Form and growth of bars in a wandering gravel‐bed river

The changing form of developing alluvial river bars has rarely been studied in the field, especially in the context of the fixed, compound, mainly alternate gravel bars that are the major morphological feature of the wandering style. Century scale patterns of three‐dimensional growth and development, and the consequent scaling relations of such bars, are examined along the gravel‐bed reach of lower Fraser River, British Columbia, Canada. A retrospective view based on maps and aerial photographs obtained through the twentieth century shows that individual bars have a life history of about 100 years, except in certain, protected positions. A newly formed gravel bar quickly assumes its ultimate thickness and relatively quickly approaches its equilibrium length. Growth continues mainly by lateral accretion of unit bars, consistent with the lateral style of instability of the river. Bar growth is therefore allometric. Mature bars approach equilibrium dimensions and volume that scale with the overall size of the channel. Accordingly, the bars conform with several published criteria for the ultimate dimensions of alternate barforms. Sand bars, observed farther downstream, have notably different morphology. Fraser River presents a typical wandering channel planform, exhibiting elements of both meandered and low‐order braided channels. Hydraulic criteria to which the Fraser bars conform illustrate why this planform develops and persists. The modest rate of bed material transfer along the channel – typical of the wandering type – determines a century‐length time scale for bar development. This time scale is consistent with estimates that have been made for change of the macroform elements that determine the overall geometry of alluvial channels. Copyright © 2009 John Wiley & Sons, Ltd.     

A field test of tubino's (1991) model of alternate bar formation

This study investigates the fluvial dynamics of straight natural stream channels. In particular, this experimental field study quantitatively assesses a physically based non-linear mathematical theory of alternate bar formation under unsteady natural flow conditions within a straight alluvial stream. The study site is an artificially straightened section of the Embarras River located approximately 16 km south of Champaign, Illinois. Data were collected on channel form, gradient, alternate bar dimensions, bed sediment size and flow stage over a 2 year study period. Both linear and non-linear steady flow hydrodynamic theories suggest that alternate bars are critical to the process of meander development. But these theories do not predict bar development for unsteady flow conditions, which typically occur in natural alluvial channels. Tubino (1991) suggests that bar evolution for a flood hydrograph can be divided into three parts: (1) a period of limited bar growth during the rising stage of the flood; (2) a stage of modest bar decay near the peak of the flood; and (3) a stage of non-linear bar growth during the prolonged falling stage of the flood. Bars developed during the falling limb of a hydrograph, and exhibited sequential development rather than the uniform growth along the reach predicted by Tubino's model. As flow stage decreased, short, low, fine-grained bars were superimposed on long, high and coarser-grained bars that developed under preceding high flow stages. These results suggest that the process of bar formation in artificially straightened natural streams with heterogeneous bed material may occur under different flow conditions and in a different manner than predicted by theoretical models. Further work should focus on attempting to isolate the physical mechanisms responsible for alternate bar formation in straight natural streams with heterogeneous bed material and flashy hydrologic flow regimes.     

Channel bar dynamics on multi‐decadal timescales in an active meandering river

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.     

Biomorphodynamics of alternate bars in a channelized,regulated river: An integrated historical and modelling analysis

The development of alternate bars in channelized rivers can be explained theoretically as an instability of the riverbed when the active channel width to depth ratio exceeds a threshold. However, the development of a vegetation cover on the alternate bars of some channelized rivers and its interactions with bar morphology have not been investigated in detail. Our study focused on the co‐evolution of alternate bars and vegetation along a 33 km reach of the Isère River, France. We analysed historical information to investigate the development of alternate bars and their colonization by vegetation within a straightened, embanked river subject to flow regulation, sediment mining, and vegetation management. Over an 80 year period, bar density decreased, bar length increased, and bar mobility slowed. Vegetation encroachment across bar surfaces accompanied these temporal changes and, once established, vegetation cover persisted, shifting the overall system from an unvegetated to a vegetated dynamic equilibrium state. The unvegetated morphodynamics of the impressively regular sequence of alternate bars that developed in the Isère following channelization is consistent with previous theoretical morphodynamic work. However, the apparent triggering dynamics of vegetation colonization needs to be investigated, based on complex biophysical instability processes. If instability related to vegetation colonization is confirmed, further work needs to focus on the relevance of initial conditions for this instability, and on related feedback effects such as how the morphodynamics of bare‐sediment alternate bars may have affected vegetation development and, in turn, how vegetation has created a new dynamic equilibrium state. Copyright © 2018 John Wiley & Sons, Ltd.     

Morphodynamics of alternate bars in the presence of riparian vegetation

Alpine gravel-bed rivers are dynamic systems that have been subjected to many anthropic alterations in the past centuries. Riparian vegetation development on previously bare sediment bedforms has been a common adjustment, raising important management issues in terms of flood risks and biodiversity. Many of these rivers are also channelized, and as a result present a pattern of alternate bars. Considering recent advances in numerical biomorphodynamic modeling, this study aims at exploring numerically the morphodynamics of alternate bars in the presence of riparian vegetation. To this end, a dynamic vegetation module has been implemented on top of an existing morphodynamic model, accounting for ecological processes of seed dispersal, seedling recruitment, growth, and mortality. Numerical simulations have been performed on a simplified reach of a gravel-bed river with free migrating alternate bars at initial state. In this work 96 scenarios have been simulated, each representing 50 years of channel evolution, with different flood regimes characterized by various peak discharges and flood durations. Yearly peak discharge variability is explicitly modeled in 48 scenarios. Model outcomes present two possible equilibrium biomorphodynamic behaviors: stationary vegetated bars, or free migrating bars in the case of frequent vegetation removal during floods. This binary behavior holds true when the stochasticity of annual peak discharges is represented, and for a wide range of parameter values included in vegetation dynamic modeling. Transient mobility of vegetated bars is observed in specific configurations where large sediment deposits deflect the flow field, eroding bar heads. Modeled bar wavelengths are in the range of values predicted for free bars by linear bar theory, and remain far from the theoretical values of hybrid, steady bars. The shift from unvegetated migrating bars to steady vegetated bars seems to show that in these simulations vegetation constitutes a hydraulic forcing, leading to a shift from free bars to forced bars, with a final configuration largely inherited from the initial state. © 2019 John Wiley & Sons, Ltd.     

Numerical modelling of alternate bar formation,development and sediment sorting in straight channels

A two‐dimensional shallow water hydro‐sediment‐morphodynamic model is applied to investigate alternate bar formation, development and sediment sorting in straight channels. The model is coupled, explicitly incorporating the flow–sediment–bed interactions by using the full mass and momentum conservation equations, which are numerically solved by a well‐balanced version of the finite volume Slope Limiter Centred (SLIC) scheme. The model is first tested against a flume experiment on alternate bars formed over a uniform sediment bed, which clearly exhibits processes of bar formation, migrating and finally approaching an equilibrium state. Then it is applied to another flume experiment on alternate bars due to non‐uniform sediment transport. The computational results are evaluated, with a focus on the longitudinal and vertical sediment sorting. It is argued for the first time that the inconsistent sediment sorting patterns observed in previous studies are determined by different sediment transport conditions, i.e. full versus partial transport. When a condition of full transport is achieved, under which all size fractions are fully mobilized and transported, the longitudinal surface sediment shows a sorting pattern of coarse‐on‐head and fine‐in‐pool, and the vertical substrate sediment exhibits an immobile‐fine‐coarse structure upwards. In contrast, for a partial transport condition, under which only finer fraction participates in the transport process, an opposite longitudinal pattern (i.e. fine‐on‐head and coarse‐in‐pool) and a different vertical structure (i.e. immobile‐coarse‐fine) are observed. Concurrently, numerical experiments with specified conditions show that the critical aspect ratio for the formation of migrating alternate bars is approximately equal to 12. With the increase of the aspect ratio, the bar length grows gradually, while the bar height increases rapidly for moderate values of the aspect ratio and then keeps nearly stable. The bar celerity, however, is weakly sensitive to the variation of this ratio. Copyright © 2016 John Wiley & Sons, Ltd.     

Alternate bars in a sandy gravel bed river: generation,migration and interactions with superimposed dunes

A field study was carried out to investigate the development of alternate bars in a secondary channel of the Loire River (France) as a function of discharge variations. We combined frequent bathymetric surveys, scour chains and stratigraphical analysis of deposits with measurements and modelling of flow dynamics. The channel exhibited migrating bars, non‐migrating bars and superimposed dunes. Possible mechanisms of bar initiation were found to be chutes associated with changes of bank direction and instability resulting from interactions between existing bars during the fall in water level after floods. We propose that the reworking of bar sediments during low flows (high width‐to‐depth ratio β), reinforced by high values of the Shields mobility parameter, can explain the formation or re‐generation of new alternate migrating bars during a subsequent flood. The migration pattern of the bars was found to be cyclic and to depend mainly on (i) channel layout and (ii) the dynamics of superimposed dunes with heights and lengths depending on location and discharge value. For instance, the hysteresis affecting the steepness of dunes influences the flow resistance of the dunes as well as the celerity of migrating bars during flood events. We compare the findings from the field with results from theoretical studies on alternate bars. This gives insight in the phenomena occurring in the complex setting of real rivers, but it also sheds light on the extent to which bar theories based on idealized cases can predict those phenomena. Copyright © 2014 John Wiley & Sons, Ltd.     

Formation and dynamics of vegetated fluvial landforms follow the biogeomorphological succession model in a channelized river

Feedback between hydrogeomorphological processes and riparian plants drives landscape dynamics and vegetation succession in river corridors. We describe the consequences of biogeomorphological feedback on the formation and dynamics of vegetated fluvial landforms based on observations from the channelized Isère River in France. The channel was laterally confined with embankments and mostly straightened. From the beginning of the 1970s to the end of the 1990s, alternate bars were progressively but heavily colonized by vegetation. This context presented an exceptional opportunity to analyse temporal adjustments between fluvial landforms and vegetation succession from bare gravel bars to mature upland forest as the consequence of biogeomorphological interactions. Based on a GIS analysis of aerial photographs (between 1948 and 1996), we show that the spatiotemporal organization of vegetated bars within the river channel observed in 1996 resulted from a bioconstruction and biostabilization effect of vegetation and interactions between bars of varying age, size and mobility. Field measurements in 1996 reflected how a strong positive feedback between sedimentary dynamics and riparian vegetation succession resulted in the construction of the vegetated bars. A highly significant statistical association of geomorphological and vegetation variables (RV of co-inertia analysis = 0.41, p < 0.001) explained 95% of the variability in just one axis, supporting the existence of very strong feedback between geomorphological changes (i.e. the transformation of small bare alternate bars to fluvial landforms covered by mature upland forest, and vegetation succession). Such dynamics reflect the fluvial biogeomorphological successions model, as described by the authors earlier. © 2020 John Wiley & Sons, Ltd.     

An opportunity to connect the morphodynamics of alternate bars with their sedimentary products

Field investigations that help clarify local sedimentary processes involved in the migration of alternate bars as a consequence of flood events are lacking. A simple approach combining scour chains, stratigraphy and frequent bathymetric surveys is proposed to connect the dynamics of free migrating alternate bars present in disconnected channels of large sandy‐gravelly rivers with their sedimentary products and vice versa. The results show that the spatial distribution of bars before a flood partly governs the scour and fill processes and that the sediment transport rates vary significantly on a single cross‐section. This can be due to preferential axes of the migration of the bars determined by their location on the cross‐section, the bank direction and the discharge. The approach allows the reconstruction of local sedimentary processes involved in alternate bar migration by combining maximum scour depths reached during a flood with frequent channel bed topography surveys and post‐flood stratigraphy. It is also possible to distinguish deposited and preserved sediments compared with sediments by‐passed during the flood. Copyright © 2011 John Wiley & Sons, Ltd.     

Channelization of a large Alpine river: what is left of its original morphodynamics?

The Adige River drains 12 200 km² of the Eastern Alps and flows for 213 km within this mountain range. Similar to other large rivers in Central Europe, the Adige River was subject to massive channelization works during the 19th century. Thanks to the availability of several historical maps, this river represents a very valuable case study to document the extent to which the morphology of the river changed due to channelization and to understand how much is left of its original morphodynamics. The study was based on the analysis of seven sets of historical maps dating from 1803–1805 to 1915–1927, on geomorphological analysis, on the application of mathematical morphodynamic theories and on the application of bar and channel pattern prediction models. The study concerns 115 km of the main stem and 29 km of its tributaries. In the pre‐channelization conditions, the Adige River presented a prevalence of single‐thread channel planforms. Multi‐thread patterns developed only immediately downstream of the main confluences. During the 19th century, the Adige underwent considerable channel adjustment, consisting of channel narrowing, straightening, and reduction of bars and islands. Multi‐thread and single‐thread reaches evolved through different evolutionary trajectories, considering both the channel width and the bar/vegetation interaction. Bar and channel pattern predictors showed good correspondence with the observed patterns, including the development of multi‐thread morphologies downstream of the confluences. Application of the free‐bar predictor helped to interpret the strong reduction – almost complete loss – of exposed sediment bars after the channelization works, quantifying the riverbed inclination to form alternate bars. This morphological evolution can be observed in other Alpine rivers of similar size and similar massive channelization, therefore, a simplified conceptual model for large rivers subjected to channelization is proposed, showing that a relatively small difference in the engineered channel width may have a strong impact on the river dynamics, specifically on bar formation. Copyright © 2017 John Wiley & Sons, Ltd.     

Field measurements of intertidal bar evolution on a high‐energy beach system

Nearshore bars play a pivotal role in coastal behaviour, helping to protect and restore beach systems particularly in post‐storm conditions. Examination of bar behaviour under various forcing conditions is important to help understand the short‐ to medium‐term evolution of sandy beach systems. This study carried out over a nine‐week period examines, the behaviour of three intertidal bars along a high energy sandy beach system in northwest Ireland using high‐frequency topographic surveys and detailed nearshore hydrodynamic modelling. Results show that, in general, there was onshore migration for all the bars during the study period, despite the variability observed between bars, which was driven mostly by wave dominated processes. Under the prevailing conditions migration rates of up to 1.83 m day^?1 and as low as 0.07 m day^?1 were observed. During higher wave energy events the migration rates of the bars decelerated in their onshore route, however, under lower wave energy conditions, they quickly accelerated maintaining their shoreward migration direction. Tidal influence appears to be subordinate in these conditions, being restricted to moderating the localized wave energy at low tides and in maintaining runnel configurations providing accommodation space for advancing slip faces. The study highlights the intricate behavioural patterns of intertidal bar behaviour along a high energy sandy coastline and provides new insights into the relative importance of wave and tidal forcing on bar behaviour over a relatively short time period. Copyright © 2016 John Wiley & Sons, Ltd.     

Short‐term intertidal bar mobility on a ridge‐and‐runnel beach,Merlimont, northern France

Digital elevation models and topographic pro?les of a beach with intertidal bar and trough (ridge‐and‐runnel) morphology in Merlimont, northern France, were analysed in order to assess patterns of cross‐shore and longshore intertidal bar mobility. The beach exhibited a pronounced dual bar–trough system that showed cross‐shore stationarity. The bars and troughs were, however, characterized by signi?cant longshore advection of sand under the in?uence of suspension by waves and transport by strong tide‐ and wind‐driven longshore currents. Pro?le changes were due in part to the longshore migration of medium‐sized bedforms. The potential for cross‐shore bar migration appears to be mitigated by the large size of the two bars relative to incident wave energy, which is modulated by high vertical tidal excursion rates on this beach due to the large tidal range (mean spring tidal range = 8·3 m). Cross‐shore bar migration is also probably hindered by the well‐entrenched troughs which are maintained by channelled high‐energy intertidal ?ows generated by swash bores and by tidal discharge and drainage. The longshore migration of intertidal bars affecting Merlimont beach is embedded in a regional coastal sand transport pathway involving tidal and wind‐forced northward residual ?ows affecting the rectilinear northern French coast in the eastern English Channel. Copyright © 2004 John Wiley & Sons, Ltd.     

Lateral sediment transport by bedforms in a meander bend

Bedforms and their migration are among the most important features determining bed load transport on point bars in sand-bedded rivers. A case study has been carried out in the Danish river Gels Å in a bend with equilibrium point bar topography. Bedforms (dunes) were repeatedly surveyed to determine the bedform migration on the point bar surface. Bedforms occur in two categories in the bend: primary dunes and secondary dunes migrating transversely in the troughs between primary dunes. The study shows that the migration of these bedforms can account for the entire lateral transport known to concentrate the transport in the convex half of the cross section at the bend exit. The bedform transport is induced by both dune categories, partly by turning of the primary dunes towards the convex bank and partly by the migration of secondary dunes. In certain areas, the two categories operate together and thus make the lateral transport particularly large; it is shown that almost 60 per cent of the entire lateral transport in the bend takes place within less than 30 per cent of the streamwise extent of the point bar.     

EXPERIMENTAL STUDY ON THE RELATION OF BED MORPHOLOGY WITH SURFACE FLOW IN MEANDER CHANNELS

1 INTRODUCTION The plane shape of a river channel is very important for river improvement planning, because it must allow floodwater to flow off safely. Natural rivers wind from side to side, which creates meandering forms. From the history of river impro…     

An experimental test of whether bar instability contributes to the formation,periodicity and maintenance of pool–riffle sequences

Pool–riffle sequences (PRSs) are periodic river‐bed morphologies with wavelengths several times the channel width. Causes of PRS formation and maintenance are not clearly understood, which may limit the effectiveness of protection and rehabilitation measures. Some confusion has existed about whether the PRS morphology is the same as or distinct from alternate bars. In this paper we investigate whether the bar instability forming alternate bars also contributes to PRS formation, periodicity and maintenance. This was unclear because bar instability occurs only when the ratio of channel width/depth exceeds a critical value, generally understood to be approximately 10, which is larger than the width‐to‐depth ratio of many PRSs. A mobile‐bed physical model is used to test whether bar instability occurs in channels characteristic of PRS morphology, with low width‐to‐depth ratio, and high relative roughness. The physical model was scaled from a prototype PRS in a gravel and cobble bed river. Alternate bars formed in the model at channel width‐to‐depth ratios as low as 3·8. The wavelength of the alternate bars formed was generally 2·2–5 times channel width, which was similar to the prototype PRS. Therefore, bar instability can occur in virtually all PRSs, and it contributes to the widespread formation of periodic PRS morphology. The model showed that maintenance of the bar height in the prototype PRS also depends on variations in channel width. It is concluded that periodic PRSs are formed and maintained by the interaction between bar instability, and flow deflections associated with variations in channel geometry such as width variation. Resonance between bar instability and three‐dimensional bed forms such as alternate bars and variations in channel geometry. Variations in channel geometry are also important in determining the location and dimensions of individual pools and bars. Copyright © 2008 John Wiley & Sons, Ltd.     

The anastomosing pattern and the extensively distributed scroll bars in the middle Amazon River

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.     

Modeling and cyclic behavior of segmental bridge column connected with shape memory alloy bars

This paper examines the quasi-static cyclic behavior,lateral strength and equivalent damping capacities of a system of post-tensioned segmental bridge columns tied with large diameter martensitic Shape Memory Alloy(SMA) link-bars.Moment-curvature constitutive relationships are formulated and analysis tools are developed for the PT column,including a modified four-spring model prepared for the SMA bars.The suggested system is exemplified using a column with an aspect ratio of 7.5 and twelve 36.5 mm diameter NiTi martensitic SMA bars.A post-tensioning force of 40% to 60% of the tendon yield strength is applied in order to obtain a self re-centering system,considering the residual stress of the martensitic SMA bars.The cyclic response results show that the lateral strength remains consistently around 10% of the total vertical load and the equivalent viscous damping ratios reach 10%-12% of critical.When large diameter NiTi superelastic SMA bars are incorporated into the column system,the cyclic response varies substantially.The creep behavior of the superelastic SMA bar is accounted for since it affects the re-centering capability of the column.Two examples are presented to emphasize the modeling sensitivities for these special bars and quantify their cyclic behavior effects within the column assembly.     

Scroll bars are inner bank levees along meandering river bends

Scroll bars across a 65-km stretch of the Trinity River in Texas, USA were studied using LiDAR data as well as with a series of 11 trenches spread out across the survey area. We conclude that scroll bars are levees that are deposited along the inner banks of these meandering river bends. Scroll bar crests were found to have similar elevations to those of outer bank levee crests, implying that they are constructional features that create positive topographic relief above the elevation of the floodplain. Trenches reveal that scroll bars are built from reworked suspended sediment, with common ripple-scale cross stratification, planar laminations and muddy bioturbated layers – characteristics often associated with levee sedimentation in other systems. LiDAR observation of the erosion of scroll bars by bed material transport during flood implies that scroll bar spacing is an imperfect proxy for estimating overall channel migration rates. In addition, interspersed lenses of coarser sediment with dune-scale cross stratification represent the stratigraphic record of these erosional events and suggest that erosion of the channel-ward edge of the scroll bar is not uncommon. Preservation of scroll bars is unlikely, given that they are responsible for an average of only the uppermost 12% of the total inner bank relief. We suggest that misidentification of point bar lateral accretion surfaces as scroll bars is common and can lead to issues with reconstructing channel properties due to systematic differences between point bar and scroll bar planform geometries. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.