Assessment of intermediate fine sediment storage in a braided river reach (southern French Prealps)

This paper presents a field investigation on river channel storage of fine sediments in an unglaciated braided river, the Bès River, located in a mountainous region in the southern French Prealps. Braided rivers transport a very large quantity of bedload and suspended sediment load because they are generally located in the vicinity of highly erosive hillslopes. Consequently, these rivers play an important role because they supply and control the sediment load of the entire downstream fluvial network. Field measurements and aerial photograph analyses were considered together to evaluate the variability of fine sediment quantity stored in a 2·5‐km‐long river reach. This study found very large quantities of fine sediment stored in this reach: 1100 t per unit depth (1 dm). Given that this reach accounts for 17% of the braided channel surface area of the river basin, the quantities of fine sediment stored in the river network were found to be approximately 80% of the mean annual suspended sediment yields (SSYs) (66 200 t year^?1), comparable to the SSYs at the flood event scale: from 1000 t to 12 000 t depending on the flood event magnitude. These results could explain the clockwise hysteretic relationships between suspended sediment concentrations and discharges for 80% of floods. This pattern is associated with the rapid availability of the fine sediments stored in the river channel. This study shows the need to focus on not only the mechanisms of fine sediment production from hillslope erosion but also the spatiotemporal dynamics of fine sediment transfer in braided rivers. Copyright © 2010 John Wiley & Sons, Ltd.     

The variability in the morphological active width: Results from physical models of gravel‐bed braided rivers

The morphological active width, defined as the lateral extent of bed material displacement over time, is a fundamental parameter in multi‐threaded gravel‐bed rivers, linking complex channel dynamics to bedload transport. Here, results are presented from five constant discharge experiments, and three event hydrographs, covering a range of flow strengths and channel configurations for which morphological change, bedload transport rates, and stream power were measured in a physical model. Changes in channel morphology were determined via differencing of photogrammetrically‐derived digital elevation models (DEMs) of the model surface generated at regular intervals over the course of ~115 h of experimental runs. Independent measures of total bedload output were made using downstream sediment baskets. Results indicate that the morphological active width increases with total and dimensionless stream power and is strongly and positively correlated with bulk change (total volume of bed material displaced over time) and active braiding intensity (ABI). Although there is considerable scatter due to the inherent variability in braided river morphodynamics, the active width is positively correlated with independent measurements of bedload transport rate. Active width, bulk change, and bedload transport rates were all negligible below a dimensionless stream power threshold value of ~ 0.09, above which all increase with flow strength. Therefore, the active width could be used as a general predictor of bulk change and bedload transport rates, which in turn could be approximated from total and dimensionless stream power or ABI in gravel‐bed braided rivers. Furthermore, results highlight the importance of the active width, rather than the morphological active depth, in predicting volumes of change and bedload transport rates. The results contribute to the larger goals of better understanding braided river morphodynamics, creating large high‐resolution datasets of channel change for model calibration and validation, and developing morphological methods for predicting bedload transport rates in braiding river systems. Copyright © 2018 John Wiley & Sons, Ltd.     

Coarse sediment transport in mountain streams in Colorado and Wyoming,USA

Since the early 1990s, US Forest Service researchers have made thousands of bedload measurements in steep, coarse‐grained channels in Colorado and Wyoming, USA. In this paper we use data from 19 of those sites to characterize patterns and rates of coarse sediment transport for a range of channel types and sizes, including step–pool, plane‐bed, pool–riffle, and near‐braided channels. This effort builds upon previous work where we applied a piecewise regression model to (1) relate flow to rates of bedload transport and (2) define phases of transport in coarse‐grained channels. Earlier, the model was tested using bedload data from eight sites on the Fraser Experimental Forest near Fraser, Colorado. The analysis showed good application to those data and to data from four supplementary channels to which the procedure was applied. The earlier results were, however, derived from data collected at sites that, for the most part, have quite similar geology and runoff regimes. In this paper we evaluate further the application of piecewise regression to data from channels with a wider range of geomorphic conditions. The results corroborate with those from the earlier work in that there is a relatively narrow range of discharges at which a substantial change in the nature of bedload transport occurs. The transition from primarily low rates of sand transport (phase I) to higher rates of sand and coarse gravel transport (phase II) occurs, on average, at about 80 per cent of the bankfull (1·5‐year return interval) discharge. A comparison of grain sizes moved during the two phases showed that coarse gravel is rarely trapped in the samplers during phase I transport. Moreover, the movement and capture of the D₁₆ to D₂₅ grain size of the bed surface seems to correspond with the onset of phase II transport, particularly in systems with largely static channel surfaces. However, while there were many similarities in observed patterns of bedload transport at the 19 studied sites, each had its own ‘bedload signal’ in that the rate and size of materials transported largely reflected the nature of flow and sediment particular to that system. Published in 2005 by John Wiley & Sons, Ltd.     

A coupled hierarchical modeling approach to simulating the geomorphic response of river systems to anthropogenic climate change

Anthropogenic climate change is expected to change the discharge and sediment transport regime of river systems. Because rivers adjust their channels to accommodate their typical inputs of water and sediment, changes in these variables can potentially alter river morphology. In this study, a hierarchical modeling approach was developed and applied to examine potential changes in reach‐averaged bedload transport and spatial patterns of erosion and deposition for three snowmelt‐dominated gravel‐bed rivers in the interior Pacific Northwest. The modeling hierarchy was based on discharge and suspended‐sediment load from a basin‐scale hydrologic model driven by a range of downscaled climate‐change scenarios. In the field, channel morphology and sediment grain‐size data for all three rivers were collected. Changes in reach‐averaged bedload transport were estimated using the Bedload Assessment of Gravel‐bedded Streams (BAGS) software, and the Cellular Automaton Evolutionary Slope and River (CAESAR) model was used to simulate the spatial pattern of erosion and deposition within each reach to infer potential changes in channel geometry and planform. The duration of critical discharge was found to control bedload transport. Changes in channel geometry were simulated for the two higher‐energy river reaches, but no significant morphological changes were found for a lower‐energy reach with steep, cohesive banks. Changes in sediment transport and river morphology resulting from climate change could affect the management of river systems for human and ecological uses. Copyright © 2015 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.     

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.     

Downstream fining in contrasting reaches of the sand‐bedded Yellow River

Compared to downstream fining of a gravel‐bedded river, little field evidence exists to support the process of downstream fining in large, fine sand‐bedded rivers. In fact, the typically unimodal bed sediments of these rivers are thought to produce equal mobility of coarse and fine grains that may discourage downstream fining. To investigate this topic, we drilled 200 sediment cores in the channel beds of two fine‐grained sand‐bedded reaches of the Yellow River (a desert reach and a lower reach) and identified a fine surface layer (FSL) developed over a coarse subsurface layer (CSL) in the 3‐m‐thick bed deposits. In both reaches downstream, the thickness of the FSL increased, while that of the CSL decreased. Comparison of the depth‐averaged median grain sizes of the CSL and the FSL separately in both reaches shows a distinct downstream fining dependence to the median grain size, which indicates that at a large scale of 600‐800 km, the CSL shows a significant downstream fining, but the FSL shows no significant trends in downstream variations in grain size. This result shows that fine sediment supply (<0·08 mm median grain size) from upstream, combined with lateral fine sediment inputs from tributaries and bank erosion, can cause a rapid fining of the downstream channel bed surface and can develop the FSL layer. However, in the desert reach, lateral coarse sediment supply (>0·08 mm median grain size) from wind‐borne sediments and cross‐desert tributaries can interrupt the FSL and coarsen the channel bed surface locally. Copyright © 2011 John Wiley & Sons, Ltd.     

Effects of helical flow in one‐dimensional modelling of sediment distribution at river bifurcations

Complex flow processes at river bifurcations and the influence of the layout of a bifurcation make it difficult to predict sediment distribution over the downstream branches in case bedload transport dominates. In one‐dimensional models we need a nodal point relationship that prescribes the distribution of sediment over the downstream branches. We have identified which factors need to be included in such a relationship for the division of bedload transport at bifurcations. Next, irrotational flow theory for idealized geometries has been used to derive a simple physics‐based nodal point relationship that accounts for the effects of helical flow in the situation that a channel takes off under an angle from a straight main channel. This first step towards a complete nodal point relationship is applicable to bedload transport situations if the flow is clearly curved and if there is no pronounced bed topography. The relationship has been tested against data from a unique set of laboratory measurements, numerical data and data from a scale model of the Rhine bifurcation at Pannerden in the Netherlands. We find that the derived model yields a reasonable prediction of the sediment division over the downstream branches, and yields better predictions than the Wang et al. model for the situation considered. Considering the relative complexity and limited accuracy of the nodal point relationship for the effect of helical flow alone, however, we conclude thatderiving a practical physics‐based 1‐D relationship including all relevant processes is not feasible. We therefore recommend 2‐D or 3‐D modelling for all cases in general where morphological evolution depends on the division of bedload transport at bifurcations. Copyright © 2012 John Wiley & Sons, Ltd.     

Hydraulic and geomorphic processes in an overbank flood along a meandering,gravel‐bed river: implications for chute formation

Hydraulic interactions between rivers and floodplains produce off‐channel chutes, the presence of which influences the routing of water and sediment and thus the planform evolution of meandering rivers. Detailed studies of the hydrologic exchanges between channels and floodplains are usually conducted in laboratory facilities, and studies documenting chute development are generally limited to qualitative observations. In this study, we use a reconstructed, gravel‐bedded, meandering river as a field laboratory for studying these mechanisms at a realistic scale. Using an integrated field and modeling approach, we quantified the flow exchanges between the river channel and its floodplain during an overbank flood, and identified locations where flow had the capacity to erode floodplain chutes. Hydraulic measurements and modeling indicated high rates of flow exchange between the channel and floodplain, with flow rapidly decelerating as water was decanted from the channel onto the floodplain due to the frictional drag provided by substrate and vegetation. Peak shear stresses were greatest downstream of the maxima in bend curvature, along the concave bank, where terrestrial LiDAR scans indicate initial floodplain chute formation. A second chute has developed across the convex bank of a meander bend, in a location where sediment accretion, point bar development and plant colonization have created divergent flow paths between the main channel and floodplain. In both cases, the off‐channel chutes are evolving slowly during infrequent floods due to the coarse nature of the floodplain, though rapid chute formation would be more likely in finer‐grained floodplains. The controls on chute formation at these locations include the flood magnitude, river curvature, floodplain gradient, erodibility of the floodplain sediment, and the flow resistance provided by riparian vegetation. Copyright © 2015 John Wiley & Sons, Ltd.     

Feedbacks between erosion and sediment transport in experimental bedrock channels

Natural bedrock rivers flow in self‐formed channels and form diverse erosional morphologies. The parameters that collectively define channel morphology (e.g. width, slope, bed roughness, bedrock exposure, sediment size distribution) all influence river incision rates and dynamically adjust in poorly understood ways to imposed fluid and sediment fluxes. To explore the mechanics of river incision, we conducted laboratory experiments in which the complexities of natural bedrock channels were reduced to a homogenous brittle substrate (sand and cement), a single sediment size primarily transported as bedload, a single erosion mechanism (abrasion) and sediment‐starved transport conditions. We find that patterns of erosion both create and are sensitive functions of the evolving bed topography because of feedbacks between the turbulent flow field, sediment transport and bottom roughness. Abrasion only occurs where sediment impacts the bed, and so positive feedback occurs between the sediment preferentially drawn to topographic lows by gravity and the further erosion of these lows. However, the spatial focusing of erosion results in tortuous flow paths and erosional forms (inner channels, scoops, potholes), which dissipate flow energy. This energy dissipation is a negative feedback that reduces sediment transport capacity, inhibiting further incision and ultimately leading to channel morphologies adjusted to just transport the imposed sediment load. Copyright © 2007 John Wiley & Sons, Ltd.     

Sediment transport in an alpine river before and after a dambreak flood event

Fluvial sediment transport in the high mountain Partnach River (Reintal Valley, Bavarian Alps) was investigated during a 10‐year observation period (2001–2010). During this period, the downstream reach of the Partnach River was decoupled from upstream sediment throughput by a rockslide deposit until 2005. In August 2005, the dam was partially breached during a flood event resulting in renewed sediment coupling between the upstream and downstream reaches. A comparison of pre‐ and post‐dambreak river sediment load data showed that the dissolved load dominated sediment transport prior to August 2005 with a switch to the dominance of bedload transport, post‐dambreak. The higher post‐dambreak bedload rates were particularly evident during the first years after the dam failure due to significant coarse material coupling between active sediment sources (undercut banks/talus cones) and the Partnach River. In the last years of the observation period (2009 and 2010) the dominance of dissolved load transport was re‐established. Copyright © 2011 John Wiley & Sons, Ltd.     

Spectral variations of underwater river sounds

Passive acoustic monitoring of the self‐generated noise of particle impacts has been shown to be correlated to bedload flux and bedload size. However, few studies have concentrated on the role of acoustic wave propagation in a river. For the first time, the river environment is modeled as a Pekeris waveguide, where a wave number integration technique is used to predict the transformation of sounds through their propagation paths. Focusing on the distance of a hydrophone from the channel bed and cutting off the low frequencies produced by impacts between gravel particles, we demonstrate that acoustic propagation modifies the spectral content of bedload‐generated sound. Acoustic signals analyzed with the proposed model are interpreted by comparison to Helley–Smith bedload data obtained during flood conditions on the large gravel‐bedded Arc‐en‐Maurienne River, France. This study shows that careful attention to acoustic propagation effects is required when estimating bedload grain size distribution with hydrophones in rivers, especially for rivers with slopes higher than 1%. Bedload monitoring with a hydrophone is particularly appropriate for large gravel‐bed rivers – especially so during large floods, when in situ sampling is difficult or impractical and the impact of acoustic propagation is weaker relative to the self‐generated noise of bedload impacts. Copyright © 2017 John Wiley & Sons, Ltd.     

Defining phases of bedload transport using piecewise regression

Differences in the transport rate and size of bedload exist for varying levels of flow in coarse‐grained channels. For gravel‐bed rivers, at least two phases of bedload transport, with notably differing qualities, have been described in the literature. Phase I consists primarily of sand and small gravel moving at relatively low rates over a stable channel surface. Transport rates during Phase II are considerably greater than Phase I and more coarse grains are moved, including material from both the channel surface and subsurface. Transition from Phase I to Phase II indicates initiation and transport of grains comprising the coarse surface layer common in steep mountain channels. While the existence of different phases of transport is generally acknowledged, the threshold between them is often poorly defined. We present the results of the application of a piecewise regression analysis to data on bedload transport collected at 12 gravel‐bed channels in Colorado and Wyoming, USA. The piecewise regression recognizes the existence of different linear relationships over different ranges of discharge. The inflection, where the fitted functions intersect, is interpreted as the point of transition from Phase I to Phase II transport; this is termed breakpoint. A comparison of grain sizes moved during the two phases shows that coarse gravel is rarely trapped in the samplers during Phase I transport, indicating negligible movement of grains in this size range. Gravel larger than about D₁₆ of the channel surface is more consistently trapped during Phase II transport. The persistence of coarse gravel in bedload samples provides good evidence that conditions suitable for coarse grain transport have been reached, even though the size of the sediment approaches the size limits of the sampler (76 mm in all cases). A relative breakpoint (R_br) was defined by the ratio between the discharge at the breakpoint and the 1·5‐year flow (a surrogate for bankfull discharge) expressed as a percentage. The median value of R_br was about 80 percent, suggesting that Phase II begins at about 80 percent of the bankfull discharge, though the observed values of R_br ranged from about 60 to 100 percent. Variation in this value appears to be independent of drainage area, median grain size, sorting of bed materials, and channel gradient, at least for the range of parameters measured in 12 gravel‐bed channels. Published in 2002 by 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.     

Analysis of bedload transport characteristics of Idaho streams and rivers

Field data are essential in evaluating the adequacy of predictive equations for sediment transport. Each dataset based on the sediment transport rates and other relevant information gives an increased understanding and improved quantification of different factors influencing the sediment transport regime in the specific environment. Data collected for 33 sites on 31 mountain streams and rivers in Central Idaho have enabled the analysis of sediment transport characteristics in streams and rivers with different geological, topographic, morphological, hydrological, hydraulic, and sedimentological characteristics. All of these streams and rivers have armored, poorly sorted bed material with the median particle size of surface layer coarser than the subsurface layer. The fact that the largest particles in the bedload samples did not exceed the median particle size of the bed surface material indicates that the armor layer is stable for the observed flow discharges (generally bankfull or less, and in some cases two times higher than bankfull discharge). The bedload transport is size‐selective. The transport rates are generally low, since sediment supply is less than the ability of flow to move the sediment for one range of flow discharges, or, the hydraulic ability of the stream is insufficient for entrainment of the coarse bed material. Detailed analyses of bedload transport rates, bedload and bed material characteristics were performed for each site. The obtained results and conclusions are used to identify different influences on bedload transport rates in analyzed gravel‐bed rivers. Copyright © 2008 John Wiley & Sons, Ltd.     

Controls of alluvial cover formation,morphology and bedload transport in a sinuous channel with a non-alluvial boundary

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

Network‐scale dynamics of grain‐size sorting: implications for downstream fining,stream‐profile concavity,and drainage basin morphology

We explore the link between channel‐bed texture and river basin concavity in equilibrium catchments using a numerical landscape evolution model. Theory from homogeneous sediment transport predicts that river basin concavity directly increases with bed sediment size. If the effective grain size on a river bed governs its concavity, then natural phenomena such as grain‐size sorting and channel armouring should be linked to concavity. We examine this hypothesis by allowing the bed sediment texture to evolve in a transport‐limited regime using a two grain‐size mixture of sand and gravel. Downstream ?ning through selective particle erosion is produced in equilibrium. As the channel‐bed texture adjusts downstream so does the local slope. Our model predicts that it is not the texture of the original sediment mixture that governs basin concavity. Rather, concavity is linked to the texture of the sorted surface layer. Two different textural regimes are produced in the experiments: a transitional regime where the mobility of sand and gravel changes with channel‐bed texture, and a sand‐dominated region where the mobility of sand and gravel is constant. The concavity of these regions varies depending on the median gravel‐ or sand‐grain size, erosion rate, and precipitation rate. The results highlight the importance of adjustments in both surface texture and slope in natural rivers in response to changes in ?uvial and sediment inputs throughout a drainage network. This adjustment can only be captured numerically using multiple grain sizes or empirical downstream ?ning rules. Copyright © 2004 John Wiley & Sons, Ltd.     

Bedload transport measurements at the Erlenbach stream with geophones and automated basket samplers

In the Erlenbach stream, a pre‐alpine steep channel in Switzerland, sediment transport has been monitored for more than 25 years. Near the confluence with the main valley river, stream flow is monitored and sediment is collected in a retention basin with a capacity of about 2000 m³. The basin is surveyed at regular intervals and after large flood events. In addition, sediment transport has been continuously monitored with piezoelectric bedload impact and geophone sensors since 1986. In 2008–2009, the measuring system in the Erlenbach stream was enhanced by installing an automatic system to obtain bedload samples. Movable metal baskets are mounted on a rail at the downstream wall of the large check dam above the retention basin, and they can be moved automatically into the flow to take bedload transport samples. The wire mesh of the baskets has a spacing of 10 mm to sample all sediment particles coarser than this size (which is about the limiting grain size detected by the geophones). The upgraded measuring system permits to obtain bedload samples over short sampling periods and to measure the grain size distribution of the transported material and its variation over time and with discharge. The analysis of calibration relationships for the geophone measuring system confirms findings from very similar measurements which were performed until 1999 with piezoelectric bedload impact sensors; there is a linear relationship between impulse counts and bedload mass passing over the sensors. Findings from flume experiments are used to discuss the most important factors which affect the calibration of the geophone signal. The bedload transport rates as measured by the moving baskets are among the highest measured in natural streams, with values of the order of several kilograms per meter per second. Copyright © 2012 John Wiley & Sons, Ltd.     

Sediment transport in a highly regulated fluvial system during two consecutive floods (lower Ebro River,NE Iberian Peninsula)

The transfer of sediment through a highly regulated large fluvial system (lower Ebro River) was analysed during two consecutive floods by means of sediment sampling. Suspended sediment and bedload transport were measured upstream and downstream of large reservoirs. The dams substantially altered flood timing, particularly the peaks, which were advanced downstream from the dams for flood control purposes. The suspended sediment yield upstream from the dams was 1 700 000 tonnes, which represented nearly 99 per cent of the total solid yield. The mean concentrations were close to 0·5 g l^?1. The sediment yield downstream from the dams was an order of magnitude lower (173 000 tonnes), showing a mean concentration of 0·05 g l^?1. The dams captured up to 95 per cent of the fine sediment carried in suspension in the river channel, preventing it from reaching the lowermost reaches of the river and the delta plain. Total bedload transport upstream from the dams was estimated to be about 25 000 tonnes, only 1·5 per cent of the total load. The median bedload rate was 100 gms^?1. Below the dams, the river carried 178 000 tonnes, around 51 per cent of the total load, at a mean rate of 250 g ms^?1. The results of sediment transport upstream and downstream from the large dams illustrate the magnitude of the sediment deficit in the lower Ebro River. The river mobilized a total of 350 000 tonnes in the downstream reaches, which were not replaced by sediment from upstream. Therefore, sediment was necessarily entrained from the riverbed and channel banks, causing a mean incision of 33 mm over the 27 km long study reach, altogether a significant step towards the long‐term degradation of the lower Ebro River. Copyright © 2005 John Wiley & Sons, Ltd.     

The role of vegetation in the retention of fine sediment and associated metal contaminants in London's rivers

Many urban rivers receive significant inputs of metal‐contaminated sediments from their catchments. Restoration of urban rivers often creates increased slack water areas and in‐channel vegetation growth where these metal‐contaminated sediments may accumulate. Quantifying the accumulation and retention of these sediments by in‐channel vegetation in urban rivers is of importance in terms of the planning and management of urban river restoration schemes and compliance with the Water Framework Directive. This paper investigates sediment properties at four sites across three rivers within Greater London to assess the degree to which contaminated sediments are being retained. Within paired restored and unrestored reaches at each site, four different bed sediment patch types (exposed unvegetated gravel, sand, and silt/clay (termed ‘fine’) sediments, and in‐channel vegetated sediments) were sampled and analysed for a range of metals and sediment characteristics. Many samples were found to exceed Environment Agency guidelines for copper (Cu), lead (Pb) and zinc (Zn) and Dutch Intervention Values for Cu and Zn. At all sites, sediments accumulating around in‐channel vegetation were similar in calibre and composition to exposed unvegetated fine sediments. Both bed sediment types contained high concentrations of pseudo‐total and acetic acid extractable metal concentrations, potentially due to elevated organic matter and silt/clay content, as these are important sorbtion phases for metals. This implies that the changed sediment supply and hydraulic conditions associated with river restoration may lead to enhanced retention of contaminated fine sediments, particularly around emergent plants, frequently leading to the development of submerged and emergent landforms and potential river channel adjustments. High pseudo‐total metal concentrations were also found in gravel bed sediments, probably associated with iron (Fe) and manganese (Mn) oxyhydroxides and discrete anthropogenic metal‐rich particles. These results highlight the importance of understanding the potential effects of urban river restoration upon sediment availability and channel hydraulics and consequent impacts upon sediment contaminant dynamics and storage. Copyright © 2014 John Wiley & Sons, Ltd.