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1.
River meanders are natural features on the surface of Earth that present some degree of regularity of form. They range from being highly dynamic to being stable under present conditions. Conventional theory is that meanders develop to an equilibrium form which is related to discharge and sediment load. Other research has demonstrated that many highly active meanders exhibit a continuous evolution over time and a non-linearity in rate of development. Ideas of autogenesis and of self-organised criticality as being an explanation of some meander changes have been proposed. In this paper data from rivers around the world are examined for further evidence of autogenic, self-organised or non-linear behaviour through analysis of change in sinuosity over time for reaches and change in individual bend form, particularly bend curvature and bend elongation. Some examples do exhibit trends of increasing sinuosity over time and a few show limits from which large decreases occur. Several case studies show non-linearity of behaviour and increasing complexity of form. Other case studies, however, do not exhibit such trends. Phase space plots are used to help uncover emergent behaviour but show a variety of patterns. The example of a reach in which multiple cut-offs occurred is analysed for mechanisms of self-organisation of the planform and in the pool-riffle pattern. Riffles are more closely spaced and also more transient in the more rapidly changing and higher sinuosity parts of the channel. Hypothetical trajectories of different meander behaviour, including for bedrock meanders, are plotted but the challenge remains to uncover the conditions for occurrence and for divergence of tendencies to stability and instability. Identification of attractors and phase space of behaviour of different meandering systems offer the potential for application to sustainable channel management. 相似文献
2.
Marcin Słowik 《Geografiska Annaler: Series A, Physical Geography》2014,96(1):61-81
Ground‐penetrating radar surveys, coring and the analyses of satellite and aerial images have been carried out to study differences in the evolution of meander bends formed in various geological conditions. The research was conducted in the lower course of the Obra River (western Poland) characterized by a complex geology: particular sections of the valley were formed in glacial, stagnant water and fluvioglacial deposits. The research was conducted in four detailed study sites representing different formation conditions for the meander bends. Four types of meanders were distinguished: laterally migrating bends characterized by frequent changes of migration direction accompanied by river bed avulsions and cutoffs; bends with traces of continuous migration limited by stagnant water basin deposits; confined meanders in a narrow valley formed in glacial till characterized by the occurrence of mid‐channel islands; and meanders with traces of complex changes of the river bed migration influenced by anthropogenic intervention followed by intensive overbank deposition. Moreover, traces of an early development of the Obra valley and remains of multi‐channel pattern were discovered. The results also show that despite being formed in different geological conditions, a similar number of the meander migration phases were recorded in the floodplain architecture during the last 7000 years at each of the sites. It is also found that the development of the studied meanders was slow compared with cases of actively migrating meandering rivers. 相似文献
3.
边界条件对曲流发育影响的过程响应模型实验研究 总被引：7，自引：0，他引：7
基于系统论模型化原理及地貌演化类比性法则的过程响应模型,有利于研究河型演化,河道过程及控制因素的作用。运用该模型所进行的边界条件对曲流发育影响的实验表明,河漫滩物质结构及河床上的抗蚀露头对曲流发育具有控制作用。 相似文献
4.
The creation of cutoffs and of oxbow lakes is a well-known phenomenon of meandering rivers, but views on the extent to which they are inherent in meander behaviour have varied. Assumptions of meander behaviour have shifted from those of stability and equilibrium to recognition of gradual evolution and increased complexity of form. Alternative explanations of cutoff occurrence are discussed here in relation to a remarkable set of cutoffs that occurred in one reach of the River Bollin, UK, for which long-term historical evidence of meander evolution existed and which has been monitored for change and processes over the last 20 years. The cutoffs occurred during the high floods of winter 2000–2001. A series of hypotheses is examined, including the occurrence of floods and effects of hydrological changes. Although the flood events actually caused the cutoffs, the long-term pattern accords with ideas of chaotic behaviour and sinuosity of a river reaching a critical state at which clustering of meander cutoffs takes place. It is suggested that the occurrence of the cutoffs can be explained as inherent in meander behaviour. 相似文献
5.
The Holocene evolution of the shallow alluvial valley occupied by the Red River was investigated at two successive river meanders near St. Jean Baptiste, Manitoba. A transect of five boreholes was sited across the flood plain at each meander to follow the path of lateral channel migration. From the cores, 24 wood and charcoal samples were AMS radiocarbon dated. The dates from the lower half of the alluvium in each core are interpreted to represent the age of the lateral accretion deposits within the flood plain at the borehole sites. The ages of these deposits increase progressively from 900 to 7900 and 1000 to 8100 cal years B.P. along each transect, respectively, from the proximal to distal portions of the flood plain. At the upstream meander, the average rate of channel migration was initially 0.35 m/year between 7900 and 7400 cal years B.P., then decreased to 0.18 m/year between 7400 and 6200 cal years B.P., and subsequently varied between 0.04 and 0.08 m/year. Net channel incision of the river since 8100 cal years B.P. is estimated to have ranged between 0.4 and 0.8 m/ky. The pre-6000-years-B.P. interval of greater channel migration is hypothesized to reflect a higher phase of sediment supply that was associated with the establishment of the river system on the former bed of glacial Lake Agassiz. Since 1000 years B.P., the outward migration of the meanders has caused a gradual enlarging of 0.7–2% in the cross-sectional area of the shallow valley at the two meanders. When considered proportionally over timescales of up to several centuries, the widening of the valley cross-section is very low to negligible and is deemed an insignificant factor affecting the modern flood hazard on the clay plain. 相似文献
6.
密云水库以上的潮白河分为白河和潮河两支,分别穿行于燕山山区。两河的上段为顺直河型或游荡河型,下段为典型的深切曲流。河势及河型受地质构造的深刻影响。潮白两河的纵比降大,来水、来沙在时间和空间上的分配不均匀,对不同河型的形成有影响,这也表明密云水库以上的潮白河具有山地河流的特点。 相似文献
7.
A. S. Zavadskii A. K. Il’yasov S. N. Ruleva V. V. Surkov L. A. Turykin R. S. Chalov 《Geography and Natural Resources》2013,34(1):26-33
We examine the transformation of a gentle segmental to a meander loop with the result that the city, founded in the 17th century, found itself on the caving bank. Comparison of cartographic materials from different times showed the occurrence of secondary bends on elongated rectilinear wings. Not only did such an evolution of the meanders lead to the caving of the terraced urban bank, but also it involved a likely emergency situation on the underwater crossing of the pipeline, and on other engineering facilities. We ascertained the causes of the ongoing channel deformations; modeling data were used in developing recommendations on meander straightening in order to divert the river away from the city. 相似文献
8.
Prediction of alluvial channel pattern of perennial rivers 总被引：2，自引：0，他引：2
Jan H. van den Berg 《Geomorphology》1995,12(4)
Purely braided, meandering and straight channels can be considered as end-members of a continuum of alluvial channel patterns. Several researchers have succeeded in separating channel patterns in fields defined by flow related parameters. However, the discriminators of the principal channel patterns derived from these diagrams all require some a priori knowledge of the channel geometry. In this paper a method is presented which enables prediction of the equilibrium conditions for the occurrence of braided and high sinuosity meandering rivers in unconfined alluvial floodplains. The method is based on two, almost channel pattern independent, boundary conditions: median grain size of the river bed material, and a potential specific stream power parameter related to bankfull discharge or mean annual flood and valley gradient. This can be regarded as a potential maximum of the available flow energy corresponding to the minimum sinuosity condition, P = 1. Based on an analysis of 228 datasets of measurement sites along rivers from many parts of the world an independent discriminating function was found that separates the occurrence of braided rivers and meandering rivers with P > 1.5. The function applies to equilibrium conditions of rivers that neither incise nor show rapid aggradation, with a bankfull or mean annual flood discharge above 10 m3/s and a median bed material grain size between 0.1 and 100 mm. 相似文献
9.
高含沙型曲流河床形成机理的初步研究 总被引：4，自引：3，他引：4
本文描述了在我国黄土高原宽谷中发现的含沙量超过100kg/m^3的曲流河床，并运用高含沙水流的作用对其成因进行了探讨。指出当水流进入高含沙范围时，水流能耗率大幅度降低，挟沙能力大大增加，这是促使河床向弯曲发展的主要原因。由于高含沙水流的稳定输送有赖于边界条件，故边界条件在这种曲流河床的形成中起了重要的作用。文中并讨论了这一地区游荡河型与弯曲河型的判别条件。 相似文献
10.
Response of active tectonics on the alluvial Baghmati River, Himalayan foreland basin, eastern India 总被引：1，自引：2，他引：1
Active tectonics in a basin plays an important role in controlling a fluvial system through the change in channel slope. The Baghmati, an anabranching, foothills-fed river system, draining the plains of north Bihar in eastern India has responded to ongoing tectonic deformation in the basin. The relatively flat alluvial plains are traversed by several active subsurface faults, which divide the area in four tectonic blocks. Each tectonic block is characterized by association of fluvial anomalies viz. compressed meanders, knick point in longitudinal profiles, channel incision, anomalous sinuosity variations, sudden change in river flow direction, river flow against the local gradient and distribution of overbank flooding, lakes, and waterlogged area. Such fluvial anomalies have been identified on the repetitive satellite images and maps and interpreted through DEM and field observations to understand the nature of vertical movements in the area. The sub-surface faults in the Baghmati plains cut across the river channel and also run parallel which have allowed us to observe the effects of longitudinal and lateral tilting manifested in avulsions and morphological changes. 相似文献
11.
A revised typology of Australian tropical rivers was applied to the complete channel network (named and major rivers) shown on 1:250 000 topographic maps for three large drainage basins in northern Australia (Daly River, NT; Fitzroy River, WA; Flinders River, Qld). Reach mapping and classification were conducted using the revised typology. The 12 major river types proposed were: (1) bedrock rivers; (2) bedrock‐confined and ‐constrained rivers; (3) low sinuosity (straight) rivers; (4) meandering rivers; (5) wandering rivers; (6) anabranching rivers; (7) chains of ponds; (8) gullies; (9) floodouts; (10) lakes, swamps, billabongs and wetlands; (11) non‐channelized valley floors; and (12) estuarine rivers. The 12 major river types were developed based on river reach mapping for more than 264 000 km2 of tropical Australian catchments. At scales larger than 1:250 000, subdivision of each major river type is recommended. In the Daly and Fitzroy catchments, confined and constrained rivers dominate, whereas in the Flinders and Fitzroy catchments, anabranching rivers dominate. The dominant river types need benchmarking with adequate numbers of control reaches so that channel changes induced by human and natural impacts can be measured by reference to the stability of these controls. Wandering rivers, floodouts and non‐channelized valley floors were rare for the 1:250 000 channel network in northern Australia but need inclusion in national parks. 相似文献
12.
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
13.
Field surveys, location-for-time reasoning, and computer modeling were used to study the evolution of slopes on valley walls of abandoned bedrock meanders on the Eastern Highland Rim, Tennessee. Hillslopes on the undercut slopes of cutoff incised meanders were ordered as to relative age by the height of their meander floors above the modern stream level. The assumption is that the undercut slope is actively eroded by the stream until abandonment of the meander, at which time the slope begins to evolve to a different form. More-advanced stages of evolution occur on walls of higher meanders that were abandoned earlier. The most rapid change in this initial form is the elimination of a free face, which occurs soon after the meander is abandoned. In addition, the hillslopes associated with even the lowest (youngest) cutoff meanders show somewhat gentler overall gradients than the actively undercut slopes. Hillslopes associated with meanders 3 to about 20 m above modern stream level maintain straight segments with angles showing only a slight decrease from the 36–38° associated with the lowest cutoffs; overall angles decrease, however, as the straight segment becomes shorter. The oldest slopes, those on cutoffs 30 m or greater above modern stream level, have developed into convex–concave slopes with maximum slopes of 15°.A hillslope evolution model based on previously published algorithms was used to simulate the transition of actively undercut hillslopes into hillslopes on abandoned meanders. Hillslope modeling is particularly useful in this setting. If the valley incision rate is known, an age can be estimated for the cutoff and hence for the hillslope. Alternatively, if hillslope process rates are known, a model age obtained for the hillslope can be used to estimate an incision rate. Even where both incision rates and hillslope process rates are poorly constrained, as in the present setting, modeling allows assumptions about specific rates to be evaluated by determining their implications for other rates. For example, for three cutoff meanders along one stream, best-fit criteria were used to select process rates for the model. Model ages of hillslopes were then obtained and compared with those calculated from a valley-incision rate measured elsewhere in the same physiographic province. For two of the hillslopes, model ages were found to be much younger than those calculated from the incision rate. In order to make the two ages agree, unreasonably low process rates had to be used in the model, thus implying that the incision rate probably underestimates the actual incision rate in this valley.Experimentation with heights of initial profiles, again using best-fit criteria, suggests that since abandonment of the highest cutoff, the plateau has been downwasting at a rate about one-fourth that of the valley incision rate, a finding in agreement with published rates of chemical denudation in the area. 相似文献
14.
Channel change is an important aspect of geomorphological evolution and habitat dynamics in large alluvial rivers. Planimetric maps of channel locations were used to investigate spatio-temporal alluvial channel changes in a geomorphic context along the Lower Mississippi River (LMR). Analyses were conducted with the aid of a time-weighted locational probability map. The locational probability map was constructed in ArcGIS and covered a period of 205 years. An examination of the pixel data from the probability maps indicates a high occurrence of low probability pixels along the Lower Mississippi River, which is in accordance with the dynamism of alluvial rivers. The northern section of the Lower Mississippi River (Columbus, KY to Memphis, TN) has been much more stable than the southern river segments (Helena, AR to Natchez, MS). Areas of high channel probability (channel stability) were often associated with alluvial channel confinement from a combination of flood-plain deposits, geologic structures and large stable islands. Low channel probability locations were found along sections exhibiting the following geomorphic characteristics: changes in meander amplitude, meander neck and chute cutoffs, meander extensional processes and islands lost in channel migrational processes. The results provide a strong foundation for understanding channel change on the Lower Mississippi River and serves as a valuable instrument for future management and restoration schemes. 相似文献
15.
The objective of this study was the formulation of fluvial morphological regularities for small rivers with a wide range of morphological and geological characteristics in North-Rhine-Westphalia (Germany) based on a statistical research methodology. Such empirical quantitative information on reference conditions is required for the restoration of small rivers in the former highly industrialised Ruhr-Area. Following the approach of some classic empirical works in fluvial morphology of the last century, several natural reference rivers in the entire research area have been observed in order to provide a statistical correlation between independent and dependent morphological variables. Regressions between valley-floor slope, bankfull discharge and stream power on the one hand and several variables describing the longitudinal profile and river planform on the other hand have shown some significant results. The regularities found are a quantitative contribution to the establishment of reference conditions as well as a useful tool for the restoration of small rivers, if the specific properties and values of the underlying random sampling are taken into account. In addition, the relation between stream power and sinuosity shows the likely existence of a threshold: Exceeding a stream power of 100 W/m, the sinuosity decreases after an increase for lower stream power values. Comparable thresholds were found for the relation between stream power and pool depth as well as stream power and step steepness. The thresholds could be explained by a change in the type of energy dissipation, due to different physio-geographical settings in highland rivers within forested v-shaped valleys. Here, large-woody debris seems to increase the channel roughness and possibly replaces the significance of coarse-grained bed material, pool depth and step steepness as contributors to energy dissipation. 相似文献
16.
At present, the Ramu River in Papua New Guinea flows directly into the Bismarck Sea, while the adjacent Keram flows into the Sepik River. There is evidence to suggest that the Ramu previously occupied the existing Keram water course, and has been gradually diverting its discharge into its present channel, probably over the last 4,000 years. The diversion process is not complete yet, and during the wet season flood water still travels across these two rivers. The most direct evidence to support this proposal of river diversion is that there are much larger meander bends on which the existing Keram stream channel is superimposed. The diversion of the Ramu is believed to be a result of relative sea‐level rise in the late Quaternary which altered the gradients of the Keram and Ramu rivers, and may have been facilitated through neo‐tectonic movement of the floodplain. 相似文献
17.
小比例尺制图中河流自动渐变绘制是地图制图研究中的常见问题,也是一个技术难点。该文针对当前河流自动渐变研究实用性较差的现状,提出一种基于河流类树结构的河流自动渐变方法:在研究河流复杂树结构基础上,提出了河流类树结构及其构建方法,并从网结构中搜索出主流和支流、河流的流向以及面状水域的处理,在补充河流上下游出图面的长度后,依据河流渐变率实现河流的渐变绘制,并基于MapGIS二次开发,实现了小比例尺地图河流的绘制。结果表明,该算法极大减少了地图制图过程中河流渐变绘制的人工干预,提高了自动化渐变绘制效率,具有可行性。 相似文献
18.
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
19.
Laura Jugaru Tiron Jrme Le Coz Mireille Provansal Nicolae Panin Guillaume Raccasi Guillaume Dramais Philippe Dussouillez 《Geomorphology》2009,106(3-4):186-197
This article analyzes the water and suspended solid fluxes through a straightened meander of the southern branch of the Danube Delta (the St. George branch) during episodic flooding. The Mahmudia study site corresponds to a vast natural meander which was cut off in 1984–1988 by an artificial canal opened to shipping. The meander correction accelerated fluxes through the artificial canal and dramatically enhanced deposition in the former meander. After his formation, the cutoff meander acted as sediment storage locations, essentially removing channel and point bar sediments from the active sediment budget of the main channel. Increases in slope and stream power in reaches upstream and downstream have also occurred, but to a lesser degree. During the one-hundred-year recurrent flood in April 2006, bathymetry, flow velocity and discharge data were acquired across several sections of both natural and artificial channels with an acoustic Doppler current profiler (aDcp Workhorse Sentinel 600 kHz, Teledyne RDI) in order to investigate the distribution of the flow and sediment and his impact on sedimentation in a channelized reach and its adjacent cutoff. The contrasting hydro-sedimentary processes at work in both channels and bifurcation/confluence nodal points are analyzed from the measured flux distribution, morphological profiles and velocity and concentration patterns. In the cutoff, a diminishing of the intensity of the flow velocity (c. 50%) and of the SSC was observed correlated with the aggradation of the river bed. In the bifurcation/confluence nodal points and in the artificial canal were observed the most intensive hydrodynamic activity (high flow velocity, SSC concentration, degradation of the river bad). Both the event-scale and long-term morphological trends of the alluvial system are discussed analyzing the boundary shear stress and SSC variability. Excess boundary shear stress in the sub-reaches directly affected by cutoffs resulted in scour that increased downstream bed material load. These high sediment loads play a key role in driving morphological adjustments towards equilibrium in the cutoff channel.The approach followed in this paper combines detailed episodic in-situ aDcp measurements and robust numerical 1D modeling in order to provide a practical comprehension of the relevant morphodynamical processes. The 1D model reproduces robustly the continuity of hydrodynamical variables along the streamwise axes of the two-channel network. The simulated are used in the paper for highlighting reach-scale morphological processes, at both event and long-term scales. 相似文献
20.
Brooks C. Pearson 《The Professional geographer》2005,57(3):376-394
This study compares the accuracy of four maps available to Union and Confederate officers during the 1862 Shenandoah Valley campaign of the American Civil War. It examines historical maps of the Valley of Virginia by the following cartographers: James W. Abert, Hermann Böÿe and Lewis von Buchholtz, Jedediah Hotchkiss, and Franz Kappner. Both simple and three‐tiered sinuosity measures are derived for reference points along study map representations of the Shenandoah river system. These data are then statistically compared to corresponding sinuosity data from USGS topographic quadrangles to identify the relative accuracies of the historical maps. This article offers evidence to refute the common historical assumption that Hotchkiss provided Maj. Gen. Thomas “Stonewall” Jackson with terrain intelligence that was far superior to that available to his Union opponents. Evidence indicates that maps by Union cartographers as well as the prewar Böÿe‐Buchholtz map were at least as accurate, or superior to, the work of Hotchkiss. 相似文献