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1.
Braun's finding that the wavelength of bedrock meanders decreases as rock hardness increases is reconciled with Hack's and Tinkler's finding that meander wavelength is greater in bedrock than alluvial channels. In populations of alluvial or bedrock channels, bank resistance controls meander wavelength via its influence on channel shape and is negatively correlated with meander wavelength. In contrast, in mixed populations of alluvial and bedrock channels, bank resistance controls meander wavelength through its effect on the recurrence interval of the channel-forming discharge and channel size and is positively correlated with meander wavelength.  相似文献   

2.
Marked differences in bedrock meander dimensions in the Appalachian Valley and Ridge province, at times in adjacent reaches of a single stream, are related to differences in relative erodability of the bedrock. Meanders cut in thick-bedded to massive lithologies, typically carbonates, are distinctly smaller overall than meanders cut into shaly lithologies. Other factors that could affect bedrock meander dimensions were considered and none appeared to offer any help in explaining the dimensional differences observed above. In plateau regions underlain by essentially horizontal strata, the meander form characteristic of one lithology may become superimposed upon another as incision progresses, producing anomalous relationships between bedrock meander dimensions and lithology. Empirical relations developed from meander geometry measurements and estimated bedrock erodability for 78 bedrock meander reaches, containing a total of 1089 individual meander loops, show that meanders cut in shaly lithologies (ML = 105 Q0.50f, where ML = meander length and Qf = most probable annual flood) are about twice the length of meanders cut in non-shaly lithologies (ML = 39.30.56f). The valley floor width of the meanders cut in shaly bedrock (VF = 28 Q0.43f) is two to three times wider than the valley floor width of the meanders cut in the more resistant non-shaly bedrock (VF = 8 Q0.43f). The mean and median meander length values for individual reaches typically differ by less than 10 per cent.  相似文献   

3.
Many models of incision by bedrock rivers predict water depth and shear stress from discharge; conversely, palaeoflood discharge is sometimes reconstructed from flow depth markers in rock gorges. In both cases, assumptions are made about flow resistance. The depth–discharge relation in a bedrock river must depend on at least two roughness length scales (exposed rock and sediment cover) and possibly a third (sidewalls). A conceptually attractive way to model the depth–discharge relation in such situations is to partition the total shear stress and friction factor, but it is not obvious how to quantify the friction factor for rough walls in a way that can be used in incision process models. We show that a single flow resistance calculation using a spatially averaged roughness length scale closely approximates the partitioning of stress between sediment and rock, and between bed and walls, in idealized scenarios. Both approaches give closer fits to the measured depth–discharge relations in two small bedrock reaches than can be achieved using a fixed value of Manning's n or the Chézy friction factor. Sidewalls that are substantially rougher or smoother than the bed have a significant effect on the partitioning of shear stress between bed and sidewalls. More research is needed on how best to estimate roughness length scales from observable or measurable channel characteristics. © 2019 John Wiley & Sons, Ltd.  相似文献   

4.
Abstract

An attractive explanation for the observed spatial growth of the Gulf Stream meanders is that the meanders are spatially growing unstable waves. The results of a calculation based on a simple two-layer model of baroclinically unstable flow presented here support this idea. The model is a familiar one with the energy for the growth of the meander perturbations coming from the potential energy available in the geostrophic tilt of the interface between the two layers due to their velocity shear. In order to distinguish between spatial and temporal growth, it IS necessary to assume that the meanders are generated in a localized region, or equivalently, that the meanders are upstream disturbances which are amplified as they enter a region of unstable flow. This assumption is implemented mathematically through the use of a Green's function which governs the propagation of the meanders. Analysis of the spatial and temporal characteristics of the Green's function leads to a criterion which must he satisfied if the meanders arc to grow spatially. This criterion is that the mean flow velocity must be sufficiently greater than the velocity shear, Um > √2 Us, in order to have spatial growth. This simply means that the growing meanders must be washed downstream faster than they spread upstream, or equivalently the spatial growth is due to downstream advection of growing disturbances. The actual Gulf Stream flow is in fair agreement with this criterion.  相似文献   

5.
A sustained dynamic inflow perturbation and bar–floodplain conversion are considered crucial to dynamic meandering. Past experiments, one-dimensional modelling and linear theory have demonstrated that the initiation and persistence of dynamic meandering require a periodic transverse motion of the inflow. However, it remains unknown whether the period of the inflow perturbation affects self-formed meander dynamics. Here, we numerically study the effect of the inflow perturbation period on the development and meander dynamics of a chute-cutoff-dominated river, which requires two-dimensional modelling with vegetation forming floodplain on bars. We extended the morphodynamic model Nays2D with growth and mortality rules of vegetation to allow for meandering. We tested the effect of a transversely migrating inflow boundary by varying the perturbation period between runs over an order of magnitude around typical modelled meander periods. Following the cutoff cascade after initial meander formation from a straight channel, all runs with sufficient vegetation show series of growing meanders terminated by chute cutoffs. This generates an intricate channel belt topography with point bar complexes truncated by chutes, oxbow lakes, and scroll-bar-related vegetation age patterns. The sinuosity, braiding index and meander period, which emerge from the inherent biomorphological feedback loops, are unrelated to the inflow perturbation period, although the spin-up to dynamic equilibrium takes a longer time and distance for weak and absent inflow perturbations. This explains why, in previous experimental studies, dynamic meandering was only accomplished with a sustained upstream perturbation in flumes that were short relative to the meander wavelength. Our modelling of self-formed meander patterns is evidence that scroll-bar-dominated and chute-cutoff-dominated meanders develop from downstream convecting instabilities. This insight extends to many more fluvial, estuarine and coastal systems in morphological models and experiments, which require sustained dynamic perturbations to form complex patterns and develop natural dynamics. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.  相似文献   

6.
YAN Bing  JIA Dong 《地震地质》2017,39(6):1127-1142
Offset river is one of the characteristic landforms along active strike-slip fault. Whereas because of various factors such as natural meander, river capture, etc, difficulties exist while interpreting slip motion and offset amount using landforms of offset rivers. In this study, we introduced the systematic offset of bedrock channels as a method to analyze offset rivers along strike-slip fault. Systematic offset of bedrock channels is the result of coupling between tectonic process and surface process. It also describes the phenomenon of synchronous accumulation both of the offset amount and the upstream length because of head-ward erosion. Based on the interpretation, measuring and statistics of the offset river landforms, it is found that systematic offset of bedrock channels have developed along the Ganzi-Yushu, Xianshuihe and eastern Kunlun fault zones on the eastern Tibetan plateau. There is a linear relationship between the upstream length (L), measured from the headwater to the fault, and the offset amount (D):D=a·L. This study provides useful implications to the role of strike-slip faults during the geomorphic evolution of the eastern Tibetan plateau.  相似文献   

7.
During the evolution of meander bends, the intra‐meander groundwater head gradients steepen and generate zones of accelerated water and nutrient intra‐meander fluxes important for ecosystem processes. This paper compares and contrasts three MODFLOW groundwater model packages based on their simulation of intra‐meander flux for two stages of meander evolution observed in a sandbox river table and one level of river bed clogging, where the hydraulic conductivity in the river bed is lower than in the adjacent aquifer. These packages are the Time‐Variant Specified Head package [constant head (CHD)], River package (RIV), and Streamflow‐Routing package (SFR2), each controlling the groundwater or river head bounding the intra‐meander region. The RIV and SFR2 packages fix river stage and allow for variation in groundwater head below the river, which is suggested for simulating intra‐meander flux for all sinuosities with and without river bed clogging whenever river bed parameters are available. The CHD package fixes below river groundwater head and fails to simulate intra‐meander head loss and flux in meanders with high sinuosity or river bed clogging. In low sinuosity meanders and in cases without river bed clogging, there were no significant differences between MODFLOW packages for simulating river intra‐meander head loss and flux. This research demonstrates why MODFLOW users need to consider the limitations of each package when simulating intra‐meander flux in reaches with river bed clogging, high sinuosity, or similarly steep hydraulic gradients. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

8.
The saltation–abrasion model predicts rates of river incision into bedrock as an explicit function of sediment supply, grain size, boundary shear stress and rock strength. Here we use this experimentally calibrated model to explore the controls on river longitudinal profile concavity and relief for the simple but illustrative case of steady‐state topography. Over a wide range of rock uplift rates we find a characteristic downstream trend, in which upstream reaches are close to the threshold of sediment motion with large extents of bedrock exposure in the channel bed, while downstream reaches have higher excess shear stresses and lesser extents of bedrock exposure. Profile concavity is most sensitive to spatial gradients in runoff and the rate of downstream sediment fining. Concavity is also sensitive to the supply rate of coarse sediment, which varies with rock uplift rate and with the fraction of the total sediment load in the bedload size class. Variations in rock strength have little influence on profile concavity. Profile relief is most sensitive to grain size and amount of runoff. Rock uplift rate and rock strength influence relief most strongly for high rates of rock uplift. Analysis of potential covariation of grain size with rock uplift rate and rock strength suggests that the influence of these variables on profile form could occur in large part through their influence on grain size. Similarly, covariation between grain size and the fraction of sediment load in the bedload size class provides another indirect avenue for rock uplift and strength to influence profile form. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

9.
Floodplain wetlands are common features of rivers in southern Africa, but they have been little studied from a geological or geomorphological perspective. Study of the upper Klip River, eastern Free State, South Africa, indicates strong geological controls on the formation of alluvial meanders and associated floodplain wetlands. Along this river, pronounced and abrupt changes in valley width are strongly linked to lithological variations. Where weakly cemented sandstone crops out, the Klip has laterally eroded bedrock and carved valleys up to 1500 m wide. In these valleys, the river meanders (sinuosity up to ～1·75) on moderate gradients (<0·001) within extensive floodplains marked by numerous oxbow lakes, backswamps and abandoned channels, many of which host substantial wetlands. In contrast, where highly resistant dolerite crops out, lateral erosion of bedrock is restricted, with the Klip tending instead to erode vertically along joints or fractures. Here, valleys are narrower (<200 m), channel‐bed gradients are steeper (>0·003), the river follows a much straighter course (sinuosity ～1·10–1·34), and floodplains are restricted in width. Long‐term landscape development in the Klip and numerous similar catchments depends on the interaction between fluvial processes in the sandstone and dolerite valleys. In the sandstone valleys, vertical erosion rates are controlled by erosion rates of the more resistant dolerites downstream. Hence, in the short‐ to medium‐term (decades to tens of thousands of years), lateral erosion dominates over vertical erosion, with the river concomitantly planing sandstone in the channel floor and reworking floodplain sediments. The thickness of alluvial fill in the sandstone valleys is limited (<4 m), but the resultant meanders are naturally dynamic, with processes such as point bar deposition, cutoff formation and channel avulsion resulting in an assemblage of fluvial landforms. In the longer term (greater than tens of thousands of years), however, vertical erosion will occur in the sandstone valleys as the downstream dolerites are lowered by erosion, resulting in channel incision, floodplain abandonment, and desiccation of the wetlands. Identification of the geological controls on meander and wetland formation provides information vital for the design of effective management guidelines for these ecologically rich habitats, and also contributes to a better understanding of rivers that are intermediate between fully alluvial and fully bedrock. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

10.
The evolution of meander bends and formation of cutoffs, including a series of cutoffs developed simultaneously in a number of bends, have been investigated by many researchers. However, relatively little is known about factors that lead to the development of multiple cutoffs that are formed subsequently at one location. The present study aims to determine the influence of meander bend development on multiple chute cutoff formation in a single bend. The research is based on the sedimentary record of meander migration and cutoffs preserved in a lowland river floodplain (the lower Obra River, Poland). Analysis of changes in meander geometry was conducted to describe the influence of their migration on cutoff formation and in other rivers where multiple cutoffs occurred. The results showed that multiple cutoffs in the lower Obra River have occurred during the last 3000 years, owing to the interaction of upstream and downstream controls: migration of meander bends in opposing directions accompanied by an increase of flood frequency and sediment supply. The flow and sediment supply has been further altered since the nineteenth century due to anthropogenic impacts: an artificial cutoff of the downstream bend and elevation of channel levées. Similar mechanisms driving the formation of multiple cutoff have been found in other river courses, despite significantly higher energy of the compared rivers. Moreover, development of a confined‐shape bend (caused by artificial barrier or autogenic bend behaviour) may also favour the formation of multiple cutoffs. However, counter migration of meanders enhanced by increased flood frequency and sediment supply are primary triggers for such events. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

11.
A large meander of the Kuroshio was generated in the region off the southern coast of Japan in August 2004 and continued until approximately July 2005. The formation and decay of the large-meander (LM) path was observed by bottom pressure (BP) sensors installed on inverted echo sounders (PIESs) and a seismic observing system off Shikoku. The variation in BP was examined focusing on the development, persistence, and decay of the LM path. The BP was found to be depressed associated with a Kuroshio path disturbance, called a small meander, and this BP depression led the sea surface height (SSH) depression by up to approximately two months. The temporal phase shift between the sea surface and deep disturbances was significantly greater than those of other small meanders that did not develop into large meanders. After the formation of the LM path, the BP beneath the Kuroshio increased with a lag of approximately two months behind the SSH elevation along with the upward displacement of the main thermocline. The increase in BP is associated with that of the positive southward BP gradient anomaly, i.e., the eastward deep Kuroshio current anomaly, which suggests an enhancement of the topographic steering and stability of the LM path. This is consistent with the fact that no small meanders occurred in the early LM period from late July 2004 to late January 2005.  相似文献   

12.
The bombardment of the oceans by asteroids and comets leads to the propagation of very large waves—superwaves. On approaching continental margins, superwaves may grow to heights large enough to spill on to the land, flooding extensive areas of continental lowland. The waters from these superfloods, in running back to the sea, would be capable of carrying out enormous amounts of work, possibly diverting rivers, cutting gorges, forming valley meanders, and leading to the widespread aggradation of coarse deposits produced by incoming superwaves stripping the soil and regolith cover and denuding weathered bedrock. Calculations of the magnitude and frequency of superfloods, based on current asteroidal impact rates, indicate that the superflooding of continental margins, including the British Isles, could have occurred several times since the start of the Pliocene. Thus, there seems to be a case for reviving some of the views of the old diluvialists, and exploring the worth of neodiluvialism as a system of Earth surface history.  相似文献   

13.
Rivers may dramatically change course on a fluvial plain. Such an avulsion temporarily leads to two active channels connected at a bifurcation. Here we study the effect of dynamic meandering at the bifurcation and the effect of channel width adjustment to changing discharge in both downstream branches on the evolution of a bifurcation and coexisting channels. As an example, we reconstructed the last major avulsion at the Rhine delta apex. We combined historical and geological data to reconstruct a slowly developing avulsion process spanning 2000 years and involving channel width adjustment and meandering at the bifurcation. Based on earlier idealised models, we developed a one‐dimensional model for long‐term morphodynamic prediction of upstream channel and bifurcates connected at the bifurcation node. The model predicts flow and sediment partitioning at the node, including the effect of migrating meanders at the bifurcation and channel width adjustment. Bifurcate channel width adaptation to changing discharge partitioning dramatically slows the pacing of bifurcation evolution because the sediment balance for width adjustment and bed evolution are coupled. The model further shows that meandering at the bifurcation modulates channel abandonment or enlargement periodically. This explains hitherto unrecognised reactivation signals in the sedimentary record of the studied bifurcation meander belts, newly identified in our geological reconstruction. Historical maps show that bifurcation migration due to meander bend dynamics increases the bifurcation angle, which increases the rate of closure of one bifurcate. The combination of model and reconstruction identifies the relevant timescales for bifurcation evolution and avulsion duration. These are the time required to fill one downstream channel over one backwater length, the time to translate one meander wavelength downstream and, for strong river banks, the adaptation timescale to adjust channel width. The findings have relevance for all avulsions where channel width can adjust to changing discharge and where meandering occurs. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

14.
Sand and gravel tailings from nineteenth century open‐pit hydraulic gold mines formed large alluvial fans at tributary con?uences in the northwestern Sierra Nevada, California. In the Bear River watershed, several of these fans were so large that they blocked main channels for decades. Some channels not only aggraded deeply, but also moved laterally and cut across the inner bends of valley spurs. Now locked in bedrock channels, these valley‐spur cutoffs impose local controls on geomorphic, hydraulic, and sedimentary processes. One cutoff has incised 25 m into bedrock over the past century (25 cm a?1) with rapid initial incision rates of up to 50 cm a?1 (1884–1890). Recognition of spur cutoffs in the geological record may help to identify large landslides and provide an analogue for a type of natural earth?ll dam spillway not prone to catastrophic failures. Tailing fans, valley‐spur cutoffs, and the sediment they trap are described from contemporary accounts and recent ?eld conditions in the Bear River watershed. These anthropogenic changes represent a major shift in the watershed from supply‐limited to transport‐limited sediment budgets and a change in geomorphic processes away from long‐term drainage evolution dominated by ingrown meanders. The large volumes of mining sediment stored in these landforms will be slowly released over the next millennium and could be signi?cant to contemporary ecological and public health issues due to recent ?ndings of high mercury loadings associated with hydraulic mines. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

15.
Aerial photographs led to the detection of a previously undescribed type of river meander. These features are characterized by nearly circular pools located at the apex of the bend and are widely distributed on small to medium sized coastal plain streams in the southeastern United States. Bathymetric evidence shows that the structure of meanders containing circular pools is distinctive. Field and laboratory evidence suggests that these features are formed by a short-lived countercurrent that develops during the flood cycle.  相似文献   

16.
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   

17.
Two methods are discussed for determining changes in ground displacement spectra and the velocity and acceleration spectra derived from these when recorded on solidly frozen bedrock (T < ?3°C) versus epicentral distance and energy class. The first of these methods characterizes the entire epicenter field of earthquakes, while the second aims at studying possible differences between spectra in different source zones. We have found the maximum spectral level as a function of epicentral distance and energy class. The calculated spectra are compared with the available records of large earthquakes. The manner in which near and comparatively small earthquakes can be used to find ground motion spectra is shown for solidly frozen bedrock for earthquakes as large as the 15–17 energy class, as well, determination of the differences between the spectra of seismic signals due to earthquakes occurring in different source zones is performed. The results can be used both directly and for the zonation and prediction of seismic hazard within the zone of solidly frozen rock, and also for the case where the temperature regime of the frozen rock has been changed or disturbed.  相似文献   

18.
An hypothesis from which a general principle covering deformation of alluvial boundaries may be developed is proposed, namely—flow of fluid past a deformable boundary will deform the boundary so as to increase the resistance to flow. Upon attainment of a local maximum of resistance to flow the boundary shape will cease to change. Evidence from channel bed forms, meander geometry and armoured beds is presented to support the hypothesis. One implication of the hypothesis is that channels will adjust so that sediment movement is maximized. The authors would like geomorphologists to test this in their work on landscape evolution.  相似文献   

19.
Research in landscape evolution over millions to tens of millions of years slowed considerably in the mid‐20th century, when Davisian and other approaches to geomorphology were replaced by functional, morphometric and ultimately process‐based approaches. Hack's scheme of dynamic equilibrium in landscape evolution was perhaps the major theoretical contribution to long‐term landscape evolution between the 1950s and about 1990, but it essentially ‘looked back’ to Davis for its springboard to a viewpoint contrary to that of Davis, as did less widely known schemes, such as Crickmay's hypothesis of unequal activity. Since about 1990, the field of long‐term landscape evolution has blossomed again, stimulated by the plate tectonics revolution and its re‐forging of the link between tectonics and topography, and by the development of numerical models that explore the links between tectonic processes and surface processes. This numerical modelling of landscape evolution has been built around formulation of bedrock river processes and slope processes, and has mostly focused on high‐elevation passive continental margins and convergent zones; these models now routinely include flexural and denudational isostasy. Major breakthroughs in analytical and geochronological techniques have been of profound relevance to all of the above. Low‐temperature thermochronology, and in particular apatite fission track analysis and (U–Th)/He analysis in apatite, have enabled rates of rock uplift and denudational exhumation from relatively shallow crustal depths (up to about 4 km) to be determined directly from, in effect, rock hand specimens. In a few situations, (U–Th)/He analysis has been used to determine the antiquity of major, long‐wavelength topography. Cosmogenic isotope analysis has enabled the determination of the ‘ages’ of bedrock and sedimentary surfaces, and/or the rates of denudation of these surfaces. These latter advances represent in some ways a ‘holy grail’ in geomorphology in that they enable determination of ‘dates and rates’ of geomorphological processes directly from rock surfaces. The increasing availability of analytical techniques such as cosmogenic isotope analysis should mean that much larger data sets become possible and lead to more sophisticated analyses, such as probability density functions (PDFs) of cosmogenic ages and even of cosmogenic isotope concentrations (CICs). PDFs of isotope concentrations must be a function of catchment area geomorphology (including tectonics) and it is at least theoretically possible to infer aspects of source area geomorphology and geomorphological processes from PDFs of CICs in sediments (‘detrital CICs’). Thus it may be possible to use PDFs of detrital CICs in basin sediments as a tool to infer aspects of the sediments' source area geomorphology and tectonics, complementing the standard sedimentological textural and compositional approaches to such issues. One of the most stimulating of recent conceptual advances has followed the considerations of the relationships between tectonics, climate and surface processes and especially the recognition of the importance of denudational isostasy in driving rock uplift (i.e. in driving tectonics and crustal processes). Attention has been focused very directly on surface processes and on the ways in which they may ‘drive’ rock uplift and thus even influence sub‐surface crustal conditions, such as pressure and temperature. Consequently, the broader geoscience communities are looking to geomorphologists to provide more detailed information on rates and processes of bedrock channel incision, as well as on catchment responses to such bedrock channel processes. More sophisticated numerical models of processes in bedrock channels and on their flanking hillslopes are required. In current numerical models of long‐term evolution of hillslopes and interfluves, for example, the simple dependency on slope of both the fluvial and hillslope components of these models means that a Davisian‐type of landscape evolution characterized by slope lowering is inevitably ‘confirmed’ by the models. In numerical modelling, the next advances will require better parameterized algorithms for hillslope processes, and more sophisticated formulations of bedrock channel incision processes, incorporating, for example, the effects of sediment shielding of the bed. Such increasing sophistication must be matched by careful assessment and testing of model outputs using pre‐established criteria and tests. Confirmation by these more sophisticated Davisian‐type numerical models of slope lowering under conditions of tectonic stability (no active rock uplift), and of constant slope angle and steady‐state landscape under conditions of ongoing rock uplift, will indicate that the Davis and Hack models are not mutually exclusive. A Hack‐type model (or a variant of it, incorporating slope adjustment to rock strength rather than to regolith strength) will apply to active settings where there is sufficient stream power and/or sediment flux for channels to incise at the rate of rock uplift. Post‐orogenic settings of decreased (or zero) active rock uplift would be characterized by a Davisian scheme of declining slope angles and non‐steady‐state (or transient) landscapes. Such post‐orogenic landscapes deserve much more attention than they have received of late, not least because the intriguing questions they pose about the preservation of ancient landscapes were hinted at in passing in the 1960s and have recently re‐surfaced. As we begin to ask again some of the grand questions that lay at the heart of geomorphology in its earliest days, large‐scale geomorphology is on the threshold of another ‘golden’ era to match that of the first half of the 20th century, when cyclical approaches underpinned virtually all geomorphological work. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

20.
An analytical modelling framework is proposed to reproduce the frequently observed but poorly studied occurrence of mid‐channel bars in meandering channels. Mid‐channel bars occur in meanders and may characterize transitional morphologies between pure meandering and braided rivers. Based on existing field and experimental observations, we propose that two different mechanisms can generate central topographical patterns in meanders. A former mechanism (‘width‐forced’) is related to spatial width oscillations which determine a laterally symmetrical bed shear stress pattern that promotes mid‐channel bars. A second mechanism (‘curvature‐forced’) can take place also in curvilinear equiwidth streams since also longitudinal variations of channel curvature can produce laterally symmetrical alterations of the sediment transport capacity. A perturbation approach is employed to model both mechanisms within a common framework, allowing reproduction, at least qualitatively, of several observed features. While width‐forced mid‐channel bars are a symmetric linear altimetric response, to reproduce curvature‐forced mid‐channel bars requires modelling nonlinear flow‐bed topography interactions at the second order of the perturbation expansion. Hypotheses on how these mechanisms operate are further discussed through an application to field cases. The amplitude of the nonlinear response can be relevant compared to that of the point bar in equiwidth meanders and the location of mid‐channel bars seldom coincides with bend apexes, mainly depending upon the intrinsic meander wavelength. Central bars tend to symmetrically divert the flow against the two banks, a process which is proposed as a possible cause of cross‐sectional overwidening, along with the asymmetry between the rates of bank erosion and of the opposite bank accretion. The outcomes of this first modelling step on the subject allow discussion of the mutual feedback processes that characterize interactions between mid‐channel bars and width variations in river meanders. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

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