, 河口上段含沙量对流域来沙减少的响应最为敏感, 而拦门沙区的泥沙粒径对流域来沙减少的响应最敏感。在长江流域来沙量减少的背景下, 河口拦门沙区域仍能维持较高的含沙量, 主要缘于河口系统内部的供沙 相似文献
5.
6.
Flow structure at an asymmetrical stream confluence 总被引：2，自引：0，他引：2
Measurements of downstream and cross-stream velocities at a small, asymmetrical stream confluence show that the structure of low-stage flows is influenced by the tributary/main stem momentum flux ratio, the total discharge of the incoming flows, and the bed morphology. Flow accelerates through the confluence during all three measured events. This acceleration is associated with a downstream reduction in channel capacity caused in part by the presence of a large bar along the inner bank of the downstream channel. As the momentum ratio increases, flow from the lateral tributary increasingly deflects flow from the main stream toward the outer channel bank within the confluence. As a result, the mixing interface between the converging flows also shifts outward. The large bar in the downstream channel deflects flow along the inner bank toward the adjacent scour hole, enhancing flow convergence downstream of the confluence and producing a region of flow separation adjacent to, or in the lee of the bar. The loci of maximum topographic deflection and flow separation vary with momentum ratio and total discharge.Secondary circulation within the downstream channel is characterized by a single large helical cell when the momentum ratio exceeds one, and weak surface-convergent helical cells on opposite sides of the mixing interface when the momentum ratio is less than one. Curvature of the flow, and thus the strength of helical motion, is greatest on the tributary side of the mixing interface. Although the flow events measured in this study did not exceed the threshold for sediment movement, the bed morphology at the confluence can be explained by the flow structure observed during these low-stage events, suggesting that formative flows may have similar downstream and cross-stream velocity fields. 相似文献
7.
Three-dimensional morphological adjustment in a chute cutoff (breach) alluvial channel is quantified using Digital Elevation Model (DEM) analysis for a ca. 0.7 km reach of the River Coquet, Northumberland, UK. Following cutoff in January 1999, channel and bar topography was surveyed using a Total Station on five occasions between February 1999 and December 2000. Analysis of planform change coupled with DEM differencing elucidates channel and barform development following cutoff, and enables quantification of sediment transfers associated with morphological adjustment within the reach. This exercise indicates an initial phase of bed scour, followed by a period characterised by extensive bank erosion and lateral channel migration where erosion (including bed scour) totalled some 15,000 m3 of sediment. The channel in the post-cutoff, disequilibrium state is highly sensitive to relatively low-magnitude floods, and provision of accommodation space by bank erosion encouraged extensive lateral bar development. Bar development was further facilitated by infilling of channels abandoned by repeated within-reach avulsion and large-scale aggradation of sediment lobes deposited by higher magnitude floods. Calculations indicate that at least 6600 m3 of sediment was deposited on emerging bars within the reach over the survey period, and >2300 m3 deposited within the channel. Sediment losses from the reach may have exceeded 6500 m3. 相似文献
8.
Confluence dynamics in the Ganga–Ramganga valley in the western Ganga plains of India has been studied through systematic mapping of channel configuration using multi-date remote sensing images and topographic sheets for a period spanning nearly 100 years (1911–2000). The study has been supplemented with a detailed analysis of the channel morphology, hydrology and sediment transport characteristics of the different rivers. Our study indicates that new confluences have been created during this period and that the confluence points have moved both upstream and downstream on a historical time scale. Apart from major avulsions, other processes that have controlled the confluence movements include river capture, cut-offs and aggradation in the confluence area. River capture occurs through lateral bank erosion and migration, encroachment by the master stream and beheading of smaller rivers resulting in upstream movement of the confluence point. Another process which influences the upstream migration of the confluence is an increase in sinuosity of one of the channels near the confluence and then a cut-off. Aggradation in the confluence area and local avulsions of the primary channel in a multi-channel system seem to be the major process controlling the downstream movement of the confluence point. Analysis of channel morphology, hydrology and sediment budget for the study period supports our interpretations. 相似文献
9.
近年来长江流域入海沙量呈现阶梯性减小趋势,三峡水库蓄水后加剧了这一减小趋势,并通过传递效应影响河口悬沙浓度变化。基于长江口1950-2013年悬沙浓度数据,结果表明：① 长江口南支河段及口外海域悬沙浓度为减小趋势,且越向海域减幅越小,同时与流域入海沙量减幅差距加大;② 北支优势流变化不大,但悬沙浓度为减小趋势,主要为南支和海域大环境悬沙浓度减小所致;③ 拦门沙河段悬沙浓度的峰值区域因径流减小、潮流相对增强,2003-2012年较1984-2002年期间峰值位置向口内上溯约1/6经度,上溯距离洪季 > 年均 > 枯季;④ 1999-2009年南槽进口悬沙浓度减小,主要是再悬浮和滩槽交换引起的悬沙浓度增量小于流域和海域悬沙浓度锐减引起的减量,中段该作用相反,悬沙浓度为增加趋势;⑤ 北槽进口由于分流分沙比减小、流域和海域悬沙浓度减小及再悬浮量减小等综合影响下,1999-2012年逐年的8月份悬沙浓度呈减小趋势,中段越堤沙量作用明显高于外部坏境引起的减小量,为增加趋势。 相似文献
10.
准确把握环境变化下前期水沙条件对当前河床形态调整的影响,建立非平衡态河床形态调整的模拟方法,对深化河床非平衡调整过程的认识至关重要。基于黄河下游花园口—利津河段1965—2015年的水沙和沿程82个大断面数据,首先统计分析了不同河段主槽断面形态参数（面积、河宽、水深和河相系数）的调整过程及其对水沙变化的响应规律;进而以水沙因子作为主槽断面形态调整的主控因素,采用滞后响应模型的多步递推模式,建立了其对前期水沙条件变化的滞后响应模型。结果表明,各河段面积、河宽和水深经历了减小—增加—减小—增加的变化过程,并且其与4 a滑动平均流量和含沙量之间分别呈正相关和负相关;而河相系数孙口以上段整体减小,孙口以下段呈增加—减小—增加—减小的变化过程,除花高段1965—1999年外,其与流量呈负相关,与含沙量呈正相关。滞后响应模型在黄河下游主槽断面形态对前期水沙条件响应过程的应用表明,各参数模型计算值与实测值符合程度均较高,模型能够很好地模拟主槽断面形态对水沙变化的响应调整过程,模型计算结果显示主槽断面形态调整受当年在内的前8 a水沙条件的累积影响,当年和前7 a水沙条件对当前断面形态的影响权重分别约为30%和70%。本文模型有助于深化前期水沙条件对当前河床形态调整影响机理的认识,并为未来不同水沙情形下主槽断面形态的预测提供了有效计算方法。 相似文献
11.
山区河流准三维水沙输运与河床演变模拟 总被引：1，自引：0，他引：1
在渠道水力学和河流动力学研究的基础上，对不规则河流形态下的纵向流速分布和弯道水流进行了讨论，并分析了任意断面和垂线位置的流场计算公式。在综合各种泥沙动力学研究成果基础之上，探讨了在天然河流中的悬移质输沙能力与沉积条件，分析了河床质起动的控制性因素及推移质输沙方程。提出了受冲淤影响的河床质粒径组成和动态递推公式。结合这些流场与泥沙计算方法及河床形态调整技术，作者提出一个准三维河流动力学模型。该模型只需要河流几何形状、人流条件以及泥沙级配参数，能够模拟出不规则几何形态的河道断面流速分布及平面流场分布，并且对于弯道水流特征与泥沙输送一沉积特征也能得到较好的表现。该模型是对有关河流演变理论和实验成果的综合应用，为研究山区高含沙水流下的河床变形以及洪水演进提供了强有力的工具。 相似文献
12.
Using the confluence nodes of broad-floodplain rivers as an example, we examine the different variants of effects of channel deformations (channel meandering of the main river and its tributary, formation of forks, and shift of meander bars) upon their configuration and reconfiguration across time. We demonstrate the diversity of these effects even where the deformations themselves are of the same type. Problems of further investigations are formulated. 相似文献
13.
黄河上游沙漠宽谷段高含沙支流对干流的淤堵影响 总被引：1，自引：1，他引：0
黄河上游沙漠宽谷段支流——十大孔兑的高含沙洪水多次淤堵黄河干流,造成灾害。基于实测水文泥沙资料和模型试验手段对孔兑高含沙洪水特点、淤堵干流过程和交汇区沙坝形态进行了研究。结果表明：孔兑洪水具有陡涨陡落、持续时间短、洪峰流量大、含沙量高、输沙量大等特点;孔兑洪水形成沙坝、淤堵黄河的过程可分为形成、相对稳定和冲刷3个阶段,支流洪水量级较大,形成的沙坝规模较大,沙坝冲刷历时也较长;孔兑高含沙水流入黄交汇区水流结构分区包括壅水区、回流区、高流速带、剪切层等组成要素,沙坝淤积形态模式包括壅水区、回流区、回流区下游淤积体和交汇区输水输沙窄槽;基于沙坝淤堵判别方法以及实测和试验数据提出了孔兑入黄交汇区沙坝判别条件。 相似文献
14.
汉江丹江口水库下游河床演变 总被引：1，自引：0，他引：1
丹江口水库建库20多年来,下游河道由堆积性河道变成了冲刷性的河道,河床物质沿程粗化,比降得以调平。水深增加幅度大于河宽,流速并不随着水深增加而加快。深槽、浅滩分布明显化,宽深比不断在减少。整个库下游变化可分为三段:1)近库段,游荡河道向单一限制性曲流转化;2)大支流影响段,河道仍保持着游荡特性;3)下游段,游荡段游荡特性减弱,弯曲段则深蚀作用加强。 相似文献
15.
黄河内蒙古段异源水沙输移特性 总被引：1，自引：1，他引：0
河流悬移质泥沙的输移方式按照水沙来源可以分为水沙同源与水沙异源两大类。黄河内蒙古段水沙来自不同的区域,冲泻质泥沙主要来源于陇西黄土高原区,而径流则主要来自唐乃亥以上的山区,呈现异源特性。通过分析内蒙古河段石嘴山与巴彦高勒水文站1951-2003年的水文数据,发现期间共发生了13次大流量和29次高含沙事件且大流量、高含沙量同时出现现象很少,多呈现大流量、低含沙量,或者小流量、高含沙量的态式。同时,通过改变传统流量-含沙量幂函数公式Ci=aQb(Ci为悬移质浓度,Q为流量)的参数项,建立了2种水沙异源情况下流量-含沙量的公式。 相似文献
16.
Three-dimensional structure of flow at a confluence of river channels with discordant beds 总被引：2，自引：0，他引：2
This paper presents three-dimensional data of the mean and turbulent structure of flow collected at a natural confluence of rivers with discordant beds to (1) describe the three-dimensional flow field of a natural junction of channels; (2) assess the role of changes in bed morphology occurring during transport-effective events on the structure of flow at a confluence; and (3) examine how the three-dimensional structure of flow varies with changes in the ratio of momentum flux between the two confluent streams. Three-dimensional measurements of velocity were reconstructed from the measurements obtained with an array of four, two-component electromagnetic current meters. Six detailed velocity profiles were taken at five cross-sections in a wide range of flow conditions. The mean field of flow is characterised by (1) the acceleration of flow in the downstream portion of the post-confluence channel, but by lower velocities upstream in the mixing layer area; (2) a stagnation zone at the apex of the junction; (3) a zone of flow deviation, and strong fluid upwelling, close to the avalanche face and at the margin of the tributary mouth bar; and (4) reduced velocities over the depositional bar at the downstream junction corner. The position and extent of these zones vary with changes in the ratio of momentum flux. Very high intensity of turbulence (peaks up to 50%) and turbulent kinetic energy were observed in the mixing layer region. Distortion of the mixing layer, characteristic of flow where bed discordance is present between the two tributary channels, was evident from mean and turbulent flow data. This field study suggests that the effects of bed discordance on flow, sediment transport, and the resultant bed morphology must be incorporated into conceptual and numeric models of these sites of complex flow. 相似文献
17.
我国河口海岸动力沉积和动力地貌研究以沉积学和地貌学的方法侧重静态描述,动力地貌相互作用的定量研究不多,与缺少动力地貌模型技术有关。本文介绍了中长时间尺度的河口海岸动力地貌模型技术及其发展和应用,讨论了决定动力地貌演变的泥沙余输运及引起泥沙余输运的主要动力因子,介绍了动力地貌模型的应用进展,进而分析了河口海岸地貌过程的机制和地貌平衡系统。本文指出对淤泥质及复杂动力条件下的河口海岸动力地貌过程还有待进一步研究。 相似文献
18.
Takeshi Mizutani 《Geomorphology》1998,24(4):353-361
The formation mechanism of multiple erosional terraces which develop in the dissection process of sedimentary valley fills was investigated by laboratory experiments and computer simulations. Sequences of unpaired terraces were produced on experimental fan-like deposits under conditions of constant water discharge, no base-level change, and no sediment supply. The terraces developed as a result of the repetition of lateral shifting and stillstand of the stream channel at each cross-section during continuous downcutting. This mode of channel migration was caused by meander growth during which amplitude and wavelength increased with time. The terrace formation process was well-reproduced by computer simulations using a meander model having a similar wave mode. 相似文献
19.
The synchronicity and difference in the change of suspended sediment concentration in the Yangtze River Estuary 总被引：2，自引：1，他引：1
The sediment discharge from the Yangtze River Basin has a stepwise decreasing trend in recent years. The impounding of the Three Gorges Reservoir exacerbated this decreasing trend and affected the change of the suspended sediment concentration (SSC) in the Yangtze River Estuary through the transmission effect. The SSC data of the Yangtze River Estuary during 1959-2012 showed that: (1) The SSC in the South Branch of the Yangtze River in the estuary and in the off-shore sea area displayed decreasing trends and decreased less towards the sea. At the same time, the difference in decreasing magnitude between SSC and sediment discharge became bigger towards the sea. (2) For the North Branch the preferential flow did not change much but the SSC tended to decrease, which was mainly caused by the decrease of SSC in the South Branch and China East Sea. (3) Due to the decreased runoff and the relatively strengthened tide, the peak area of the SSC in the bar shoal section in 2003-2012 moved inward for about 1/6 longitude unit compared with that in 1984-2002, and the inward-moving distance was in the order of flood season > annual average > dry season. (4) In the inlet of the South Passage, the SSC decreased mainly because the increase caused by resuspension and shore-groove exchange was less than the decrease caused by the sharp SSC decrease in the basin and the sea areas. The reverse was true in the middle section, where the SSC showed an increasing trend. (5) In the inlet of the North Passage, under the combined influence of decreased flow split and sediment split ratios, the decreased SSC in the basin and the sea area and decreased amount of resuspension, the SSC displayed a decreasing trend. In the middle section, because the increased amount caused by sediment going over the dyke was markedly more than the decreased amount caused by external environments, the SSC tended to increase. Holistically, the sharp decrease in sediment discharge caused synchronized SSC decreases in the Yangtze River Estuary. But there were still areas, where the SSC displayed increasing trends, indicating synchronicity and difference in the response of SSC to the sharp decrease in sediment discharge from the basin. 相似文献
20.
Changes in bed morphology and sedimentology at stream confluences in relation to specific hydrological events have not been documented extensively. Moreover, few, if any, studies have investigated the effect of net hydrological conditions on channel change at a confluence over a period of many years. Repeat cross section surveys and sampling of bed material at a small, asymmetrical stream confluence reveal the short-term effects of hydrological variability on bed morphology and spatial patterns of bed material texture. Survey data over a 15-year period illustrate the long-term response of channel morphology at the confluence to net hydrological conditions. Results confirm that this confluence is a dynamic fluvial environment, involving shifts in the zone of scour and repeated deposition and erosion of a bar complex at the downstream junction corner as the discharge ratio changes in response to hydrological variability in the tributary streams. Persistent faster runoff-response times of the lateral tributary compared to the main stem have resulted in long-term net erosion of the channel bank opposite this tributary. The documented morphological and sedimentological changes are consistent with and can be explained by results of experimental studies and by past investigations at this confluence that have documented changes in patterns of mean flow structure and turbulence in response to variation in the discharge ratio of contributing streams. 相似文献