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1.
Channel incision is part of denudation, drainage-network development, and landscape evolution. Rejuvenation of fluvial networks by channel incision often leads to further network development and an increase in drainage density as gullies migrate into previously non-incised surfaces. Large, anthropogenic disturbances, similar to large or catastrophic “natural” events, greatly compress time scales for incision and related processes by creating enormous imbalances between upstream sediment delivery and available transporting power. Field examples of channel responses to antrhopogenic and “natural” disturbances are presented for fluvial systems in the mid continent and Pacific Northwest, USA, and central Italy. Responses to different types of disturbances are shown to result in similar spatial and temporal trends of incision for vastly different fluvial systems. Similar disturbances are shown to result in varying relative magnitudes of vertical and lateral (widening) processes, and different channel morphologies as a function of the type of boundary sediments comprising the bed and banks. This apparent contradiction is explained through an analysis of temporal adjustments to flow energy, shear stress, and stream power with time. Numerical simulations of sand-bed channels of varying bank resistance and disturbed by reducing the upstream sediment supply by half, show identical adjustments in flow energy and the rate of energy dissipation. The processes that dominate adjustment and the ultimate stable geometries, however, are vastly different, depending on the cohesion of the channel banks and the supply of hydraulically-controlled sediment (sand) provided by bank erosion.The non-linear asymptotic nature of fluvial adjustment to incision caused by channelization or other causes is borne out in similar temporal trends of sediment loads from disturbed systems. The sediments emanating from incised channels can represent a large proportion of the total sediment yield from a landscape, with erosion from the channel banks generally the dominant source. Disturbances that effect available force, stream power or flow energy, or change erosional resistance such that an excess of flow energy occurs can result in incision. Channel incision, therefore, can be considered a quintessential feature of dis-equilibrated fluvial systems. 相似文献
2.
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
3.
Impacts of colonial settlement upon catchment-scale fluvial geomorphic relationships are reported for a relatively small catchment in northern Rio de Janeiro State, Brazil. Structural controls have induced the type and patterns of rivers in Macaé Basin. Fault block activity has resulted in steep, incised headwater streams above the escarpment. Confined and partly confined rivers in mid-catchment reaches of the rounded foothills have limited potential for geomorphic adjustment. Fluvial, estuarine and marine sediments in low relief landscapes of the lowland plain have supported the development of meandering sand bed rivers, with many cut-and-fill (intact valley fill) deposits in tributary systems. Indigenous people exerted relatively minor, localized impacts upon the geomorphology of this river system. Portuguese settlement since the sixteenth century brought about clearance of much of the Atlantic Forest of lowland reaches, and subsequent establishment of sugar cane and coffee plantations. Lowland reaches were channelized from the 1940s-1980s for flood protection and to support the expansion of pastoral agriculture. Significant adjustments have occurred to these geomorphologically sensitive reaches. In contrast, although rivers in the rounded foothills were impacted by forest clearance, the limited availability of sediment stores along these reaches has limited the extent of geomorphic responses to human disturbance. Relatively inaccessible upland reaches were even less impacted, and are now major conservation areas. Building on principles of the River Styles framework, catchment-scale evolutionary trajectories of rivers in the Macaé Basin are assessed based on analysis of patterns of river types, their capacity for adjustment and connectivity relationships, and responses to disturbance events. From this, three future scenarios of prospective evolutionary traits are developed: a ‘steady as she goes’ scenario, an optimistic (effective, proactive management) scenario, and a ‘doomsday’ scenario. 相似文献
4.
The influence of river training on mountain channel changes (Polish Carpathian Mountains) 总被引：2，自引：0，他引：2
The purpose of this paper is to explain the influence of river training on channel changes in mountain rivers. Also considered are the causes of failure of different training schemes. The research was conducted on the regulated Mszanka and Porębianka Rivers, belonging to the Raba River drainage basin in the Polish Flysh Carpathian Mountains. Channel mapping carried out in 2004 drew attention to the contemporary morphology of the channels and the development of their dynamic typology. General changes in channel morphometry and land cover were identified by comparing cartographic sources from various years. Archive material from Cracow's Regional Water Management Authority (RZGW) was used to analyse the detailed channel changes caused by each regulation structure. The material consisted of technical designs of individual training works, as well as plans, longitudinal profiles and cross-sections of trained channel reaches. A series of minimum annual water stages at the Mszana Dolna gauging station was used to determine the tendency of channel bed degradation over 53 years. During the first half of the 20th century, the middle and lower courses of the Mszanka and Porębianka Rivers had braided patterns. The slopes, mostly covered with crops, were an important source of sediment delivery to the river channels. Today, both channels are single-threaded, narrow and sinuous. Downcutting is the leading process transforming the channels. They cut down to bedrock along about 60% of their lengths. The main type of channel is an erosion channel, which occurs also in the middle and lower courses of the rivers. The channel sediment deficit is an important cause for river incision. Sediment supply to the channels was reduced after a replacement of crops on the slopes by meadows or forests. Gravel mining has also caused channel downcutting. The rapid channel changes began after 1959, as systematic training was introduced. Channel regulation seems therefore to be a major factor determining channel adjustment. Debris dams and groynes were built before 1980 and these caused the greatest change of channel pattern, increase of channel gradient and magnitude of river incision. After that date the measures mostly involved drop structures. From then on, the rate of downcutting decreased considerably, but has not ceased. The rivers continued to incise until bedrock was exposed or training structures were destroyed. After that, a tendency to lateral migration and local braiding were observed in the deepened channel. The channels displayed a tendency to return to their morphology and dynamic from before the training. The results demonstrate that river training distorts the equilibrium of channel systems. A channel becomes divided into artificial reaches, which later follow different evolutionary patterns. Most training schemes on mountain channels are ineffective in the long term, as river managers seem to consider a channel at a reach scale only. Individual channel reaches, however, are not independent but rather form a system that must be managed at the entire channel scale. 相似文献
5.
Jiongxin Xu 《Geomorphology》2004,57(3-4):321-330
Through an analysis of data collected from the Yellow River and its tributaries on the Loess Plateau of China, the phenomenon of double-thresholds in scour–fill processes of wide-range water-sediment two-phase flows has been shown. Thresholds located in non-hyperconcentrated flows may be called the lower threshold, and that in hyperconcentrated flows the upper threshold. This double-threshold phenomenon leads to complicated sediment transport behavior of heavily sediment-laden rivers. With an increase in suspended sediment concentration, the channel sediment delivery ratio increases initially and becomes higher than 1, followed by a decrease and finally becomes lower than 1 again.Controlled by the double-thresholds in the scour–fill processes, channel adjustment of the lower Yellow River is non-linear and complex. When the suspended concentrations were lower than the lower threshold or higher than the upper threshold, scour or bed downcutting was the dominant channel-forming process. Channel shape tends to be narrower and deeper, and the channel thalweg became more sinuous. When the suspended concentrations lay between the lower and upper thresholds, deposition of sediment was the dominant channel-forming process; channel shape tended to be shallower and wider, and channel thalweg became less sinuous. 相似文献
6.
The River Ganga in the central Gangetic plain shows the incision of 20 m of Late Quaternary sediments that form a vast upland terrace (T2). The incised Ganga River Valley shows two terraces, namely the river valley (terrace-T1) and the present-day flood plain (terrace-T0). Terrace-T1 shows the presence of meander scars, oxbow lakes, scroll plains, which suggests that a meandering river system prevailed in the past. The present-day river channel flows on terrace-T0 and is braided, sensu stricto. It is thus inferred that the River Ganga experienced at least two phases of tectonic adjustments: (1) incision and (2) channel metamorphosis from meandering to braided.Optical dating of samples from three different terraces has bracketed the phase of incision to be <6 and 4 ka. Different ages of the top of terrace-T2 show that this surface experienced differential erosion due to tectonic upwarping in the region, which also caused the river incision. River metamorphosis occurred some time during 4 and 0.5 ka. 相似文献
7.
基于对Leopold-Wolman关系修正的河床河型判别 总被引：1，自引：0，他引：1
在广泛收集中国和世界上冲积河流资料的基础上,以近200条河流的大样本对著名的、国际上沿用40余年的 Leopold-Wolman关系,即用于河型判别的比降-流量关系进行了检验。结果发现,该关系不能用于包括砾石与砂质河床在内的河型判别。这是由于比降-流量关系主要反映砾石河床与砂质河床之间差异,其次才反映分汊与弯曲河型之间的差异。为此,对于Leopold-Wolman关系进行了改进,提出了以比降和河宽来判别河型的新关系。这一关系综合反映了河流在纵向上的能耗、阻力与输沙特性与在横向上的流场与环流分布特性的组合关系,因而具有更好的河型判别效果,可以用于包括砾石与砂质河床在内的河型判别。 相似文献
8.
Channel bed adjustments following major aggradation in a steep headwater setting: findings from Oyabu Creek, Kyushu, Japan 总被引：1，自引：0，他引：1
A typhoon in 1993 induced major aggradation along Oyabu Creek, a steep, gravel bed mountain stream in Kyushu, Japan. Processes of sediment reworking are inferred from a 7-year monitoring program that measured adjustments to channel cross-sections, the longitudinal profile, and the extent/distribution of bedrock outcrops along a 3-km study reach. Over time, the reach adopted a riffle and pool structure, with notable increase in the area of exposed bedrock on the bed. This adjustment process was characterised by progressive reduction in sediment storage change per unit flow. The relaxation pathway following disturbance induced by the typhoon was shaped by the magnitude and frequency of subsequent rainfall events, the capacity of these events to transport available sediments, and physical linkages between reaches. Adjacent subreaches demonstrated differing relaxation pathways in response to these influences, induced by spatial and temporal variability in threshold conditions along the channel. Longer-term evidence indicates that responses to major disturbance, such as the 1993 typhoon, occur as ‘cycles’ of around 20-year duration. A relaxation period of 7 years is required to attain a quasi-equilibrium bed configuration and rate of sediment flux. The timeframe of cycles is considered to reflect changes to hillslope–channel bed coupling, marking the period required to generate sufficient sediment stores to reactivate phases of aggradation and subsequent degradation. 相似文献
9.
While studies on gravel mantled and mixed alluvial bedrock rivers have increased in recent decades, few field studies have focused on spatial distributions of bedrock and alluvial reaches and differences between reach types. The objective of this work is to identify the spatial distribution of alluvial and bedrock reaches in the Upper Guadalupe River. We compare reach length, channel and floodplain width, sinuosity, bar length and spacing, bar surface grain size, and slope in alluvial and bedrock reaches to identify whether major differences exist between channel reach types. We find that local disturbances, interaction of the channel and valley sides, variation in lithology, and regional structural control contribute to the distribution of bedrock reaches in the largely alluvial channel. Alluvial and bedrock channel reaches in the Upper Guadalupe River are similar, particularly with respect to the distribution of gravel bars, surface grain size distributions of bars, and channel slope and width. Our observations suggest that the fluvial system has adjusted to changes in base level associated with the Balcones Escarpment Fault Zone by phased incision into alluvial sediment and the underlying bedrock, essentially shifting from a fully alluvial river to a mixed alluvial bedrock river. 相似文献
10.
Terrace remnants are commonly used to reconstruct longitudinal profiles of rivers and floodplains, and to establish temporal correlations of events in fluvial systems. In most cases, it is assumed that the terrace remnants represent time-equivalent surfaces. Our observations of terrace formation in flume experiments and in a degrading braided river, Ash Creek, Arizona, suggest that this assumption is not always valid. Degradation resulted from a reduction in upstream sediment delivery to braided channels. In both the flume and Ash Creek, degradation in the upstream reach produced a number of inset terraces, while the production of sediment in the degrading reach simultaneously caused further aggradation downstream. Thus, stratigraphically lower surfaces in the upstream reaches are temporally equivalent to higher surfaces in downstream reaches. The downstream progression of the wave of incision produced more terraces upstream than downstream, and terrace surfaces could not be correlated on the basis of relative position or elevation above the channel bed. Furthermore, a physically continuous terrace tread was produced by longitudinal accretion of temporally non-equivalent depositional segments, as the locus of deposition progressed downstream. Therefore, in some instances, physically continuous terrace treads may not be time-equivalent surfaces that represent former channel bed or floodplain profiles. [Key words: terrace development, degradation, braided channels, channel pattern change.] 相似文献
11.
The style and degree of channel narrowing in aggrading reaches downstream from large dams is dependent upon the dominant geomorphic processes of the affected river, the magnitude of streamflow regulation, and the post-dam sediment transport regime. We measured different magnitudes of channel adjustment on the Green River downstream from Flaming Gorge Dam, UT, USA, that are related to these three factors. Bankfull channel width decreased by an average of about 20% in the study area. In reaches with abundant debris fans and eddy deposited sand bars, the amount of channel narrowing was proportional to the decrease in specific stream power. The fan–eddy-dominated reach with the greatest decrease in stream power narrowed by 22% while the reach with the least decrease in stream power narrowed by 11%. In reaches with the same magnitude of peak flow reduction, meandering reaches narrowed by 15% to 22% and fan–eddy-dominated reaches narrowed by 11% to 12%. Specific stream power was not significantly affected by flow regulation in the meandering reaches.In the diverse array of reach characteristics and deposit types found in the study area, all pre- and post-dam deposits are part of a suite of topographic surfaces that includes a terrace that was inundated by rare pre-dam floods, an intermediate bench that was inundated by rare post-dam floods, and a post-dam floodplain that was inundated by the post-dam mean annual flood. Analysis of historical photographs and tree-ring dating of Tamarix sp. shows that the intermediate bench and post-dam floodplain are post-dam landforms in each reach type. Although these two surfaces occur at different levels, they are forming simultaneously during flows of different magnitude. And while the relative elevation and sedimentologic characteristics of the deposits differ between meandering reaches and reaches with abundant debris fans and eddies, both reach types contain deposits at all of these topographic levels.The process of channel narrowing varied between fan–eddy-dominated and meandering reaches. In the meandering reaches, where stream power has not changed, narrowing was accomplished by essentially the same depositional processes that operated prior to regulation. In fan–eddy-dominated reaches, where significant reductions in stream power have occurred, channel narrowing has been accompanied by a change in dominant depositional processes. Mid-channel sand deposits are aggrading on deposits that, in the pre-dam era, were active gravel bars. These deposits are creating new islands and decreasing the presence of open-framework gravel bars. In eddies, bare sand bars are replaced with vegetated bars that have a simpler topography than the pre-dam deposits. 相似文献
12.
N. I. Alekseevskii K. M. Berkovich R. S. Chalov S. R. Chalov 《Geography and Natural Resources》2012,33(3):192-199
We assess the spatiotemporal changes in channel processes on rivers of Russia, determine the causes for vertical (incision or directional sediment accumulation) and horizontal (displacement of channel forms) deformations and show the distribution of stream channel of different morphodynamical types and with a different reconfiguration rate. The conditions are revealed, under which the channel types change over time. Particular emphasis is placed on the analysis of spatiotemporal changes in channels caused by anthropogenic disturbances and by direct technogenic interferences in the life of rivers (hydroelectric schemes, quarries in the river channels, and waterway dredging). 相似文献
13.
以西辽河红山水库修建后上游河道调整为例，说明了除来水来沙等的变化所引起的河流系统调整外，存在由于河流系统可调要素之间的相互联系，相互作用，在水库水位上升后造成的溯源淤积发展中，主、支流河道、上、下游河段冲淤相互影响，槽，滩淤积相互转换，地生态系统调整的影响下，产生的可调要素的复杂多向的调整过程。 相似文献
14.
Analysis of controls upon channel planform at the First Great Bend of the Upper Yellow River, Qinghai-Tibet Plateau 总被引：3，自引：3，他引：0
Zhiwei Li Zhaoyin Wang Baozhu Pan Jun Du Gary Brierley Guo-an Yu Brendon Blue 《地理学报(英文版)》2013,23(5):833-848
The 270 km long section of the Upper Yellow River at the First Great Bend is comprised of single channel and multiple channel systems that alternate among anastomosing, anabranching, meandering and braided reaches. The sequence of downstream pattern changes is characterized as： anastomosing-anabranching, anabranching-meandering, meandering-braided and braided-meandering. Remote sensing images, DEM data and field investigations are used to assess ahd interpret controls on these reach transitions. Channel slope and bed sediment size are key determinants of transitions in channel planform. Anas- tomosing reaches have a relatively high bed slope （0.86‰） and coarser sediment bed material （d50 = 3.5 mm）. In contrast, meandering reaches have a low slope （0.30‰） and fine sediment bed material （d50 = 0.036 mm）. The transition from a meandering to braided pattern is characterized by an increase in channel width-depth ratio, indicating the important role of bank strength （i.e. cohesive versus non-cohesive versus channel boundaries）. Interestingly, the braided-meandering and meandering-braided transitions are coincident with variable flow inputs from tributary rivers （Baihe and Heihe rivers respectively）. Theoretical analysis of the meandering-braided transition highlights the key control of channel width-depth ratio as a determinant of channel planform. 相似文献
15.
In-stream wetlands and their significance for channel filling and the catchment sediment budget, Jugiong Creek, New South Wales 总被引：2，自引：0，他引：2
Evidence is presented here of recent and extensive infilling of the incised channel network of the Jugiong Creek catchment, SE Australia. The present channel network resulted from widespread stream and gully incision in the period between 1880 and 1920. Our survey shows that gully floors have been colonised extensively by emergent macrophyte vegetation since before 1944, forming continuous, dense, in-stream wetlands, which now cover 25% of the channel network in the 2175 km2 catchment and have so far trapped almost 2,000,000 t of nutrient-enriched, fine sediments. This mass of sediments represents the equivalent of 4.7 years of annual sediment production across the catchment and in some tributaries, more than 20 years of annual yield is stored within in-stream wetlands.Previous work on the late Quaternary stratigraphy of the region has shown that there were repeated phases of channel incision in the past following which the channels quickly stabilised by natural means and then filled with fine-grained sediment to the point of channel extinction, creating unchannelled swampy valley floors. The current formation and spread of in-stream wetlands is interpreted to be the onset of the next infill phase but it is not known whether present conditions will allow complete channel filling and reformation of the pre-existing swampy valley floors. Nevertheless, further spread of in-stream wetlands is likely to increase the sediment trapping capacity and further reduce the discharge of sediments and nutrients into the Murrumbidgee River. The in-stream wetlands may provide a significant capacity to buffer erosion from gullied catchments of considerable size (up to 300 km2) as an adjunct to current riparian management options. They may also assist the recovery of sediment-impacted channels downstream. 相似文献
16.
The Platte River in Nebraska has evolved in the twentieth century from a predominantly braided river pattern to a mélange of meandering, wandering, anastomosed, island braided, and fully braided reaches. Identifying the factors that determine the occurrence of a fully braided main channel was the objective of this study. Aerial photography, gage flow data, ground-surveyed cross sections, bed material samples, and the results of sediment transport modeling were used to examine factors that control spatial change in main river pattern of the central Platte River. Valley confinement is identified as the determining factor of braided river in nine of eleven divisions of the central Platte River. Flow reduction and the interruption of sediment supply are identified as determining factors preventing fully braided river in the remaining two of eleven reaches.Valley confinement, the topography which limits the width of the floodplain, was initially measured as width between historical banks (predevelopment river banks). This metric was later refined to width between confining features (historical banks, remnant bars, bridge abutments, protected banks and levees). Under existing conditions, the main channel of the central Platte River is fully braided when valley confinement (width between confining features) is 600 m or less and begins to divide into the multiple channels of an anastomosed pattern when valley confinement (width between confining features) exceeds 600 m When Platte River flow is divided between two to four major anabranches, a fully braided pattern in the main channel of the main anabranch requires a more confined valley of 400 m or less.Valley confinement is demonstrated to be the dominant factor in determining river pattern in the central Platte River, although this factor is not normally considered in the continuum of channel pattern model. Conclusions from this study can be used to increase the occurrence of fully braided main channel in the central Platte River, to aid habitat recovery for endangered or threatened bird species that favor this river pattern. Consideration of valley confinement with river continuum factors can aid river managers by improving predictions of river pattern in response to management actions. 相似文献
17.
This study focuses on the importance of different stages of bed formation and transportation processes along the partly managed lowland section of Maros River (South Hungary). The role of different stages was determined, applying a series of cross-sectional data. Besides, major morphological zones of over-widened, braided sections were defined, along with their role and function in sediment storage and transport processes.During the rising stage of floods a great volume of sediment is eroded from and transported through the cross-sections. On the falling stage accumulation will be the dominant process. However, concerning the total amount of sediment deposited during falling stages, braids store overwhelmingly more, than straight, narrow reaches. During long-lasting, low-stage periods the cross-sectional area increases, which means net erosion and an increase in mean depth. Therefore, braids have a significant role in falling stage sediment storage and low-stage release.Nevertheless, the location of accumulation within braids is different, influenced by their structure. In braids which are characterised by islands, the sediment is deposited at their upstream end. Islands create a transportation zone towards the downstream end of the braid by increasing stream power. Over-widened sections without islands are more likely to experience deposition at their downstream end.Constant narrowing of the studied reach was also observed. Braids on unmanaged reaches proved to be the most prominent places in this sense. Changes in the regime of the river, and altered land-use help to explain this process, as the frequency of floods has decreased, while the length of extreme low stages has increased. Therefore, the need for the sediment storing and releasing function of braids is less necessary, which leads to their narrowing. 相似文献
18.
黄河下游河道断面形态参数变化及其水沙过程响应 总被引：1，自引：0，他引：1
基于1965—2015年黄河下游花园口、高村、泺口站的逐年水文和汛前河道断面的实测资料,分析了河道断面形态参数（河道断面面积,河道宽深比等）的变化,以及对河道断面形态与来水来沙间的关系做出定量化分析。结果表明：主槽断面形态参数与水沙搭配以及前期断面形态密切相关,沿程3个断面形态参数调整方式存在显著差异。河宽调整幅度沿程减小,辫状河段变幅最大,尤其在1986—1999年,辫状河段萎缩程度最为严重,其次为弯曲河段,顺直河段横向调整幅度最小。受到前期断面形态的影响,辫状河段河道断面调整方式既有横向展宽（萎缩）又有垂直加深（淤积）;弯曲河段河道宽深比与流量呈较弱的正相关关系,具有横向和垂向的调整方式;而顺直河段的宽深比与流量呈负相关关系,与来沙系数呈正相关关系,河道以垂直加深（淤积）为主。 相似文献
19.
Xin Shan Shengli Li Shunli Li Xinghe Yu Li Wan Lina Jin Tianyi Wang 《Journal of Paleolimnology》2018,59(2):245-261
Sedimentary characteristics and genesis of a sandy, topset-dominated braided river delta from Huangqihai Lake were investigated using trenching and a ground penetrating radar survey. Ten lithofacies types were identified with assistance of grain size distribution to record a broad range of depositional processes within an overall coarsening upward sequence. Four distinct architectural elements, including channel fill, compound bar, sand sheet, and river mouth bar, built up this delta. No obvious foreset can be identified in this braided river delta. A gentle slope (0.2°) and shallow basin (<10 m deep), young age (~25 years), and low sediment supply caused by human disturbance and semi-arid climate, are believed to allow this type of topset-dominated delta to form. 相似文献
20.
A catastrophic flood ire 1965 on Plum Creek, a perennial sandbed stream in the western Great Plains, removed most of the bottomland vegetation and transformed the single-thalweg stream into a wider, braided channel. Following eight years of further widening associated with minor high flows, a process of channel narrowing began in 1973; narrowing continues today. The history of channel narrowing was reconstructed by counting the annual rings of 129 trees and shrubs along a 5-km reach of Plum Creek near Louviers, Colorado. Sixty-three of these plants were excavated in order to determine the age and elevation of the germination point. The reconstructed record of channel change was verified from historical aerial photographs, and then compared to sediment stratigraphy and records of discharge and bed elevation from a streamflow gaging station in the study reach. Channel narrowing at Plum Creel: occurs in two ways. First, during periods of high flow, sand and fine gravel are delivered to the channel, temporarily raising the general bed-level. Subsequently, several years of uninterrupted low flows incise a narrower channel. Second, during years of low flow, vegetation becomes established on the subaerial part of the present channel bed. In both cases, surfaces stabilize as a result of vegetation growth and vertical accretion of sediment. 相似文献