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1.
This paper describes the morphology, sequential development and general sedimentology of concave-bank benches on the Murrumbidgee River of southeastern Australia, and also notes their important role in floodplain formation on certain meandering rivers in western Canada. Benches form against the concave bank (cut-bank) of abruptly curving bends immediately upstream of the point of maximum curvature. As a result of flow deflection against the upstream limb of the convex bank, the channel widens here and produces a zone of expanded flow facilitating flow separation near the upstream limb of the concave bank. Sedimentation within this zone starts with a longitudinal-shaped bar of medium sand forming a platform isolated even at low flow by a narrow secondary channel against the concave bank. Aggradation of the longitudinal-shaped bar with fine sand, mud and organic matter permits the establishment of trees. Further sedimentation, particularly around the young trees, results in the formation of a fully developed bench isolated by the secondary channel from the remainder of the floodplain only during high flows. Observations on confined meandering rivers in western Canada provide evidence of substantial floodplain formation by concave-bank bench accretion, a process distinctly different in character to the more familiar mechanism of lateral point-bar accretion. Furthermore, the preservation of abundant organic debris means that extensive bench deposits may be a source of locally useful natural gas from within floodplain sediments.  相似文献   

2.
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   

3.
Upland swamp channels with low width/depth ratios (w/d), armoured beds, minimal sediment loads, tightly curving bends and an absence of point bars provide a striking contrast to the flow characteristics of larger channels with higher w/d ratios. Two subsets of these bends were examined in relation to their patterns of cross‐stream flow relative to the channel boundary. The first, with mean w/d = 2·0 and gentle barforms, exhibited even velocity distributions at bend entrances but developed vertically stacked pairs of maximum velocity filaments (MVFs). Cross‐stream circulation increased with decreasing curvature before essentially ceasing in the tightest bend due to the conservation of angular momentum and reduced vertical velocity differentials; bed friction has more limited influence in narrow deep channels relative to bank friction. In the second subset of bends, with larger w/d (mean 4·8) and much steeper barforms, the MVFs were laterally paired and strongly helical flow was partly driven by the vertical confinement of flow due to large, stable barforms at the bend entrances. In one bend, the velocity profile became inverted immediately past the apex and caused helical flow to abruptly reverse. Point bars in relatively wide bedload channels appear to greatly distort secondary flow patterns. In narrow, deep, sediment‐starved channels, separation zones against the convex and/or the concave bank deliver the flow confinement that would otherwise be provided by point bars or concave‐bank benches. In these channels, separation zones are important for protecting both the channel bed and banks from scour. Three‐dimensional near bankfull flow fields are presented for one bend with a meander pool; inward shifting of the MVF and limited sediment supply are proposed as mechanisms for the development and maintenance of these features. These flow data in narrow and deep peatland channels demonstrate very different flow patterns and morphological characteristics relative to the more commonly studied wide, shallow channels with more abundant sediment. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

4.
Occurrence and development of channel bars are major components of the morphodynamics of rivers and their relation to river meandering has been much explored through theory and experimentation. However, field and documentary data of characteristics and evolution over timescales from years to several decades are lacking. Four sets of aerial photographs in the period 1984–2007 were used to map and quantify bar numbers and areas in GIS on an active meandering reach. Bar types were classified. Additional temporal resolution was provided by annual ground photography and mapping for 1981–2010. Analysis was extended backward by use of large scale Ordnance Survey maps from 1873 onwards. As expected, point bars are the most common type but ‘free’ bars of several types are major components of bar deposition. Point bars and attached bars are significantly larger in size than mid‐channel and side bars. Spatial distribution of bars varies down the reach and over time but is related to channel sinuosity, gradient and mobility and to bend evolution. Different types of bar occur in distinctive channel locations, with point and concave‐bend bars in zones of high curvature. Bar activity shows a relation with discharge events and phases and possibly with changing riparian conditions, but superimposed on this is a common sequence of bar evolution from incipient gravel mid‐channel bars to full floodplain integration. This life‐cycle is identified as 7–9 years on average. No evidence for mobility of free bars within the course is found. The results are compared with bar and bend theory; the bars are forced and conform in general to bend theory but detailed variation relates to geomorphic factors and to autogenic sequences of bends and bars. Mid‐channel bars are width induced. Variability of bar occurrence needs to be taken into account in river management and ecological evaluation, including for the EU WFD. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

5.
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   

6.
《水文科学杂志》2013,58(4):883-892
Abstract

Contour benches are earthen structures constructed across cultivated slopes, at intervals down the slope, largely used in semi-arid zones. The results of an experiment to monitor water and sediment balance inside a contour bench terrace system are presented. The study site, located in the El-Gouazine watershed (central Tunisia), includes two terraced plots of approximately 3000 m2, one of which was left fallow for several years, while the other was tilled. The characteristics of rainfall—runoff processes and erosion inside both terraced plots during a two-year period (2004–2006) are described. Ploughing reduced runoff by 75%. Erosion was monitored following runoff episodes that produced observable deposits in the bench channel. After ploughing, erosion was reduced by 44% between July 2004 and July 2005 and by 50% between October 2005 and July 2006. However, erosion per millimetre of runoff was about twice as great on the tilled soil as on the fallow. Even though ploughing weakens the soil, it seems to reduce erosion by increasing infiltration. For the studied rain events, ploughing used in combination with contour bench terraces seems to have limited erosion and enhanced the effectiveness of contour bench terrace management.  相似文献   

7.
The evolution of meandering river floodplains is predominantly controlled by the interplay between overbank sedimentation and channel migration. The resulting spatial heterogeneity in floodplain deposits leads to variability in bank erodibility, which in turn influences channel migration and planform development. Despite the potential significance of these feedbacks, few studies have quantified their impact upon channel evolution and floodplain construction in dynamic settings (e.g. locations characterized by rapid channel migration and high rates of overbank sedimentation). This study employs a combination of field observations, geographic information system (GIS) analysis of satellite imagery and numerical modelling to investigate these issues along a 375 km reach of the Rio Beni in the Bolivian Amazon. Results demonstrate that the occurrence of clay‐rich floodplain deposits promotes a significant reduction in channel migration rates and distinctive styles of channel evolution, including channel straightening and immobilization of bend apices leading to channel narrowing. Clay bodies act as stable locations limiting the propagation of planform disturbances in both upstream and downstream directions, and operate as ‘hinge’ points, around which the channel migrates. Spatial variations in the erodibility of clay‐rich floodplain material also promote large‐scale (10–50 km) differences in channel sinuosity and migration, although these variables are also likely to be influenced by channel gradient and tectonic effects that are difficult to quantify. Numerical model results suggest that spatial heterogeneity in bank erodibility, driven by variable bank composition, may force a substantial (c. 30%) reduction in average channel sinuosity, compared to situations in which bank strength is spatially homogeneous. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

8.
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9.
Casuarina cunninghamiana Miq. is an important rheophytic tree in New South Wales, Australia because it is fast growing and can tolerate flood disturbance. Widden Brook is an active sand‐bed stream that has widened substantially since initial European settlement in the early 1800s and is characterized by high flood variability and multi‐decadal periods of alternating high and low flood frequency, called flood‐ and drought‐dominated regimes. Channel contraction by bench formation is currently occurring. Conversion of coarse‐grained point bars to benches is an important process of channel contraction. When point bars grow to a height where suspended sediment is first deposited to thicknesses of at least 50 mm by sub‐bankfull floods, rapid establishment of C. cunninghamiana occurs. As the trees grow they partially block bankside flows, thereby locally reducing flow velocity and inducing further deposition on the benches. Such synergistic relationships between bar height and inundation, fine‐grained sediment deposition, tree establishment and the development of a bankside low current velocity zone are fundamental for bench development. Size‐class frequency data demonstrate that C. cunninghamiana on the benches consists of pure even‐aged stands with most trees clustering near the average diameter. Two benches have similar size class frequency distributions but a third has significantly smaller trees. Recruitment on benches is episodic, may occur in areas open to grazing and is dependent on favourable conditions that allow tree survival. These favourable conditions include high seed availability, low levels of competition, deposition of fine sediments and adequate moisture for tree growth. Although C. cunninghamiana germinates on bars, seedlings are eliminated by prolonged inundation or flood scour and do not reach maturity. Recurring catastrophic floods or a sequence of large floods in rapid succession episodically destroy benches by substantial channel widening and initiate a new phase of bar and bench development. A conceptual model of the conversion of point bars to benches by thick mud deposition and C. cunninghamiana recruitment has been developed for sand‐bed streams draining similar sandstone catchments. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

10.
Previous studies of alluvial rivers have shown that channel patterns form a continuum controlled by interactions among factors such as gradient, discharge, sediment size, and bank strength. Data from channels in the permanent wetlands of the Okavango Delta add to these ?ndings by focusing on pattern transitions in channels with banks formed by sedges and grasses that are rooted in peat and underlain by unconsolidated sediment. Channels are well de?ned, and transport ?ne–medium sand as bedload between the vertical, vegetation‐lined banks. Water depths, velocities, grain sizes, and bankline vegetation do not vary signi?cantly or systematically downstream, but the permeable banks allow water to leak from the channels, contributing to an overall downstream decrease in discharge and width. In addition, as the Okavango River ?ows from the <12 km wide ‘Panhandle’ and splits into distributaries in the broader ‘Fan’, valley gradient steepens by c. 60 per cent. These downstream changes result in channel pattern adjustments. In the Panhandle, the Okavango River is a relatively wide (c. 30–100 m), actively meandering, sinuous channel (P > 2·0), but further downstream in the Fan, the narrower (<40 m) distributaries follow laterally stable, less sinuous (‘straight’) courses (P < 1·75). Some channel pattern discrimination diagrams based on simple indices of gradient, discharge, sediment size or stream power are inadequate for analysing the meandering–straight transition in the Okavango but Parker's (1976) approach, based on ratios of depth–width and slope–Froude number, accurately characterizes the transition. Our ?eld observations, combined with the results from previous experimental studies, suggest that in relatively wide channels (w/d > 10), thalweg meandering results in scour of the unconsolidated sediment at the bank base, leading to undermining and collapse of the vegetation, and to slow meander migration. However, as channels narrow downstream (w/d < 10) with discharge losses, proportionally increasing sidewall drag exerted by bankline vegetation suppresses thalweg meandering and bank scour, and channels follow stable, less sinuous courses. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

11.
Slow earth sliding is pervasive along the concave side of Red River meanders that impinge on Lake Agassiz glaciolacustrine deposits. These failures form elongated, low‐angled (c. 6 to 10°) landslide zones along the valleysides. Silty overbank deposits that accumulated during the 1999 spring freshet extend continuously along the landslide zones over hundreds of metres and aggraded the lower slopes over a distance 50 to 80 m from the channel margin. The aggradation is not obviously related to meander curvature or location within a meander. Along seven slope profiles surveyed in 1999 near Letellier, Manitoba, the deposits locally are up to 21 cm thick and generally thin with increasing distance from, and height above, the river. Local deposit thickness relates to distance from the channel, duration of inundation of the landslide surface, mesotopography, and variations in vegetation cover. Immediately adjacent to the river, accumulated overbank deposits are up to 4 m thick. The 1999 overbank deposits also were present along the moderately sloped (c. 23 to 27°) concave banks eroding into the floodplain, but the deposits are thinner (locally up to c. 7 cm thick) and cover a narrower area (10 to 30 m wide) than the deposits within the landslide zones. Concave overbank deposition is part of a sediment reworking process that consists of overbank aggradation on the landslide zones, subsequent gradual downslope displacement from earth sliding, and eventually reworking by the river at the toe of the landslide. The presence of the deposits dampens the outward migration of the meanders and contributes to a low rate of contemporary lateral channel migration. Concave overbank sedimentation occurs along most Red River meanders between at least Emerson and St. Adolphe, Manitoba. © Her Majesty the Queen in right of Canada.  相似文献   

12.
Haiyan Yang 《水文研究》2020,34(17):3702-3717
Gravel-bed braided rivers are highly energetic fluvial systems characterized by frequent in-channel avulsions, which govern the morphodynamics of such rivers and are essential for them to maintain a braided planform. However, the avulsion mechanisms within natural braided rivers remain unclear due to their complicated hydraulic and morphodynamic processes. Influenced by neighbouring channels, avulsions in braided rivers may differ from those of bifurcations in single-thread rivers, suggesting that avulsions should be studied within the context of the entire braid network. In this study, braiding evolution processes in gravel-bed rivers were simulated using a physics-based numerical model that considers graded bed-load transport by dividing sediment particles into multiple size fractions and vertical sediment sorting by dividing the riverbed into several vertical layers. The numerical model successfully produced braiding processes and avulsion activities similar to those observed in a laboratory river. Results show that bend evolution of the main channel was the fundamental process controlling the occurrence of avulsions in the numerical model, with a cyclic process of channel meandering by lateral migration that transitioned to a straight channel pattern by avulsion. The radius of bend curvature for triggering avulsions in the numerical model was measured and it was found that the highest probability for a channel bend to generate an avulsion occurs when its radius of curvature is approximately 2.0–3.3 times the average anabranch width. Other types of avulsion were also observed that did not occur specifically at meander bends, but upstream meander evolution indirectly influenced such avulsions by altering channel pattern and discharge to those locations. This study explored the processes and mechanisms of several types of avulsion, and proposed factors controlling their occurrence, namely increasing channel curvature, high shear stress, tributary discharge, riverbed gradient and upstream channel pattern, with high shear stress being a direct indicator. Furthermore, avulsions in a typical gravel-bed braided river, the Waimakariri River in New Zealand, were analysed using sequential Google Earth maps, which confirmed the conclusions derived from the numerical simulation.  相似文献   

13.
The alluvial cover in channels with non-alluvial beds is a major morphologic feature in these rivers and has important geomorphic and ecologic functions. Although controls on the extent of the alluvial cover have been previously researched, little is known about the role of channel meanders in shaping the three-dimensional morphology and bedload transport rates in these rivers. Flume experiments were conducted in a fixed-bed sinuous channel scaled from an engineered urban river. A fully graded sediment supply mixture was fed into the bare channel at rates ranging between 0.3 and 1.2 times the estimated channel capacity under constant discharge. The three-dimensional morphology and surface texture of the alluvial cover were captured using photogrammetry, and the sediment output was periodically measured and sieved. A stable alluvial cover was achieved under all sediment supply conditions that coincided with a sediment transport equilibrium. The sediment supply rate controlled the final areal extent, mass and volume of the alluvial cover, while cover developed as a periodic series of stable bars ‘fixed’ by the channel planform. The alluvial cover development followed consistent trajectories relative to angular position around bends but developed to a greater degree and higher elevation with increasing sediment supply. The stable cover extent had a logarithmic relationship with the relative sediment supply, while the final mass, volume and bar height had linear relationships. The final channel morphology was characterized by fine-textured point bars with flat tops and steep margins connected by coarse riffle features. The outside of banks between bend apexes remained bare, even at sediment supply conditions exceeding the channel capacity. The length of the exposed outer banks followed predictable linear relationships with the total cover extent. Insights from this study can provide guidance for the management of channels with non-alluvial boundaries and provide validation for models of sinuous bedrock channel abrasion. © 2020 John Wiley & Sons, Ltd.  相似文献   

14.
Extensive valley fills at the base of the escarpment in upper Wolumla Creek, on the south coast of New South Wales, Australia, have formed from a combination of ‘cut and fill’ processes. The valley fills comprise series of alternating, horizontally bedded sand and mud units, reflecting reworking of detritus from deeply weathered granites of the Bega Batholith. Sand units are deposited as sand sheets or splays on floodplain surfaces or in floodouts that form atop intact valley fill surfaces downstream of discontinuous gullies. Alternatively, sands are deposited from bedload and form bars or part of the valley floor within channel fills. Organic-rich mud units are deposited from suspension in swamps or in seepage zones at the distal margin of floodouts. Within 5 km of the escarpment, valley deposits grade downstream from sand sheet and splay deposition in floodouts, to mud deposition in swamp and seepage zones. Radiocarbon dates indicate that virtually the entire valley fill of upper Wolumla Creek was excavated prior to 6000 years BP . Remnant terraces are evident at valley margins. The valley subsequently filled between 6000 years BP and 1000 years BP producing valley fills around 12 m deep, but no greater than 300 m wide. Reincision into the valley fill, on a scale smaller than the present incision phase, is indicated at around 1000 years BP , following which the channel refilled. Portion plans dated from 1865 refer to the study area as ‘Wolumla Big Flat’, and show large areas of swampy terrain, suggesting that the valley fill had re-established by this time. Within a few decades of European settlement the valley fill incised once more. Upper Wolumla Creek now has a channel over 10 m deep and 100 m wide in places, draining a catchment area of less than 20 km2. © 1998 John Wiley & Sons, Ltd.  相似文献   

15.
Six plains cottonwoods along the axis of a meander were excavated to determine if dendrochronology could identify the year and location of germination and date past overbank sedimentation events. Samples from all excavated trees showed clear anatomical changes associated with burial, including increased vessel size, decreased definition of annual ring boundaries, and decreased ring widths. Some of these burial signatures were created by deposition of only a few centimeters of sediment, and most burial events were detected by multiple samples from the same tree. Four of the trees germinated at or near the upper surfaces of bar deposits, while two germinated within thin overbank deposits draped over bar deposits, indicating that germination is closely associated with bars. Dates and inferred thicknesses of overbank sedimentation events are consistent with repeated topographic surveys and data obtained from cesium-137 (137Cs) analyses. However, the record of overbank sedimentation extracted from the trees does not entirely reflect the history of past peak discharges documented by stream gaging, largely because individual trees are progressively less likely to be flooded through time as the river migrates farther away. Germination dates and locations closely track past positions of the river channel. Germination elevations and the elevations of the tops of point bars appear to be decreasing with time as the bend migrates, implying vertical incision by Powder River at a rate of 7.1 ± 4.3 mm/yr. The rate of floodplain growth determined by elevation changes decreases progressively through time, ultimately reaching an apparent plateau after 0.8–1.3 m of vertical accretion. While similar patterns of vertical accretion have previously been interpreted as resulting from decreasing flood probability with increasing floodplain elevation, distance from the channel is also a first-order control on vertical floodplain growth. © 2019 John Wiley & Sons, Ltd.  相似文献   

16.
In meandering rivers cut into bedrock, erosion across a channel cross‐section can be strongly asymmetric. At a meander apex, deep undercutting of the outer bank can result in the formation of a hanging cliff (which may drive hillslope failure), whereas the inner bank adjoins a slip‐off slope that connects to the hillslope itself. Here we propose a physically‐based model for predicting channel planform migration and incision, point bar and slip‐off slope formation, bedrock abrasion, the spatial distribution of alluvial cover, and adaptation of channel width in a mixed bedrock‐alluvial channel. We simplify the analysis by considering a numerical model of steady, uniform bend flow satisfying cyclic boundary conditions. Thus in our analysis, ‘sediment supply’, i.e. the total volume of alluvium in the system, is conserved. In our numerical simulations, the migration rate of the outer bank is a specified parameter. Our simulations demonstrate the existence of an approximate state of dynamic equilibrium corresponding to a near‐solution of permanent form in which a bend of constant curvature, width, cross‐sectional shape and alluvial cover distribution migrates diagonally downward at constant speed, leaving a bedrock equivalent of a point bar on the inside of the bend. Channel width is set internally by the processes of migration and incision. We find that equilibrium width increases with increasing sediment supply, but is insensitive to outer bank migration rate. The slope of the bedrock point bar varies inversely with both outer bank migration rate and sediment supply. Although the migration rate of the outer bank is externally imposed here, we discuss a model modification that would allow lateral side‐wall abrasion to be treated in a manner similar to the process of bedrock incision. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

17.
Many lowland fluvial systems are suspected to possess a morphological legacy from a long history of channel modifications as a consequence of limited energy and sediment supply to facilitate recovery. We explore the extent of such modifications using a regionally extensive dataset of physical habitat surveys compiled by non-specialist surveyors. Representative photographs for each surveyed site were used to quality check channel width, depth and bed grain size information derived from Modular River Physical (MoRPh) surveys. Following checking, 1659 surveys were retained for analysis from alluvial sites, almost entirely in England. The photographs were also inspected for evidence of clear ‘overdeepening’ that would preclude frequent overtopping of the lower bank top. Results indicated that almost one-third of the sites were overdeepened, that width-to-depth ratios defined using the active bed width showed stronger discrimination of overdeepening than bankfull width, that highly statistically significant identification of overdeepened channels was possible in channels up to 10 m wide and with only minimal differences attributable to channel bed materials. Stepwise regression analysis estimated relationships between channel width-to-depth ratios and channel size for overdeepened and non-overdeepened channels. We demonstrate that large data sets collected by numerous non-specialist surveyors can, with careful filtering, generate statistically robust results of geomorphological value over areas larger than is otherwise practicable. Furthermore, we reveal a notable legacy of overdeepening in the analysed lowland rivers, which presents a significant ‘hydromorphological’ management challenge.  相似文献   

18.
The Pitt River is a meandering river channel linking the Fraser River estuary and Pitt Lake. The lake acts as a temporary reservoir for tidally diverted Fraser River flow. Stage level can fluctuate 2 m in Pitt River and as much as 1.2 m in Pitt Lake on a tidal cycle. Stage data from three locations in the system, used in conjunction with velocity measurements (profiles and tethered meter), revealed large tidal and seasonal variations in discharge. Calculations indicate that during the flood, basal shear stress peaks earlier in the cycle and reaches higher values than during the ebb. Thus, sediment moves farther forward on a flood flow than it moves back on the succeeding ebb. An upstream movement of sediment in Pitt River from the Fraser River is indicated by: (1) the identical mineralogy of the two rivers, (2) a decrease in median grain size from the Fraser to Pitt Lake, and (3) a predominance of flood-oriented bedforms in the river channel. A delta, 12 km2 area, has accumulated at the lower (draining) end of the lake. Studies of the river channel using hydrographic charts revealed regular meanders (λM = 6100 m) and evenly spaced riffles and pools which appear to be scaled to the strongest flow, winter flood current (2400 m3/s). The winter flood is thus considered to be the effective discharge. Meander point bars are accreting on the ‘upstream’ side indicating deposition by the flood-oriented flow. The three dimensional geometry of the large-scale bedforms which cover the sandy thalweg of both river and delta channel was determined by echo sounding and side-scan sonar. Three distinct sizes (height/spacing = 0.8 m/10–15 m; 1.5 m/25–30m; 3m/50–60 m) of large-scale bedforms (sand waves) were found; their linear relationship of height vs. spacing on a log-log plot suggests a common genesis. Their occurrence by size does not appear to be related to depth of flow but rather to their position in the channel with respect to large scale features which alter flow.  相似文献   

19.
Meander bends of many large, sand‐bed meandering rivers are partitioned by chute channels that convey permanent flow, and co‐exist with the mainstem for decades. As a first step toward understanding the dynamics and morphodynamic implications of these ‘bifurcate meander bends’, this study applied binary logistic regression analysis to determine whether it is possible to predict chute initiation based on attributes of meander bend character and dynamics. Regression models developed for the Strickland River, Papua New Guinea, the lower Paraguay River, Paraguay/Argentina, and the Beni River, Bolivia, revealed that the probability of chute initiation at a meander bend is a function of the bend extension rate (the rate at which a bend elongates in a direction perpendicular to the valley axis trend). Image analyses of all rivers and field observations from the Strickland suggest that the majority of chute channels form during scroll–slough development. Rapid extension is shown to favour chute initiation by breaking the continuity of point bar deposition and vegetation encroachment at the inner bank, resulting in widely‐spaced scrolls with intervening sloughs that are positively aligned with primary over‐bar flow. The rivers plot in order of increasing chute activity on an empirical meandering‐braided pattern continuum defined by potential specific stream power (ωpv) and bedload calibre (D50). Increasing stream power is considered to result in higher bend extension rates, with implications for chute initiation. In addition, chute stability is shown to depend on river sediment load relative to flow discharge (Qs/Q), such that while the Beni may plot in the region of highly braided rivers by virtue of a high potential specific stream power, the formation of stable chute channels is suppressed by the high sediment load. This tendency is consistent with previous experimental studies, and results in a planform that is transitional between single‐thread meandering and braided. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

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