Active width of gravel‐bed braided rivers

Previous analyses have identified the active width of braided rivers, the bed area over which bed load flux and short‐term morphological change occurs, as an important element of braiding dynamics and predictions of bed load flux. Here we compare theoretical predictions of active width in gravel‐bed braided rivers with observations from Sunwapta River, and from a generic physical model of gravel braided rivers, to provide general observations of the variation in active width, and to develop an understanding of the causes of variation. Bed topography was surveyed daily along a 150 m reach of the pro‐glacial Sunwapta River for a total of four weeks during summer when flow was above threshold for morphological activity. In the laboratory, detailed digital elevation models (DEMs) were derived from photogrammetric survey at regular intervals during a constant discharge run. From the field and flume observations there is considerable local and circumstantial variation in active width, but also a general trend in average active width with increasing discharge. There is also a clear relationship of active width with active braiding index (number of active branches in the braided channel network), and with dimensionless stream power, which appears to be consistent across the range of data from field and physical models. Thus there is a link between active width and the river morphology and dynamics, and the possibility of a general relationship for estimating active width from channel pattern properties or reach‐scale stream power values, from which approximate bedload flux calculations may be made. The analysis also raises questions about differences between hydraulically‐based numerical model computations of instantaneous active width and observation of time‐integrated morphological active width. Understanding these differences can give insight into the nature of bedload transport in braided rivers and the relationship to morphological processes of braiding. Copyright © 2011 John Wiley & Sons, Ltd.     

Particle path length distributions in meandering gravel‐bed streams: results from physical models

In gravel‐bed rivers with well‐de?ned pool–bar morphology, the path length of transported bed particles must be, at least during ‘channel‐forming’ ?ows, equal to the length scale of the morphology. This is the basis for some methods for estimating bed material transport rates. However, previous data, especially from ?eld tests, are often strongly positively skewed with mean much shorter than the pool–bar spacing. One possible explanation is that positively skewed distributions occur only in channels lacking distinct pool–bar topography or only at lower discharges in pool–bar channels. A series of ?ume experiments using ?uorescent tracers was used to measure path length distributions in low‐sinuosity meandering channels to assess the relation with channel morphology and ?ow conditions. At channel‐forming ?ows, 55 to 75 per cent of the tracer grains were deposited on the ?rst point bar downstream of the point of tracer input, with 15 per cent passing beyond the ?rst bar. Path length distributions are symmetrical with mean equal to the pool–bar spacing and can be described with a Cauchy distribution. In some cases there was a secondary mode close to the point of tracer introduction; this bimodal distribution ?ts a combined gamma–Cauchy distribution. Only when discharge was reduced below the channel‐forming ?ow were frequency distributions unimodal and positively skewed with no relation to the pool–bar spacing. Thus, path length distributions become more symmetrical, and mean path length increases to coincide with pool–bar spacing, as ?ow approaches channel‐forming conditions. This is a substantial modi?cation of existing models of particle transfer in gravel‐bed rivers. Copyright © 2003 John Wiley & Sons, Ltd.     

A two‐dimensional morphodynamic model of gravel‐bed river with floodplain vegetation

Artificially straight river channels tend to be unstable, and ultimately develop into river meanders through bank erosion and point‐bar deposition. In this paper account is taken of the effects of riparian and floodplain vegetation on bank strength, floodplain flow resistance, shear stress partitioning, and bedload transport. This is incorporated into an existing 2D hydrodynamic‐morphological model. By applying the new model to an initially straight and single‐threaded channel, the way that its planform and cross‐sectional geometry evolve for different hydraulic and floodplain vegetation conditions is demonstrated. The results show the formation and upstream migration of gravel bars, confluence scouring and the development of meandering and braiding channel patterns. In cases where the channel becomes unstable, the instability grows out of bar formation. The resulting braiding patterns are similar to analytical results. The formation of a transition configuration requires a strong influence from vegetation. Copyright © 2010 John Wiley & Sons, Ltd.     

Particle clusters in gravel‐bed rivers: an experimental morphological approach to bed material transport and stability concepts

Structured gravel river beds clearly exert a major influence on bed stability. Indexing structural stability by field measurements of bed strength neglects the processes operating to entrain and transport bed material in different parts of each structure. This study takes a morphological approach to interpreting the critical processes, using particle tracing to determine the movement of individual cluster particles over a range of flood event magnitudes and durations. The experiment was carried out on the River South Tyne, UK; it uses flow hydrographs measured nearby and also benefits from previous studies of historical development, channel morphology and sediment transport at the same site. More than 30 clusters were monitored over a seven‐month period during which clusters occupied 7–16 per cent of the bed. Threshold flows delimiting three apparently contrasting bed sediment process regimes for cluster particles are tentatively set at 100 m³ s^?1 and 183 m³ s^?1; durations of flow at these levels are critical for cluster development, rather than flow peak values. Wake particles are transported most easily. Flow straightening in the wandering channel planform reduces the stability of clusters, since mechanical strength is markedly reduced by this change of direction. The overall area covered by clusters between significant transport events varies little, implying a dynamic equilibrium condition. Copyright © 2005 John Wiley & Sons, Ltd.     

Life of a bifurcation in a gravel‐bed braided river

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.     

A coupled sediment routing and lateral migration model for gravel‐bed rivers

We describe additions made to a multi‐size sediment routing model enabling it to simulate width adjustment simultaneously alongside bed aggradation/incision and fining/coarsening. The model is intended for use in single thread gravel‐bed rivers over annual to decadal timescales and for reach lengths of 1–10 km. It uses a split‐channel approach with separate calculations of flow and sediment transport in the left and right sides of the channel. Bank erosion is treated as a function of excess shear stress with bank accretion occurring when shear stress falls below a second, low, threshold. A curvature function redistributes shear stress to either side of the channel. We illustrate the model through applications to a 5·6‐km reach of the upper River Wharfe in northern England. The sediment routing component with default parameter values gives excellent agreement with field data on downstream fining and down‐reach reduction in bedload flux, and the width‐adjustment components with approximate calibration to match maximum observed rates of bank shifting give plausible patterns of local change. The approach may be useful for exploring interactions between sediment delivery, river management and channel change in upland settings. Copyright © 2011 John Wiley & Sons, Ltd.     

Relation between flow,surface‐layer armoring and sediment transport in gravel‐bed rivers

This study investigates trends in bed surface and substrate grain sizes in relation to reach‐scale hydraulics using data from more than 100 gravel‐bed stream reaches in Colorado and Utah. Collocated measurements of surface and substrate sediment, bankfull channel geometry and channel slope are used to examine relations between reach‐average shear stress and bed sediment grain size. Slopes at the study sites range from 0·0003 to 0·07; bankfull depths range from 0·2 to 5 m and bankfull widths range from 2 to 200 m. The data show that there is much less variation in the median grain size of the substrate, D_50s, than there is in the median grain size of the surface, D₅₀; the ratio of D₅₀ to D_50s thus decreases from about four in headwater reaches with high shear stress to less than two in downstream reaches with low shear stress. Similar trends are observed in an independent data set obtained from measurements in gravel‐bed streams in Idaho. A conceptual quantitative model is developed on the basis of these observations to track differences in bed load transport through an idealized stream system. The results of the transport model suggest that downstream trends in total bed load flux may vary appreciably, depending on the assumed relation between surface and substrate grain sizes. Copyright © 2007 John Wiley & Sons, Ltd.     

Insights into bedload transport processes of a large regulated gravel‐bed river

A comprehensive monitoring programme focusing on bedload transport behaviour was conducted at a large gravel‐bed river. Innovative monitoring strategies were developed during five years of preconstruction observations accompanying a restoration project. A bedload basket sampler was used to perform 55 cross‐sectional measurements, which cover the entire water discharge spectrum from a 200‐year flood event in 2013 to a rare low flow event. The monitoring activities provide essential knowledge regarding bedload transport processes in large rivers. We have identified the initiation of motion under low flow conditions and a decrease in the rate of bedload discharge with increasing water discharge around bankfull conditions. Bedload flux strongly increases again during high flood events when the entire inundation area is flooded. No bedload hysteresis was observed. The effective discharge for bedload transport was determined to be near mean flow conditions, which is therefore at a lower flow discharge than expected. A numerical sediment transport model was able to reproduce the measured sediment transport patterns. The unique dataset enables the characterisation of bedload transport patterns in a large and regulated gravel‐bed river, evaluation of modern river engineering measures on the Danube, and, as a pilot project has recently been under construction, is able to address ongoing river bed incision, unsatisfactory ecological conditions for the adjacent national park and insufficient water depths for inland navigation. Copyright © 2017 John Wiley & Sons, Ltd.     

The active layer in gravel-bed rivers: An empirical appraisal

The vertical position of the streambed–water boundary fluctuates during the course of sediment transport episodes, due to particle entrainment/deposition and bedform migration, amongst other hydraulic and bedload mechanisms. These vertical oscillations define a topmost stratum of the streambed (i.e. the ‘active layer or active depth’), which usually represents the main source of particles entrained during long and high-magnitude bedload transport episodes. The vertical extent of this layer is hence a capital parameter for the quantification of bedload volumes and a major driver of stream ecology in gravel-bed rivers. However, knowledge on how the active depth scales to flow strength and the nature of the different controls on the relation between the flow strength and the active depth is still scarce. In this paper we present a meta-analysis over active depth data coming from ~130 transport episodes extracted from a series of published field studies. We also incorporate our own field data for the rivers Ebro and Muga (unpublished), both in the Iberian Peninsula. We explore the database searching for the influence of flow strength, grain size, streambed mobility and channel morphology on the vertical extent of the active layer. A multivariate statistical analysis (stepwise multiple regression) confirms that the set of selected variables explains a significant amount of variance in the compiled variables. The analysis shows a positive scaling between active depth and flow strength. We have also identified some links between the active depth and particle travel distances. However, these relations are also largely modulated by other fluvial drivers, such as the grain size of the bed surface and the dominant channel macro-bedforms, with remarkable differences between plane-bed, step-pool and riffle-pool channels. © 2020 John Wiley & Sons, Ltd.     

Gravel‐bed rivers: from particles to patterns

The papers in this special issue reflect several of the major themes and topics from the 7^th International Workshop on Gravel‐Bed Rivers. The papers focus primarily on aspects of bed material transport in gravel‐bed rivers and larger scale morpho‐dynamics. Research in gravel‐bed rivers is increasingly integrating processes over a wide range of temporal and spatial scales by combining field observation, lab experimentation, numerical modeling and theory testing in a range of river types, aided by new technological developments in particle tracking, computational modeling and high resolution spatial data. This is leading to greater understanding of the processes leading to distinctive morpho‐dynamics of river types and a more reliable basis for river management. Copyright © 2012 John Wiley & Sons, Ltd.     

Regime theories in gravel‐bed rivers: models,controlling variables,and applications in disturbed Italian rivers

Downstream hydraulic geometry relationships describe the shape of alluvial channels in terms of bankfull width, flow depth, flow velocity, and channel slope. Recent investigations have stressed the difference in spatial scales associated with these variables and thus the time span required for their adjustment after a disturbance. The aim of this study is to explore the consequences in regime models considering the hypothesis that while channel width and depth adjust quickly to changes in water and sediment supply, reach slope requires a longer time span. Three theoretical models were applied. One model incorporates an extremal hypothesis (Millar RG. 2005. Theoretical regime equations for mobile gravel‐bed rivers with stable banks. Geomorphology 64 : 207–220), and the other two are fully physically based (Ikeda S, Parker G, Kimura Y. 1988. Stable width and depth of straight gravel rivers with heterogeneous bed materials. Water Resources Research 24 : 713–722; Parker G, Wilcock PR, Paola C, Dietrich W, Pitlick J. 2007. Physical basis for quasi universal relations describing bankfull hydraulic geometry of single‐thread gravel‐bed rivers. Journal of Geophysical Research 112 , DOI: 10.1029/2006JF000549). In order to evaluate the performance of models introducing the slope as an independent variable, we propose two modifications to previous models. The performance of regime models was tested against published data from 142 river reaches and new hydraulic geometry data from gravel‐bed rivers in Patagonia (Argentina) and north‐eastern Italy. Models that assume slope as a control (Ikeda et al., 1988; or Millar, 2005) predict channel depth and width reasonably well. Parker et al.'s (2007) model improved predictions because it filters the scatter in slope data with a relation slope–discharge. The extremal hypothesis model of Millar (2005) predicts comparably to the other physically based models. Millar's model was chosen to describe the recent changes in the Piave and Brenta rivers due to human intervention – mainly in‐channel gravel mining. The change in sediment supply and recovery was estimated for these rivers. This study supports the interpretation that sediment supply is the key factor guiding morphological changes in these rivers. Copyright © 2013 John Wiley & Sons, Ltd.     

Rates of planimetric change in a proglacial gravel-bed braided river: Field measurement and physical modelling

Planimetric change was measured on daily hydrographs over two meltwater seasons using time-lapse images of the proglacial, gravel, braided, Sunwapta River, Canada. Significant planimetric change occurred on 10–15 days per year. Area of planimetric change correlated with peak and total daily meltwater hydrograph discharge. A clear threshold discharge can be identified below which no planform activity occurs, an intermediate range over which change occurs conditionally, and a peak flow range at which significant change always occurs. Field conditions were reproduced in a physical model in a laboratory flume. Photogrammetric DEMs of bed morphology and measurements of bedload output were made for each hydrograph experimental run. The physical model results for planimetric change had a threshold discharge for change, and trend with discharge, similar to the field data. The model data also show that planimetric change correlates strongly with volumes of erosion/deposition measured from successive DEMs, and with bedload transport rate. The relation between planimetric change and topographic change is also apparent from previous cross-section surveys at the field site. The results highlight the planimetric dynamics of braiding rivers in relation to discharge forcing, and the relationship between planimetric change, morphological change, and bedload transport in braided rivers. This also points to the potential use of measurements of planimetric change from time-lapse imagery as a low-cost method for high-frequency monitoring for braiding dynamics and also a surrogate for bedload transport measurement. © 2018 John Wiley & Sons, Ltd.     

Benefits of design modifications to the Birkbeck bedload sampler illustrated by flash‐floods in an ephemeral gravel‐bed channel

The benefits of three simple modifications to the design of a Birkbeck bedload slot‐sampling system that has been continuously operating in Nahal Eshtemoa, Israel, since the early 1990s are demonstrated. The modifications include the deployment of a removable slot cover which delays the accumulation of sediment, so allowing sampling at late stages of a flood and, in conjunction with other samplers, extending the period of sampling during a flood wave; inclusion of a slot the size of which is adjustable so that that the probability of sampling the largest clast sizes in transit as bedload can be increased post‐installation, once knowledge is gained about the bedload grain‐size distribution; and a sampler side‐wall door that allows stratification and textural changes within the accumulated bedload to be identified, so promoting intelligent sampling of the deposit for grain‐size determination. Results from seven flash‐floods are presented and discussed, with recommendations for bedload monitoring, particularly in rivers where sediment flux is high and dynamic sediment records are inevitably short because of instrumental limitations. Copyright © 2006 John Wiley & Sons, Ltd.     

The role of channel morphology on the mobility and dispersion of bed sediment in a small gravel‐bed stream

This study uses a unique 10‐year tracer dataset from a small gravel‐bed stream to examine bed mobility and sediment dispersion over long timescales and at a range of spatial scales. Seasonal tracer data that captured multiple mobilizing events was examined, while the effects of morphology on bed mobility and sediment dispersion were captured at three spatial scales: within morphological units (unit scale), between morphological units (reach scale) and between reaches with different channel morphologies (channel scale). This was achieved by analyzing both reach‐average mobility and travel distance data, as well as the development of ‘mobility maps’ that capture the spatial variability in tracer mobility within the channel. The tracer data suggest that sediment transport in East Creek remains near critical the majority of the time, with only rare large events resulting in high mobility rates and grain travel distances large enough to move sediment past dominant bedforms. While a variable capturing both the magnitude and frequency of flow events within a season yielded a better predictor to sediment mobility and dispersion than peak discharge alone, the distribution of events of different magnitude within the season played a large role in determining tracer mobility rates and travel distances. The effects of morphology differed depending on the analysis scale, demonstrating the importance of scale, and therefore study design, when examining the effect of morphology on sediment transport. Copyright © 2016 John Wiley & Sons, Ltd.     

Comparison of methods for quantifying active layer dynamics and bedload discharge in armoured gravel‐bed rivers

Several methods were employed in the Ardennian rivers (Belgium) to determine the depth of the active layer mobilized during floods and to evaluate the bedload discharge associated with these events. The use of scour chains has shown that the depth of the active layer is systematically less than the b‐axis of the average particle size (D₅₀) of the elements which compose the surface layer of the riffles. This indicates that only a partial transport exists during low magnitude floods. The bedload discharge has been evaluated by combining data obtained using the scour chains technique and the distance covered by tracers. Quantities of sediment transported during frequent floods are relatively low (0·02 t km^–2) due to the armour layer which protects the subsurface material. These low values are also related to the fact that the distance calculated for mobilized bedload only applies to tracers fitted with PIT (passive integrated transponder)‐tags (diameter > 20 mm), whereas part of the bedload discharge is composed of sand and fine gravel transported over greater distances than the pebbles. The break‐up of the armour layer was observed only once, for a decennial discharge. During this event, the bedload discharge increased considerably (2 t km^–2). The use of sediment traps, data from dredging and a Helley–Smith sampler confirm the low bedload transport in Ardennian rivers in comparison to the bedload transport in other geomorphological contexts. This difference is explained by the presence of an armoured layer but also by the imbricated structures of flat bed elements which increase the resistance to the flow. Finally, the use of the old iron industry wastes allowed to quantify the thickness of the bed reworked over the past centuries. In the Lembrée River, the river‐bed contains slag elements up to a depth of about 50 cm, indicating that exceptional floods may rework the bed to a considerable depth. Copyright © 2012 John Wiley & Sons, Ltd.     

Effect of changes in fine‐grained matrix on bedload sediment transport in a gravel‐bed river

While clay and silt matrices of gravel‐bed rivers have received attention from ecologists concerned variously with the deteriorating environments of benthic and hyporheic organisms, their impact on sediment entrainment and transport has been explored less. A recent increase of such a matrix in the bed of Nahal Eshtemoa, an ephemeral river of the northern Negev, has more than doubled the boundary shear stress needed to initiate bedload, from 7 N m^‐2 (τ^* = 0.027) during the flash floods of 1991–2001 to 15 N m^‐2 (τ^* = 0.059) during those of 2008–2009. The relation between bedload flux and boundary shear stress continues to be well‐defined, but it is displaced. The matrix now contains a significant amount of silt and clay size material. The reasons for the increased entrainment threshold of bedload are explored. Large‐scale laser scanning of the dry bed reveals a reduction in grain‐scale morphological roughness, while artificial in situ tests of matrix integrity indicate considerable cohesion. The implications for adopting bed material sampling strategies that account for matrix development are assessed. Copyright © 2012 John Wiley & Sons, Ltd.     

Evolution of grain size distributions and bed mobility during hydrographs in gravel-bed braided rivers

Evolution of bed material mobility and bedload grain size distributions under a range of discharges is rarely observed in braiding gravel-bed rivers. Yet, the changing of bedload grain size distributions with discharge is expected to be different from laterally-stable, threshold, channels on which most gravel bedload theory and observation are based. Here, simultaneous observations of flow, bedload transport rate, and morphological change were made in a physical model of a gravel-bed braided river to document the evolution of grain size distributions and bed mobility over three experimental event hydrographs. Bedload transport rate and grain size distributions were measured from bedload samples collected in sediment baskets. Morphological change was mapped with high-resolution (~1 mm precision) digital elevation models generated from close-range digital photogrammetry. Bedload transport rates were extremely low below a discharge equivalent to ~50% of the channel-forming discharge (dimensionless stream power ~70). Fractional transport rates and plots of grain size distributions indicate that the bed experienced partial mobility at low discharge when the coarsest grains on the bed were immobile, weak selective mobility at higher discharge, and occasionally near-equal mobility at peak channel-forming discharge. The transition to selective mobility and increased bedload transport rates coincided with the lower threshold for morphological change measured by the morphological active depth and active width. Below this threshold discharge, active depths were of the order of D₉₀ and active widths were narrow (< 3% of wetted width). Above this discharge, both increased so that at channel-forming discharge, the active depth had a local maximum of 9D₉₀ while active width was up to 20% of wetted width. The modelled rivers approached equal mobility when rates of morphological change were greatest. Therefore, changes in the morphological active layer with discharge are directly connected to the conditions of bed mobility, and strongly correlated with bedload transport rate. © 2018 John Wiley & Sons, Ltd.     

Physics‐based numerical modelling of large braided rivers dominated by suspended sediment

Physics‐based models have been increasingly developed in recent years and applied to simulate the braiding process and evolution of channel units in braided rivers. However, limited attention is given to lowland braided rivers where the transport of suspended sediment plays a dominant role. In the present study, a numerical model based on the basic physics laws of hydrodynamics and sediment transport is used to simulate the evolution process of a braided river dominated by suspended load transport. The model employs a fractional method to simulate the transport of graded sediments and uses a multiple‐bed‐layer approach to represent the sediment sorting process. An idealized braided river has been produced, with the hydrodynamic, sediment transport and morphological processes being analysed. In particular, the formation process of local pool–bar units in the predicted river has been investigated. A sensitivity analysis has also been undertaken to investigate the effects of grid resolution and an upstream perturbation on the model prediction. A variety of methods are applied to analyse the geometrical and topographical properties of the modelled river. Self‐organizing characteristics related to river geometry and topography are analysed by state‐space plots, which indicate a close relationship with the periodical erosion and deposition cycles of braiding. Cross‐sectional topography and slope frequency display similar geometries to natural rivers. Scaling characteristics are found by correlation analysis of bar parameters. Copyright © 2014 John Wiley & Sons, Ltd.     

Bar dynamics and bifurcation evolution in a modelled braided sand‐bed river

Morphodynamics in sand‐bed braided rivers are associated with simultaneous evolution of mid‐channel bars and channels on the braidplain. Bifurcations around mid‐channel bars are key elements that divide discharge and sediment. This, in turn, may control the evolution of connected branches, with effects propagating to both upstream and downstream bifurcations. Recent works on bifurcation stability and development hypothesize major roles of secondary flow and gradient advantage. However, this has not been tested for channel networks within a fully developed dynamic braided river. A reason for this is a lack of detailed measurements with sufficient temporal and spatial length, covering multiple bifurcations. Therefore we used a physics‐based numerical model to generate a dataset of bathymetry, flow and sediment transport of an 80 km river reach with self‐formed braid bars and bifurcations. The study shows that bar dissection due to local transverse water surface gradients is the dominant bifurcation initiation mechanism, although conversion of unit bars into compound bars dominates in the initial stage of a braided river. Several bifurcation closure mechanisms are equally important. Furthermore, the study showed that nodal point relations for bifurcations are unable to predict short‐term bifurcation evolution in a braided river. This is explained by occurrence of nonlinear processes and non‐uniformity within the branches, in particular migrating bars and larger‐scale backwater‐effects, which are not included in the nodal point relations. Planform morphology, on the other hand, has predictive capacity: bifurcation angle asymmetry and bar‐tail limb shape are indicators for near‐future bifurcation evolution. Remote sensing data has predictive value, for which we developed a conceptual model for interactions between bars, bifurcations and channels in the network. We conducted a preliminary test of the conceptual model on satellite images of the Brahmaputra. Copyright © 2015 John Wiley & Sons, Ltd.