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1.
Our objective is to understand general causes of different river channel patterns. In this paper we compare an empirical stream power‐based classification and a physics‐based bar pattern predictor. We present a careful selection of data from the literature that contains rivers with discharge and median bed particle size ranging over several orders of magnitude with various channel patterns and bar types, but no obvious eroding or aggrading tendency. Empirically a continuum is found for increasing specific stream power, here calculated with pattern‐independent variables: mean annual flood, valley gradient and channel width predicted with a hydraulic geometry relation. ‘Thresholds’, above which certain patterns emerge, were identified as a function of bed sediment size. Bar theory predicts nature and presence of bars and bar mode, here converted to active braiding index (Bi). The most important variables are actual width–depth ratio and nonlinearity of bed sediment transport. Results agree reasonably well with data. Empirical predictions are somewhat better than bar theory predictions, because the bank strength is indirectly included in the empirical prediction. In combination, empirical and theoretical prediction provide partial explanations for bar and channel patterns. Increasing potential‐specific stream power implies more energy to erode banks and indeed correlates to channels with high width–depth ratio. Bar theory predicts that such rivers develop more bars across the width (higher Bi). At the transition from meandering to braiding, weakly braided rivers and meandering rivers with chutes are found. Rivers with extremely low stream power and width–depth ratios hardly develop bars or dynamic meandering and may be straight or sinuous or, in case of disequilibrium sediment feed, anastomosing. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
2.
An analytical modelling framework is proposed to reproduce the frequently observed but poorly studied occurrence of mid‐channel bars in meandering channels. Mid‐channel bars occur in meanders and may characterize transitional morphologies between pure meandering and braided rivers. Based on existing field and experimental observations, we propose that two different mechanisms can generate central topographical patterns in meanders. A former mechanism (‘width‐forced’) is related to spatial width oscillations which determine a laterally symmetrical bed shear stress pattern that promotes mid‐channel bars. A second mechanism (‘curvature‐forced’) can take place also in curvilinear equiwidth streams since also longitudinal variations of channel curvature can produce laterally symmetrical alterations of the sediment transport capacity. A perturbation approach is employed to model both mechanisms within a common framework, allowing reproduction, at least qualitatively, of several observed features. While width‐forced mid‐channel bars are a symmetric linear altimetric response, to reproduce curvature‐forced mid‐channel bars requires modelling nonlinear flow‐bed topography interactions at the second order of the perturbation expansion. Hypotheses on how these mechanisms operate are further discussed through an application to field cases. The amplitude of the nonlinear response can be relevant compared to that of the point bar in equiwidth meanders and the location of mid‐channel bars seldom coincides with bend apexes, mainly depending upon the intrinsic meander wavelength. Central bars tend to symmetrically divert the flow against the two banks, a process which is proposed as a possible cause of cross‐sectional overwidening, along with the asymmetry between the rates of bank erosion and of the opposite bank accretion. The outcomes of this first modelling step on the subject allow discussion of the mutual feedback processes that characterize interactions between mid‐channel bars and width variations in river meanders. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
3.
This paper analyses types and rates of change in river meander morphology and the links between mechanisms of change and emergent behaviour of planform morphology. It uses evidence of four dates of aerial photography combined with annual field mapping and ground photography to examine the morphological changes and mechanisms of change in a series of bends on an active meandering river, the River Dane in NW England, over a 25 year period. This unique data set allows insight into the spatial and temporal variability of bank line movement and component processes. Bank lines were mapped photogrametrically from air photos of 1984, 1996, 2001 and 2007 and the digitised courses compared in ArcGIS to produce calculations of erosional and depositional areas and rates. Most bends exhibit morphological change that largely follows the autogenic sequence, identified in qualitative models of meander development, from low sinuosity curves through simple symmetric and asymmetric bends to compound forms with lobe development in the apex region. Rates of erosion and bankline movement increase through this sequence until the compound phase. Relationships of amounts of movement to various curvature measures of bend morphology are complex. Several new loops, distinct from compound bend behaviour, have developed during the study period in formerly straight sections. Mechanisms of morphological change are illustrated for four types of bends: new, rapid growth bend; sharp‐angled bend with mid‐channel bar development; symmetric migrating bend; and simple to compound bend development. The changes take place in phases that are not simply related to discharge but to inherent sequences and feedbacks in development of bars and bend morphology and timescales for these are identified. Overall, emergent behaviour of systematic planform change, moderated by channel confinement and boundary features, is produced from spatially and temporally varied channel processes. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
4.
Morphodynamics in sand‐bed braided rivers are associated with simultaneous evolution of mid‐channel bars and channels on the braidplain. Bifurcations around mid‐channel bars are key elements that divide discharge and sediment. This, in turn, may control the evolution of connected branches, with effects propagating to both upstream and downstream bifurcations. Recent works on bifurcation stability and development hypothesize major roles of secondary flow and gradient advantage. However, this has not been tested for channel networks within a fully developed dynamic braided river. A reason for this is a lack of detailed measurements with sufficient temporal and spatial length, covering multiple bifurcations. Therefore we used a physics‐based numerical model to generate a dataset of bathymetry, flow and sediment transport of an 80 km river reach with self‐formed braid bars and bifurcations. The study shows that bar dissection due to local transverse water surface gradients is the dominant bifurcation initiation mechanism, although conversion of unit bars into compound bars dominates in the initial stage of a braided river. Several bifurcation closure mechanisms are equally important. Furthermore, the study showed that nodal point relations for bifurcations are unable to predict short‐term bifurcation evolution in a braided river. This is explained by occurrence of nonlinear processes and non‐uniformity within the branches, in particular migrating bars and larger‐scale backwater‐effects, which are not included in the nodal point relations. Planform morphology, on the other hand, has predictive capacity: bifurcation angle asymmetry and bar‐tail limb shape are indicators for near‐future bifurcation evolution. Remote sensing data has predictive value, for which we developed a conceptual model for interactions between bars, bifurcations and channels in the network. We conducted a preliminary test of the conceptual model on satellite images of the Brahmaputra. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
5.
Channel bars and banks strongly affect the morphology of both braided and meandering rivers. Accordingly, bar formation and bank erosion processes have been greatly explored. There is, however, a lack of investigations addressing the interactions between bed and bank morphodynamics, especially over short timescales. One major implication of this gap is that the processes leading to the repeated accretion of mid‐channel bars and associated widenings remain unsolved. In a restored section of the Drau River, a gravel‐bed river in Austria, mid‐channel bars have developed in a widening channel. During mean flow conditions, the bars divert the flow towards the banks. One channel section exhibited both an actively retreating bank and an expanding mid‐channel bar, and was selected to investigate the morphodynamic processes involved in bar accretion and channel widening at the intra‐event timescale. We repeatedly surveyed riverbed and riverbank topography, monitored riverbank hydrology and mounted a time‐lapse camera for continuous observation of riverbank erosion processes during four flow events. The mid‐channel bar was shown to accrete when it was submerged during flood events, which at the subsequent flow diversion during lower discharges narrowed the branch along the bank and increased the water surface elevation upstream from the riffle, which constituted the inlet into the branch. These changes of bed topography accelerated the flow along the bank and triggered bank failures up to 20 days after the flood events. Four analysed flow events exhibited a total bar expansion from initially 126 m2 to 295 m2, while bank retreat was 6 m at the apex of the branch. The results revealed the forcing role of bar accretion in channel widening and highlighted the importance of intra‐event scale bed morphodynamics for bank erosion, which were summarized in a conceptual model of the observed bar–bank interactions. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
6.
Formation and development of alternate bars in an engineered mountainous reach of the Arc River, France, is studied using photo analysis, 1D modelling and by applying theoretical and empirical models for alternate bar systems. Alternate bars already existed in the 80s in the form of a stable confined wandering system. In 1994, the river bed was flattened after engineering works. However, aerial photographs and cross-sectional profiles show that bars rapidly recovered within a few years. The alternate bar system evolved rapidly with a reduction of the number of bars and so an increasing bar length. The width-to-depth ratio, the slope change, the bend upstream of the reach, and the sediment supplies are the main controls of bar formation and evolution. The system appears to lead to force bars due to the bend but also due to a bridge in the downstream part of the reach. Nevertheless some free mobile bars are still observed in the middle of the reach. A discussion on the alternate bar formation is provided using empirical and analytical models. Finally, impacts of low flows and vegetation seem to be significant in the stabilization of the system toward a confined wandering system as observed before the engineering works. 相似文献
7.
The anastomosing pattern and the extensively distributed scroll bars in the middle Amazon River 总被引：4，自引：0，他引：4
The middle Amazon River, between the confluences of the Negro and Madeira Rivers in Brazil, shows an anastomosing morphology with relatively stable, multiple interconnected channels that locally enclose floodbasins. Additionally, this system is characterized by sinuous secondary channels with meander development, discontinuous natural levees concentrated on the concave banks and extensively distributed scroll bars mainly in the islands, related to subrecent and present‐day migration of mainly secondary channels. This distinguishes the Amazon from many other anastomosing rivers that have laterally stable, non‐meandering channels. We analyzed sedimentary processes using field data, morphology and channel changes trough a temporal analysis using remote sensing data and obtained optically stimulated luminescence (OSL) dating to understand the genesis of this large anastomosing river and the development of its meandering secondary channels. Scroll bars have developed in a multichannel river system at least since 7.5 ± 0.85 ka. Avulsion is inferred to have played a minor role in the formation of this anastomosing system, with only one documented case while mid‐channel bar formation and chute cut‐offs of the main and secondary channels are the main formative mechanisms of anastomosis in this system. Differences in resistance to erosion control the relatively straight main channel and allow secondary channels to develop a meandering platform. Vegetation contributes to the relative stability of islands and the floodplain. Low gradient and high average aggradation rate (1.1 mm yr?1) are conditions which favor the development of anastomosis. Additionally, stable external conditions, low abandonment rate of older channels and independence from high avulsion frequency suggest a long‐lived, semi‐static type of anastomosing river in this reach of the Amazon. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
8.
Vittoria Scorpio Simone Zen Walter Bertoldi Nicola Surian Marco Mastronunzio Elena Dai Prá Guido Zolezzi Francesco Comiti 《地球表面变化过程与地形》2018,43(5):1044-1062
The Adige River drains 12 200 km2 of the Eastern Alps and flows for 213 km within this mountain range. Similar to other large rivers in Central Europe, the Adige River was subject to massive channelization works during the 19th century. Thanks to the availability of several historical maps, this river represents a very valuable case study to document the extent to which the morphology of the river changed due to channelization and to understand how much is left of its original morphodynamics. The study was based on the analysis of seven sets of historical maps dating from 1803–1805 to 1915–1927, on geomorphological analysis, on the application of mathematical morphodynamic theories and on the application of bar and channel pattern prediction models. The study concerns 115 km of the main stem and 29 km of its tributaries. In the pre‐channelization conditions, the Adige River presented a prevalence of single‐thread channel planforms. Multi‐thread patterns developed only immediately downstream of the main confluences. During the 19th century, the Adige underwent considerable channel adjustment, consisting of channel narrowing, straightening, and reduction of bars and islands. Multi‐thread and single‐thread reaches evolved through different evolutionary trajectories, considering both the channel width and the bar/vegetation interaction. Bar and channel pattern predictors showed good correspondence with the observed patterns, including the development of multi‐thread morphologies downstream of the confluences. Application of the free‐bar predictor helped to interpret the strong reduction – almost complete loss – of exposed sediment bars after the channelization works, quantifying the riverbed inclination to form alternate bars. This morphological evolution can be observed in other Alpine rivers of similar size and similar massive channelization, therefore, a simplified conceptual model for large rivers subjected to channelization is proposed, showing that a relatively small difference in the engineered channel width may have a strong impact on the river dynamics, specifically on bar formation. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
9.
We develop a new method for analysis of meandering channels based on planform sinuosity. This analysis objectively identifies three channel‐reach lengths based on sinuosity measured at those lengths: the length of typical, simple bends; the length of long, often compound bends; and the length of several bends in sequence that often evolve from compound bends to form multibend loops. These lengths, when normalized by channel width, tend to fall into distinct and clustered ranges for different natural channels. Mean sinuosity at these lengths also falls into distinct ranges. That range is largest for the third and greatest length, indicating that, for some streams, multibend loops are important for planform sinuosity, whereas for other streams, multibend loops are less important. The role of multibend loops is seldom addressed in the literature, and they are not well predicted by previous modelling efforts. Also neglected by previous modelling efforts is bank–flow interaction and its role in meander evolution. We introduce a simple river meandering model based on topographic steering that has more in common with cellular approaches to channel braiding and landscape evolution modelling than to rigorous, physics‐based analyses of river meandering. The model is sufficient to produce reasonable meandering channel evolution and predicts compound bend and multibend loop formation similar to that observed in nature, in both mechanism and importance for planform sinuosity. In the model, the tendency to form compound bends is sensitive to the relative magnitudes of two lengths governing meander evolution: (i) the distance between the bend cross‐over and the zone of maximum bank shear stress, and (ii) the bank shear stress dissipation length related to bank roughness. In our simple model, the two lengths are independent. This sensitivity implies that the tendency for natural channels to form compound bends may be greater when the banks are smoother. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
10.
The changing form of developing alluvial river bars has rarely been studied in the field, especially in the context of the fixed, compound, mainly alternate gravel bars that are the major morphological feature of the wandering style. Century scale patterns of three‐dimensional growth and development, and the consequent scaling relations of such bars, are examined along the gravel‐bed reach of lower Fraser River, British Columbia, Canada. A retrospective view based on maps and aerial photographs obtained through the twentieth century shows that individual bars have a life history of about 100 years, except in certain, protected positions. A newly formed gravel bar quickly assumes its ultimate thickness and relatively quickly approaches its equilibrium length. Growth continues mainly by lateral accretion of unit bars, consistent with the lateral style of instability of the river. Bar growth is therefore allometric. Mature bars approach equilibrium dimensions and volume that scale with the overall size of the channel. Accordingly, the bars conform with several published criteria for the ultimate dimensions of alternate barforms. Sand bars, observed farther downstream, have notably different morphology. Fraser River presents a typical wandering channel planform, exhibiting elements of both meandered and low‐order braided channels. Hydraulic criteria to which the Fraser bars conform illustrate why this planform develops and persists. The modest rate of bed material transfer along the channel – typical of the wandering type – determines a century‐length time scale for bar development. This time scale is consistent with estimates that have been made for change of the macroform elements that determine the overall geometry of alluvial channels. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
11.
Meandering rivers have dynamic evolution characteristics of lateral migration and longitudinal creeping movement, and studies on the migration rate of meandering rivers have both scientific and practical significance for understanding the evolution process. A river source region often is sparsely populated and lacks long-term monitoring data, making it difficult to estimate the migration rate of river bends. In the source region of the Yellow River, located in the northeastern part of the Qinghai-Tibet Plateau, China, meandering rivers have extensively developed. Combined with field investigation and sampling in the source region in 2016 and 2017, 9 river bends in the middle Baihe River were selected to attempt estimation of migration rates of the river bends using tree ring analysis. The tree core and disc samples were collected using an increment borer and a crosscut saw, and the ages of the trees were estimated based on tree ring analysis. A method for estimating the migration rate of river bends based on the relation between positions and ages of trees grown on the point bars in inner banks is proposed. The estimated migration rates of the 9 river bends of the Baihe River ranged 0.38–6.10 m/yr, and the migration rates were found to be related to the flow rate, channel slope, height of the outer bank, and width of the river valley. The maximum migration rate was determined to be at the No. 9 River Bend where the ratio of the meander-bend radius to the channel width (R/W) was 2.31, which is consistent with previous findings that the bend migration is most rapid in the ‘migration phase’. The proposed method for estimating the migration rate of river bends provides a potential alternative option for future study on the morphodynamic process of a meandering river. 相似文献
12.
Hydraulic and geomorphic processes in an overbank flood along a meandering,gravel‐bed river: implications for chute formation 
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下载免费PDF全文 Hydraulic interactions between rivers and floodplains produce off‐channel chutes, the presence of which influences the routing of water and sediment and thus the planform evolution of meandering rivers. Detailed studies of the hydrologic exchanges between channels and floodplains are usually conducted in laboratory facilities, and studies documenting chute development are generally limited to qualitative observations. In this study, we use a reconstructed, gravel‐bedded, meandering river as a field laboratory for studying these mechanisms at a realistic scale. Using an integrated field and modeling approach, we quantified the flow exchanges between the river channel and its floodplain during an overbank flood, and identified locations where flow had the capacity to erode floodplain chutes. Hydraulic measurements and modeling indicated high rates of flow exchange between the channel and floodplain, with flow rapidly decelerating as water was decanted from the channel onto the floodplain due to the frictional drag provided by substrate and vegetation. Peak shear stresses were greatest downstream of the maxima in bend curvature, along the concave bank, where terrestrial LiDAR scans indicate initial floodplain chute formation. A second chute has developed across the convex bank of a meander bend, in a location where sediment accretion, point bar development and plant colonization have created divergent flow paths between the main channel and floodplain. In both cases, the off‐channel chutes are evolving slowly during infrequent floods due to the coarse nature of the floodplain, though rapid chute formation would be more likely in finer‐grained floodplains. The controls on chute formation at these locations include the flood magnitude, river curvature, floodplain gradient, erodibility of the floodplain sediment, and the flow resistance provided by riparian vegetation. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
13.
Pool–riffle sequences (PRSs) are periodic river‐bed morphologies with wavelengths several times the channel width. Causes of PRS formation and maintenance are not clearly understood, which may limit the effectiveness of protection and rehabilitation measures. Some confusion has existed about whether the PRS morphology is the same as or distinct from alternate bars. In this paper we investigate whether the bar instability forming alternate bars also contributes to PRS formation, periodicity and maintenance. This was unclear because bar instability occurs only when the ratio of channel width/depth exceeds a critical value, generally understood to be approximately 10, which is larger than the width‐to‐depth ratio of many PRSs. A mobile‐bed physical model is used to test whether bar instability occurs in channels characteristic of PRS morphology, with low width‐to‐depth ratio, and high relative roughness. The physical model was scaled from a prototype PRS in a gravel and cobble bed river. Alternate bars formed in the model at channel width‐to‐depth ratios as low as 3·8. The wavelength of the alternate bars formed was generally 2·2–5 times channel width, which was similar to the prototype PRS. Therefore, bar instability can occur in virtually all PRSs, and it contributes to the widespread formation of periodic PRS morphology. The model showed that maintenance of the bar height in the prototype PRS also depends on variations in channel width. It is concluded that periodic PRSs are formed and maintained by the interaction between bar instability, and flow deflections associated with variations in channel geometry such as width variation. Resonance between bar instability and three‐dimensional bed forms such as alternate bars and variations in channel geometry. Variations in channel geometry are also important in determining the location and dimensions of individual pools and bars. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
14.
Pu QI Gouting LIANG Zangying SUN Honghai QI Senior Engineer Institute of Hydraulic Research Yellow River Conservancy Commission Zhengzhou P.R. China Engineer Ins 《国际泥沙研究》2002,17(1)
1 INTRODUCTION It is well known that the river channel patterns are determined by long-term water-sediment conditions. However, quantitative expressions for river channel patterns and their fluvial processes still remain not clear. Existing theories in this aspects, such as geomophological theory, theory of maximum energy consuming rate, stability theory, theory of probability, and statistic analysis, are developed based on certain simplified assumptions and can not be successfully used t… 相似文献
15.
Artificially straight river channels tend to be unstable, and ultimately develop into river meanders through bank erosion and point‐bar deposition. In this paper account is taken of the effects of riparian and floodplain vegetation on bank strength, floodplain flow resistance, shear stress partitioning, and bedload transport. This is incorporated into an existing 2D hydrodynamic‐morphological model. By applying the new model to an initially straight and single‐threaded channel, the way that its planform and cross‐sectional geometry evolve for different hydraulic and floodplain vegetation conditions is demonstrated. The results show the formation and upstream migration of gravel bars, confluence scouring and the development of meandering and braiding channel patterns. In cases where the channel becomes unstable, the instability grows out of bar formation. The resulting braiding patterns are similar to analytical results. The formation of a transition configuration requires a strong influence from vegetation. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
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17.
1 INTRODUCTION Meanders are the fundamental planform of alluvial rivers and bank erosion in the meander bends is a common feature. In most of the practical cases, the eroded material from the outer bank deposits along the inner bank to develop sand bars. Very often, bank erosion is drastic and channel width increases in an order of several hundred meter per year at some reaches of large scale meandering rivers, for example, the Ganges river inside Bangladesh, as noticed by Muramoto and… 相似文献
18.
In this paper we use multiple field surveys spanning several decades to systematically evaluate the geomorphic consequences of a change in flow hydraulics from uniform flow to backwater flow for the lower Trinity River in east Texas, USA. Spatial changes in lateral migration rate, channel geometry, and point bar size correspond to two distinct geomorphic zones. Within the upstream uniform flow reach, the river channel is defined by fully developed point bars and a high rate of lateral channel migration. This zone transitions where the median channel bottom elevation drops below sea level. At this point flow is affected by the backwater influence of the Trinity Bay water surface elevation, as opposed to being bed slope control dominated. The change in hydraulics within the backwater zone is reflected in the channel morphology, which is characterized by smaller point bars, narrower and more symmetrical cross-sectional channel geometry, lower channel migration rates, and little to no bend deformation or cutoffs. Studying the connection between channel geometry, river bend kinematics, sediment transport, and fluid mechanics in each zone provides a deeper understanding of the relationship between channel shape and river mechanics. © 2019 John Wiley & Sons, Ltd. 相似文献
19.
River meandering has been successfully modelled using vector based methods, but these can not simulate multiple or braided channels. Conversely, cellular braided river models fail to replicate meandering. This paper describes a new method to simulate river meandering within a cellular model (CAESAR). A novel technique for determining bend radius of curvature on a cell by cell basis is described, that importantly allows regional information on bend curvature to be transferred to local points. This local curvature is then used to drive meandering and lateral erosion through two methods. Key difficulties are identified, including the deposition of material on point bars and cut off development, but the method illustrates how meandering can be integrated within a cellular framework. This demonstrates the potential to produce a single model that can incorporate both meandering and braiding. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
20.
Air photo interpretation and field survey were used to examine rates and patterns of planform change over the last 40 years on an 80 km reach of the Luangwa River, Zambia. The river, a tributary of the Zambezi, is a 100–200 m wide, medium sinuosity sand‐bed river (sinuosity index 1·84). High rates of channel migration (<33 m a−1) and cutoffs on meandering sections are frequent. Some meandering reaches, however, have remained relatively stable. A form of anastomosing with anabranches up to 14 km in length is also a characteristic. Patterns of meander development vary between bends but all can be described in relation to traditional geomorphic models; change occurs by translation, rotation, double‐heading, concave bank bench formation and cutoff causing river realignment. At the local scale spatial variability in bank resistance, induced by floodplain sedimentology, controls rate of bank erosion, and valley‐side channel ‘deflection’ is also apparent. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献