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1.
The present study explores the evolutionary trajectory of the glacier-fed Mareit River (South Tyrol, Italian Alps), where a large restoration programme was implemented in 2008–2009. River corridor changes before and after the restoration works were assessed using historical maps, recent field observations, topographic surveys and topographic differencing. Trends of anthropic (forest cover, channel works, gravel mining) and natural (glacial cover, precipitation, flow regime) factors controlling channel morphology – at both catchment and reach scales – were reconstructed. From the mid-19th century, the evolutionary trajectory of the Mareit River followed a degradational trend, characterized by channel narrowing, bed incision and planform simplification. Direct, in-channel human alterations – mainly in the form of bank protections (in the late 19th century), gravel mining (mostly in the 1970s) and grade-control works (since the 1980s) – dominated the historical adjustments before the restoration. In 2008–2009, a segment of the Mareit was restored by widening the channel, partly removing the check-dams and shaping a braided pattern within a laterally constrained corridor. Post-work monitoring shows that the restoration improved both the morphological quality and the geomorphic diversity. At present, the channel is subject to narrowing and slight bed level incision, with islands and floodplains progressively expanding at the expenses of the active channel. This trend is likely to continue in the next decades based on the expected future flow regime, and indeed the Mareit River seems to be attaining a ‘miniaturized’ version of the anabranching pattern of the mid-19th century. Overall, this restoration approach and the associated evolutionary trajectory is considered positive, because it leads to a complex mosaic of geomorphic units, dynamically self-adjusting to the time-varying driving variables. The formation of a morphodynamically active corridor, while keeping artificially non-erodible boundaries, represents an optimal strategy to integrate ecological improvements with flood risk mitigation in the densely populated Alpine valleys. © 2020 John Wiley & Sons, Ltd.  相似文献   

2.
When studying the evolution of landscape, it is difficult to discriminate the influence of anthropogenic from natural causes, or recognise changes caused by different sources of human action. This is especially challenging when the influence of certain sources is overprinted. For instance, although dam closure is the most common method of altering river courses, dam construction is often preceded by hydro‐technical works such as channel straightening, embankment construction or sediment mining. Both dam construction and the hydro‐technical works that precede dam closure can result in changes in the balance between sediment supply and transport capacity, and often, changes in river planform. The main objective of this study was to verify whether the works preceding dam closure are an important driver of river planform changes on the lower Drava River (Hungary). The case study is based on geological and geophysical surveys, as well as the analysis of historical maps covering an anabranching, 23 km long valley section. We show that channel straightening conducted prior to dam closure resulted in a transition from a meandering to sinuous planform with channel bars. Dam construction itself then caused enhanced incision, exposure of bar surfaces, vegetation encroachment and the formation of an anabranching planform. Based on this study, we developed models of alluvial island and channel planform evolution downstream of dams. Dam construction enhances channel incision, narrowing, and the reduction of flow caused by earlier hydro‐technical works. Many rivers downstream of dams experience episodes of anabranching or wandering, with a multi‐thread pattern replacing sinuous, braided and meandering courses. When incision continues, river patterns evolve from anabranching to sinuous via the attachment of alluvial islands to floodplains. However, the timing and sequence of these changes depend on hydrological and sediment supply regimes, geomorphic settings and anthropogenic actions accompanying dam construction. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

3.
Anabranching rivers evolve in various geomorphic settings and various river planforms are present within these multi‐channel systems. In some cases, anabranches develop meandering patterns. Such river courses existed in Europe prior to intensive hydro‐technical works carried out during the last 250 years. Proglacial stream valleys, inherited from the last glaciation, provided a suitable environment for the development of anabranching rivers (wide valleys floors with abundant sand deposits). The main objective of the present study is to reconstruct the formation of an anabranching river planform characterized by meandering anabranches. Based on geophysical and geological data obtained from field research and a reconstruction of palaeodischarges, a model of the evolution of an anabranching river formed in a sandy floodplain is proposed. It is demonstrated that such a river system evolves from a meandering to an anabranching planform in periods of high flows that contribute to the formation of crevasse splays. The splay channels evolve then into new meandering flow paths that form ‘second‐order’ crevasses, avulsions and cutoffs. The efficiency of the flow is maintained by the formation of cutoffs and avulsions preventing the development of high sinuosity channels, and redirecting the flow to newly formed channels during maximum flow events. A comparison with other anabranching systems revealed that increased discharges and sediment loads are capable of forming anabranching planforms both in dryland and temperate climate zones. The sediment type available for transport, often inherited from older sedimentary environments, is an important variable determining whether the channel planform is anabranching, with actively migrating channels, or anastomosing, with stable, straight or sinuous branches. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

4.
Field observations on small rivers of the Amazon basin are less common due to their remote location and difficult accessibility. Here we show, through remote sensing analysis and field works, the planform evolution and riverbed topography of a small river located in the upper foreland Amazon basin, the Ichilo River. By tracking planform changes over 30 years, we identified the factors that control meander migration rates in the Ichilo River: cutoffs, climate and human interventions. The data suggest that neck cutoffs are the main controls in the Ichilo River, with an annual density of 0.022 cutoffs/km. In addition, climate controls have been identified in the form of high-precipitation events that may have promoted cutoffs, an increase in meander migration rate and channel widening. The width distribution of the Ichilo River is well represented by general extreme value and inverse Gaussian distributions. The spatiotemporal variability of meandering migration rates in the Ichilo River is analysed in two locations where neck cutoffs are expected. Analysing the distance across the neck in these two points, we predict the occurrence of a new cutoff. The combined methodology of bathymetric surveys and structure from motion photogrammetry shows us the Ichilo riverbed topography and banks at high resolution, where two scour holes were identified. Finally, we discuss the impact of planform changes of the Ichilo River on communities that are established along its riverbanks.  相似文献   

5.
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6.
Channel bifurcation is a key element in braided rivers, determining the water and sediment distribution and hence controlling the morphological evolution. Recent theoretical and experimental findings, as well as field observations, showed that bifurcations in gravel‐bed braided rivers are often asymmetrical and highly unstable. In this paper field data are presented on a bifurcation in the Tagliamento River, northeast Italy. The planform configuration of the bifurcation and its temporal evolution was monitored by an automatic digital camera during a series of seven floods with different magnitudes. This remote sensing technique allowed a high temporal resolution (pictures were acquired every hour) that was proved to be essential in a highly dynamic system as the one considered here. Digitized maps of the channels provided information on the location of the bifurcation, the width of the anabranches, the angle between them, along with the occurrence and migration of sediment bars. Data were acquired at two different water levels, giving the possibility to compare low and high flow conditions. The monitored bifurcation is largely unstable and shows sudden changes in the water distribution, mainly driven by the bar migrating in the upstream channel and entering the distributaries. A relationship between width asymmetry and flood magnitude was observed, confirming previous analyses. Moreover, recent theoretical findings were applied, in order to test the possibility to estimate general trends in bifurcation evolution. The analysis pointed out the relevance of a correct assessment of the characteristic temporal scales, as the bifurcation evolves on a timescale similar to that of bar migration and flood duration. Understanding the interactions between these processes is therefore crucial in order to increase the ability to model and predict the morphological evolution of a braided network. Copyright © 2012 John Wiley & Sons, Ltd.  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.
Small river channels within the Humber Basin of Southern Ontario exhibit irregular meandering patterns; however, contemporary river hydrology appears capable of controlling the scale of regular waveform development. Recent changes in river planform are assessed through a bi‐temporal comparison of channel planform using orthorectified aerial photography and statistical analysis of curvature series based on autocorrelation and spectral analysis. Calculated meander wavelengths are acceptable given traditional relationships between wavelength and mean annual flood discharge. It is also evident that changes in stream power are highly correlated with changes in the dominant wavelengths between channel reaches in this study. Gradual development of small scale waveforms following a rare hydrological and geomorphological event in 1954 further confirms that these forms can be attributed to the typical discharge regime. This paper argues that the scaling of wavelengths with discharge can be considered a strong factor controlling planform evolution on some small meandering river systems, despite manifest irregular planforms. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

9.
This paper focuses on one aspect of riparian zone change associated with channel incision. It presents information on sedimentation within the riparian zone of the River Garonne, France, derived from both historical and contemporary sources. The riparian zone investigated is morphologically complex, containing a remnant island and cut‐off channel. Information drawn from historical and contemporary sources permits the reconstruction of channel planform and cross‐profile change, as well as an assessment of changes in riparian zone sedimentation rates and patterns. Very significant changes are identified over the last 50 years: a change from a multiple thread to single thread channel planform; high rates of riparian zone sedimentation of the order of 0·5 to 2·5 cm a−1; clear spatial patterns in over‐bank sedimentation, reflecting topographic and vegetational variations and flood event magnitude; and a reduction in morphological and vegetational diversity through the period, which is expected to continue as the zone is transformed into an increasingly homogeneous sediment sink. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

10.
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   

11.
Identifying the relative contributions of physical and ecological processes to channel evolution remains a substantial challenge in fluvial geomorphology. We use a 74‐year aerial photographic record of the Hoh, Queets, Quinault, and Elwha Rivers, Olympic National Park, Washington, USA, to investigate whether physical or trophic‐cascade‐driven ecological factors – excessive elk impacts after wolves were extirpated a century ago – are the dominant drivers of channel planform in these gravel‐bed rivers. We find that channel width and braiding show strong relationships with recent flood history. All four rivers widened significantly after having been relatively narrow in the 1970s, consistent with increased flood activity since then. Channel planform also reflects sediment‐supply changes, evident from landslide response on the Elwha River. We surmise that the Hoh River, which shows a multi‐decadal trend toward greater braiding, is adjusting to increased sediment supply associated with rapid glacial retreat. These rivers demonstrate transmission of climatic signals through relatively short sediment‐routing systems that lack substantial buffering by sediment storage. Legacy effects of anthropogenic modification likely also affect the Quinault River planform. We infer no correspondence between channel evolution and elk abundance, suggesting that trophic‐cascade effects in this setting are subsidiary to physical controls on channel morphology. Our findings differ from previous interpretations of Olympic National Park fluvial dynamics and contrast with the classic example of Yellowstone National Park, where legacy effects of elk overuse are apparent in channel morphology; we attribute these differences to hydrologic regime and large‐wood availability. Published 2016. This article is a U.S. Government work and is in the public domain in the USA  相似文献   

12.
This study presents the first detailed field‐based analysis of the morphology of bifurcations within anabranching cobble–gravel rivers. Bifurcations divide the flow of water and sediment into downstream anabranches, thereby influencing the characteristics of the anabranches and the longevity of river islands. The history, morphology, bed grain size, and flow vectors at five bifurcations on the Renous River, New Brunswick, Canada, were studied in detail. The angles of bifurcations within five anabranching rivers in the Miramichi basin were investigated. The average bifurcation angle was 47°, within the range of values cited for braided river bifurcations. Bifurcation angle decreased when anabranches were of similar length. Shields stresses in channels upstream of bifurcations were lower than reported values for braided rivers. Stable bifurcations displayed lower Shields stresses than unstable bifurcations, contrary to experimental results from braided river bifurcations. Bifurcations in anabranching rivers are stabilized by vegetation that slows channel migration and helps to maintain a uniform upstream flow field. The morphology of stable bifurcations enhances their stability. A large bar, shaped like a shallow ramp that increases in elevation to floodplain level, forms at stable bifurcations. Floodplains at stable bifurcations accrete upstream at rates between 0·9 and 2·5 m a?1. Bars may also form within the entrance of an anabranch downstream of the bifurcation node. These bars are associated with bifurcation instability, forming after a period of stability or an avulsion. Channel abandonment occurs when a bar completely blocks the entrance to one anabranch. The stability of channels upstream of bifurcations and the location of bars at bifurcations influence bifurcation stability and the maintenance of river anabranching in the long term. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

13.
Anastomosing rivers have multiple interconnected channels that enclose flood basins. Various theories potentially explain this pattern, including an increased discharge conveyance and sediment transport capacity of multiple channels, deltaic branching, avulsion forced by base‐level rise, or a tendency to avulse due to upstream sediment overloading. The former two imply a stable anabranching channel pattern, whereas the latter two imply disequilibrium and evolution towards a single‐channel pattern in the absence of avulsion. Our objective is to test these hypotheses on morphodynamic scenario modelling and data of a well‐documented case study: the upper Columbia River. Proportions of channel and floodplain sediments along the river valley were derived from surface mapping. Initial and boundary conditions for the modelling were derived from field data. A 1D network model was built based on gradually varied flow equations, sediment transport prediction, mass conservation, transverse slope and spiral meander flow effects at the bifurcations. The number of channels and crevasse splays decreases in a downstream direction. Also, measured sediment transport is higher at the upstream boundary than downstream. These observations concur with bed sediment overloading from upstream, which can have caused channel aggradation above the surrounding floodplain and subsequent avulsion. The modelling also indicates that avulsion was likely caused by upstream overloading. In the model, multi‐channel systems inevitably evolve towards single‐channel systems within centuries. The reasons are that symmetric channel bifurcations are inherently unstable, while confluenced channels have relatively less friction than two parallel channels, so that more discharge is conveyed through the path with more confluences and less friction. Furthermore, the present longitudinal profile curvature of the valley could only be reproduced in the model by temporary overfeeding. We conclude that this anastomosing pattern is the result of time‐varying sediment overloading and is not an equilibrium pattern feature, and suggest this is valid for many anastomosing rivers. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

14.
Channel erosion along the Carmel river,Monterey county,California   总被引：1，自引：0，他引：1  
Historic maps, photographs, and channel cross-sections show that the channel of the Carmel River underwent massive bank erosion, channel migration, and aggradation in a major flood in 1911, then narrowed and incised by 1939. The channel was stable until 1978 and 1980, when bank erosion affected some reaches but not others. The narrowing and incision were in response to a lack of major floods after 1914 and construction in 1921 of a dam that cut off sediment supply from the most actively eroding half of the basin. Localized erosion in 1978 and 1980 occurred during low magnitude events along reaches whose bank strength had been reduced by devegetation. These events illustrate that the stability of a fluvial system can be disrupted either by application of a large erosive force in a high magnitude event (the 1911 flood) or in a low magnitude event, by reducing the resistance to erosion (bank devegetation). The Carmel River is a potentially unstable system. Its discharge and slope characteristics place it near the threshold between meandering and braided. On the Lower Carmel, the presence of bank vegetation can make the difference between a narrow, stable meandering channel and a wide shifting channel with braided reaches.  相似文献   

15.
Morphological features of braided rivers (bars, channels and pools) experience major changes in area, shape and spatial distribution as a response to (i) the pulsation of discharge during a flood and (ii) the bed evolution induced by floods. In this work, at‐a‐station relationships between water level and planform configuration were investigated on the Tagliamento River, a large gravel‐bed braided river in northeast Italy, over a 2‐year study period comprising three bankfull events and several small‐to‐medium floods. The analysis was performed on two 1‐km‐long reaches, characterized by different riparian vegetation cover. Ground‐based images with an hourly temporal resolution were acquired using software‐controlled, digital cameras. Bars, channels, pools and vegetated patches were manually digitized on more than 100 rectified images. Sequences of constant‐level images spanning the study period were used to quantify the impact of floods on the stability of at‐a‐station relationships and on the turnover rate of water bodies. The analysis shows that wetted area increased almost linearly with water level in both reaches. The average number of branches per cross‐section peaked at intermediate flow levels, increasing from 2 at low flow up to 6–7. The number of branches displayed the largest fluctuations over time, with significant changes produced also by moderate floods. Turnover rates were high in both reaches, with more than 30% of wetted areas at low flow converting into bare gravel in less than 2 months. Vegetation colonization was found to limit the mobility of the low flow channels over time by concentrating the flow in fewer, deeper anabranches. The number of channels per cross‐section was 30–40% less in the vegetated reach and the proportion of low flow water bodies in the same position after 12 months increased from 3% to 14%. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

16.
The Dead Run catchment in Baltimore County, Maryland, has undergone intense urbanization since the late 1950s. Reconstruction of the channel planform from topographic maps dating back to the 1890s and aerial photographs dating back to the 1930s indicates that the channel has remained stable in planform since at least the 1930s. The relative stability of Dead Run contrasts with the alterations in channel morphology reported for other urbanizing streams in the Piedmont physiographic province of the eastern United States. Trend analyses of discharge records in Dead Run show that urban development and stormwater control measures have had significant impacts on the hydrologic response of the catchment. The flood hydraulics of the Dead Run catchment are examined for the event that occurred on 22 June 1972 in association with Hurricane Agnes. A two‐dimensional hydraulic model, TELEMAC‐2D, was used with a finite‐element mesh constructed from a combination of high‐resolution LiDAR topographic data and detailed field survey data to analyse the distribution of boundary shear stress and unit stream power along the channel and floodplain during flooding from Hurricane Agnes. The spatial and temporal distributions of these parameters, relative to channel gradient and channel/valley bottom geometry, provide valuable insights on the stability of the Dean Run channel. The stability of Dead Run's channel planform, in spite of extreme flooding and decades of urban development, is most likely linked to geological controls of channel and floodplain morphology. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

17.
River islands are vital geomorphic units in alluvial rivers, and the variation of their morphology and position plays a significant role in regulating flow-sediment transport and channel stability. Based on the theories of minimum energy dissipation theory of fluid movement and river morphodynamics, this study uses the river islands in anabranching channels to analyze the relationship between the shape coefficient of river island and the flow-sediment dynamics under stable equilibrium conditions...  相似文献   

18.
《国际泥沙研究》2016,(2):131-138
Lateral migration is an important form of morphological changes on the Middle Yangtze River (MYR), particularly for the lower Jingjiang reach. The Three Gorges Reservoir (TGR) has substantially reduced sediment supply to the downstream river channels since its impoundment in June 2003. The scientific understanding of how decrease of sediment influences the processes of bank erosion and channel adjustments is complex and limited. In the present paper, the morphological responses in a typical meandering and island-braided river segment of the MYR to the filling of the TGR were investigated by a 3-D morphodynamic model. The potential of the 3-D model has been demonstrated by the observed data. The morphological evolutions in the Shishou bend during the first 12 years of the TGR impound-ment were predicted. The effects of the TGR operation on the planform evolutions in the study reach were analyzed based on the simulated results. Sediment load is decreased by 75%due to the early filling of the TGR. The magnitude of bed degradation with less sediment load due to the TGR operation is increased compared with the pre-dam situation. Qualitatively, the overall planform evolution trends in the Shishou bend after the TGR operation are similar to that without the TGR operation. The magnitude of lateral migration has been increased in some part of the channel bend, where the morphological response of the TGR operation exhibits more lateral migration rather than vertical degradation. Scouring at the bank toe enhances bank failure. Decrease of sediment load and weak bank anti-scour ability as well as the significant helical flow can be responsible for intensified bank erosion in the channel bend.  相似文献   

19.
In mixed bedrock–alluvial rivers, the response of the system to a flood event can be affected by a number of factors, including coarse sediment availability in the channel, sediment supply from the hillslopes and upstream, flood sequencing and coarse sediment grain size distribution. However, the impact of along-stream changes in channel width on bedload transport dynamics remains largely unexplored. We combine field data, theory and numerical modelling to address this gap. First, we present observations from the Daan River gorge in western Taiwan, where the river flows through a 1 km long 20–50 m wide bedrock gorge bounded upstream and downstream by wide braidplains. We documented two flood events during which coarse sediment evacuation and redeposition appear to cause changes of up to several metres in channel bed elevation. Motivated by this case study, we examined the relationships between discharge, channel width and bedload transport capacity, and show that for a given slope narrow channels transport bedload more efficiently than wide ones at low discharges, whereas wider channels are more efficient at high discharges. We used the model sedFlow to explore this effect, running a random sequence of floods through a channel with a narrow gorge section bounded upstream and downstream by wider reaches. Channel response to imposed floods is complex, as high and low discharges drive different spatial patterns of erosion and deposition, and the channel may experience both of these regimes during the peak and recession periods of each flood. Our modelling suggests that width differences alone can drive substantial variations in sediment flux and bed response, without the need for variations in sediment supply or mobility. The fluctuations in sediment transport rates that result from width variations can lead to intermittent bed exposure, driving incision in different segments of the channel during different portions of the hydrograph. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd  相似文献   

20.
The study analyses the morphological response of a gravel‐bed river to discharges of different magnitude (from moderate events that occur several times a year to a 12‐year flood) and so defines the range of formative discharges for single morphological units (channels, bars, islands) and a range of magnitude of morphological activity from the threshold discharges for gravel transport and minor bar modification up to flows causing major morphological changes. The study was conducted on the Tagliamento River, a large gravel‐bed river in north‐eastern Italy, using two different methods, analysis of aerial photographs and field observation of painted gravel particles. The available photographs (five flights from August 1997 to November 2002) and the two commissioned flights (June 2006 and April 2007) do not define periods with a single flood event, but the intervals are short enough (11 to 22 months) to have a limited number of flood events in each case. The fieldwork, which involved cross‐section survey, grain‐size analysis and observation of painted sediments, complemented the aerial surveys by allowing analysis of channel response to single flood events. Substantial morphological changes (e.g. bank erosion of several tens of metres up to more than 100 m) associated with flood events with a recurrence interval between 1·1 year and 12 years have been documented. Multiple forming discharges were defined based on the activity of different morphological units. Discharges equal to 20–50% of the bankfull discharge are formative for the channels, whereas the bankfull discharge (1·1 year flood in this case of the Tagliamento River) is formative for low bars. Larger floods, but still relatively frequent (with a recurrence interval less than five years), are required for full gravel transport on high bars and significant morphological changes of islands. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

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