Gravel transport and morphological change in braided sunwapta river,Alberta, Canada

The relation between morphological change and patterns of variation in bedload transport rate in braided streams was observed by repeated, daily topographic surveys over a 25 day study period in a 60 m reach of the proglacial Sunwapta River, Alberta, Canada. There are two major periods of morphological change, each lasting several days and each involving the complete destruction and reconstruction of bar complexes. Bar complex destruction was caused by redirection of the flow and by downstream extension of the confluence scour zone upstream. Reconstruction involved accretion of unit bars on bar head, flank and tail and in one case was initiated by disection of a large, lobate unit bar. High rates of sediment movement, measured from net scour and fill of the cross-sections, coincided with these morphological changes. Sediment was supplied from both bed and bank erosion, and patterns and distances of transfer were highly variable. Rates of transport estimated by matching upstream erosional volumes with downstream deposition were much greater than those estimated from either a step-length approach or a sediment budget. Measurements of scour and fill and observations of morphological change indicate that step lengths (virtual transport distances) were typically 40–100m during a diurnal discharge cycle. Shorter step lengths occurred when transfer was confined to a single anabranch and longer steps involved channel changes at the scale of the entire reach. Sediment budgeting was used to describe the spatial patterns of sediment transport associated with the morphological changes and to estimate minimum daily reach-averaged transport rates. Mean bedload transport rates correlate with discharge, but with considerable scatter. The largest deviations from the mean relation can be tied to phases of channel incision, bank erosion, scour hole migration, bar deposition and channel filling apparently controlled by changes and fluctuations in sediment supply from upstream, independent of discharge. These are interpreted as field evidence of ‘autopulses’ or ‘macropulses’ in bedload transport, previously observed only in laboratory models of braided streams.     

Annual bed‐elevation regime in the alluvial channel of Squamish River,Southwestern British Columbia,Canada

The aim of this study is to examine the annual regime of channel scour and fill by monitoring bed‐elevation changes in a reach of Squamish River in southwestern British Columbia, Canada. Sonar surveys of 13 river cross‐sections in a sandy gravel‐bed single‐channel study reach were repeated biweekly over a full hydrologic year (1995/6). The survey results show that bedload movement occurs as waves or pulses forming bedwaves that appear to maintain an overall coherence with movement downstream. These bedwaves propagate downstream by a mode here termed pulse scour and pulse fill, a process distinguished from the conventional mode of scour and fill commonly associated with flood events (here termed local scour and local fill). Bedwave celerity was estimated to be about 15·5 m d⁻¹ corresponding to a bedwave residence time in the study reach of almost one hydrologic year. The total amount of local bed‐elevation change ranged between 0·22 m and 2·41 m during the period of study. Analysis of the bed‐elevation and flow data reveals that, because of the bedwave phenomenon, there is no simple relation between the mean bed‐elevation and discharge nor any strong linear correlation among cross‐sectional behaviour. The bed‐elevation data also suggest that complex changes to the bed within a cross‐section are masked when the bed is viewed in one dimension, although no definitive trends in bed behaviour were found in the two‐dimensional analysis. Although a weak seasonal effect is evident in this study, the bed‐elevation regime is dominated by sediment supply‐driven fluctuations in bedload transport occurring at timescales shorter than the seasonal fluctuation in discharge. The study also indicates that bed‐elevation monitoring on Squamish River, and others like it, for purposes of detecting and measuring aggradation/degradation must take into account very considerable and normal channel‐bed variability operating at timescales from hours to months. Copyright © 2000 John Wiley & Sons, Ltd.     

The influence of channel morphology on bedload path lengths: Insights from a survival process model

Tracer studies are a commonly used tool to develop and test Einstein-type stochastic bedload transport models. The movements of these tracers are controlled by many factors including grain characteristics, hydrologic forcing, and channel morphology. Although the influence of these sediment storage zones related to morphological features (e.g., bars, pools, riffles) have long been observed to “trap” bedload particles in transport, this influence has not been adequately quantified. In this paper we explore the influence of channel morphology on particle travel distances through the development of a Bayesian survival process model. This model simulates particle path length distributions using a location-specific “trapping probability” parameter (p_i ), which is estimated using the starting and ending locations of bedload tracers. We test this model using a field tracer study from Halfmoon Creek, Colorado. We find that (1) the model is able to adequately recreate the observed multi-modal path length distributions, (2) particles tend to accumulate in trapping zones, especially during large floods, and (3) particles entrained near a trapping zone will travel a shorter distance than one that is further away. Particle starting positions can affect path lengths by as much as a factor of two, which we confirm by modelling “starting-location-specific” path length probability distributions. This study highlights the importance of considering both tracer locations and channel topography in examinations of field tracer studies. © 2020 John Wiley & Sons, Ltd.     

The effects of survey frequency on estimates of scour and fill in a braided river model

An erratum has been published for this article in Earth Surface Processes and Landforms 27(7) 2002, 795. Estimates of scour and fill in rivers that are derived by differencing topographic surfaces are known to be negatively biased by local compensation of scour and fill between surveys but the magnitude of bias is not well known. This study examines the effect of survey frequency on volumes of scour and fill over a period of active channel braiding in a small‐scale river model. A 100 min, high temporal resolution time series of digital elevation models is artificially coarsened by selectively removing models. The resulting four overlapping time series have survey intervals of 10 min, 20 min, 50 min and 100 min. Cumulative scour and fill volumes for the 100 min period are compared between the four series. It is concluded that the decay in measured volumes of scour and fill with increased survey interval can be described using inverse functions. Cumulative scour–fill volumes are approximately 420 per cent greater over the study period for 10 min survey intervals than for a 100 min interval. After the 100 min period of competent flow, nearly 65 per cent of the channel area experienced significant compensation of scour and fill. Several compensation mechanisms were identified in association with braided channel kinetics, including lateral channel migration, the migration of bed forms, and channel avulsion. It is demonstrated that by negatively biasing scour, fill and net estimates, this error significantly affects morphological approaches to the estimation of bed load sediment transport. Copyright © 2002 John Wiley & Sons, Ltd.     

Estimating bed material fluxes upstream and downstream of a controlled large bifurcation - the Mississippi-Atchafalaya River diversion

Bed material transport at river bifurcations is crucial for channel stability and downstream geomorphic dynamics. However, measurements of bed material transport at bifurcations of large alluvial rivers are difficult to make, and standard estimates based on the assumption of proportional partitioning of flow and bedload transport at bifurcations may be erroneous. In this study, we employed a combined approach based on observed topographic change (erosion/deposition) and bed material transport predicted from a one-dimensional model to investigate bed material fluxes near the engineering-controlled Mississippi-Atchafalaya River diversion, which is of great importance to sediment distribution and delivery to Louisiana's coast. Yang's (1973) sediment transport equation was utilized to estimate daily bed material loads upstream, downstream, and through the diversion during 2004–2013. Bathymetric changes in these channels were assessed with single beam data collected in 2004 and 2013. Results show that over the study period, 24% of the Mississippi River flow was diverted into the Atchafalaya River, while the rest remained in the mainstem Mississippi. Upstream of the diversion, the bed material yield was predicted to be 201 million metric tons (MT), of which approximately 35 MT (i.e., 17%) passed through the bifurcation channel to the Atchafalaya River. The findings from this study reveal that in the mainstem Mississippi, the percentage of bed material diversion (83%) is larger than the percentage of flow diversion (76%); Conversely, the diversion channel receives a disproportionate amount of flow (24%) relative to bed material supply (17%). Consequently, severe bed scouring occurred in the controlled Outflow Channel to the Atchafalaya River, while riverbed aggradation progressed in the mainstem Mississippi downstream of the diversion structures, implying reduced flow capacity and potential risk of a high backwater during megafloods. The study demonstrates that Yang's sediment transport equation provides plausible results of bed material fluxes for a highly complicated large river diversion, and that integration of the sediment transport equation with observed morphological changes in riverbed is a valuable approach to investigate sediment dynamics at controlled river bifurcations.     

Numerical modeling of local scour of non-uniform graded sediment for two arrangements of pile groups

A three-dimensional(3D) non-hydrostatic numerical model is established to investigate local scour around four aligned circular piles in uniform and non-uniform sediment mixtures and to provide information for improving scour countermeasures design. In the current study, unsteady Reynolds averaged Navier-Stokes(URANS) equations along with a Re-normalization Group(RNG) k-ε model were applied to simulate the flow field. A non-uniform sediment transport model was applied to estimate the bedload tran...     

Bedload tracing in a high‐sediment‐load mountain stream

This paper reports a radiofrequency identification (RFID) tracing experiment implemented in a high‐sediment‐load mountain stream typical of alpine gravel‐bed torrents. The study site is the Bouinenc Torrent, a tributary to the Bléone River in southeast France that drains a 38·9‐km² degraded catchment. In spring 2008, we deployed 451 tracers with b‐axis ranging from 23 to 520 mm. Tracers were seeded along eight cross‐sections located in the upstream part of the lowest 2·3 km of the stream. Three tracer inventories were implemented in July 2008, 2009 and 2010. Recovery rates calculated for mobile tracers declined from 78% in 2008 to 45% in 2009 and 25% in 2010. Observations of tracer displacement revealed very high sediment dispersion, with frontrunners having travelled more than 2 km only three months after their deployment. The declining recovery rate over time was interpreted as resulting from rapid dispersion rather than deep burial. We evaluated that 64% of the tracers deployed in the active channel were exported from the 2·3‐km study reach three years after the onset of the tracing experiment. Travel distances were characterized by right‐skewed and heavy‐tailed distributions, correctly fitted by a power‐law function. This supports the idea that in gravel‐bed rivers with abundant sediment supply relative to transport capacity, bedload transport can be viewed as a superdiffusive sediment dispersion process. It is also shown that tracers initially deployed in the low‐flow channel were characterized by a 15‐ to 30‐fold increase of mobility compared to tracers deployed in gravel bars. Copyright © 2011 John Wiley & Sons, Ltd.     

Width control on event-scale deposition and evacuation of sediment in bedrock-confined channels

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

Lithology‐controlled evolution of stream bed sediment and basin‐scale sediment yields in adjacent mountain watersheds,Idaho, USA

The composition, grain‐size, and flux of stream sediment evolve downstream in response to variations in basin‐scale sediment delivery, channel network structure, and diminution during transport. Here, we document downstream changes in lithology and grain size within two adjacent ~300 km² catchments in the northern Rocky Mountains, USA, which drain differing mixtures of soft and resistant rock types, and where measured sediment yields differ two‐fold. We use a simple erosion–abrasion mass balance model to predict the downstream evolution of sediment flux and composition using a Monte Carlo approach constrained by measured sediment flux. Results show that the downstream evolution of the bed sediment composition is predictably related to changes in underlying geology, influencing the proportion of sediment carried as bedload or suspended load. In the Big Wood basin, particle abrasion reduces the proportion of fine‐grained sedimentary and volcanic rocks, depressing bedload in favor of suspended load. Reduced bedload transport leads to stronger bed armoring, and coarse granitic rocks are concentrated in the stream bed. By contrast, in the North Fork Big Lost basin, bedload yields are three times higher, the stream bed is less armored, and bed sediment becomes dominated by durable quartzitic sandstones. For both basins, the geology‐based mass balance model can reproduce within ~5% root‐mean‐square error the composition of the bed substrate using realistic erosion and abrasion parameters. As bed sediment evolves downstream, bedload fluxes increase and decrease as a function of the abrasion parameter and the frequency and size of tributary junctions, while suspended load increases steadily. Variable erosion and abrasion rates produce conditions of variable bed‐material transport rates that are sensitive to the distribution of lithologies and channel network structure, and, provided sufficient diversity in bedrock geology, measurements of bed sediment composition allow for an assessment of sediment source areas and yield using a simple modeling approach. Copyright © 2016 John Wiley & Sons, Ltd.     

How sediment composition and flow rate influence downstream channel morphodynamics upon dam removal

The process of dam removal establishes the channel morphology that is later adjusted by high-flow events. Generalities about process responses have been hypothesized, but broad applicability and details remain a research need. We completed laboratory experiments focused on understanding how processes occurring immediately after a sediment release upon dam removal or failure affect the downstream channel bed. Flume experiments tested three sediment mixtures at high and low flow rates. We measured changes in impounded sediment volume, downstream bed surface, and rates of deposition and erosion as the downstream bed adjusted. Results quantified the process responses and connected changes in downstream channel morphology to sediment composition, temporal variability in impounded sediment erosion, and spatial and temporal rates of bedload transport. Within gravel and sand sediments, the process response depended on sediment mobility. Dam removals at low flows created partial mobility with sands transporting as ripples over the gravel bed. In total, 37% of the reservoir eroded, and half the eroded sediment remained in the downstream reach. High flows generated full bed mobility, eroding sands and gravels into and through the downstream reach as 38% of the reservoir eroded. Although some sediment deposited, there was net erosion from the reach as a new, narrower channel eroded through the deposit. When silt was part of the sediment, the process response depended on how the flow rate influenced reservoir erosion rates. At low flows, reservoir erosion rates were initially low and the sediment partially exposed. The reduced sediment supply led to downstream bed erosion. Once reservoir erosion rates increased, sediment deposited downstream and a new channel eroded into the deposits. At high flows, eroded sediment temporarily deposited evenly over the downstream channel before eroding both the deposits and channel bed. At low flows, reservoir erosion was 17–18%, while at the high flow it was 31–41%.     

SCOUR RATE FORMULA

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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.     

Unexpected sedimentation patterns upstream and downstream of the Three Gorges Reservoir: Future risks

This paper summarizes the latest developments, future prospects, and proposed countermeasures of reservoir sedimentation and channel scour downstream of the Three Gorges Reservoir (TGR) on the Yangtze River in China. Three key results have been found.(1) The incoming sediment load to the TGR has been significantly lower than expected.(2) The accumulated volume of sediment deposition in the TGR is smaller than expected because the overall sediment delivery ratio is relatively low, and the deposition in the near-dam area of the TGR is still developing.(3) River bed scour in the river reaches downstream of the Gezhouba Dam is still occurring and channel scour has extended to reaches as far downstream as the Hukou reach. Significantly, sedimentation of the TGR is less problematic than expected since the start of operation of the TGR on the one hand;on the other hand, the possible increases in sediment risks from dependence on upstream sediment control, deposition in the reservoir, and scour along middle Yangtze River should be paid more attention.(1) Sediment trapped by dams built along the upper Yangtze River and billion tons of loose materials on unstable slopes produced by the Wenchuan Earthquake could be new sediment sources for the upper Yangtze River. More seriously, possible release of this sediment into the upper Yangtze River due to new earthquakes or extreme climate events could overwhelm the river system, and produce catastrophic consequences.(2) Increasing sediment deposition in the TGR is harmful to the safety and efficiency of project operation and navigation.(3) The drastic scour along the middle Yangtze River has intensified the down-cutting of the riverbed and erosion of revetment, it has already led to increasing risk to flood control structures and ecological safety. It is suggested to continue the Field Observation Program, to initiate research programs and to focus on risks of sedimentation.     

Geomorphic response to gravel augmentation and high‐flow dam release in the Trinity River,California

The geomorphic effect of introducing a gravel augmentation totaling 520 m³ into a gravel‐bed stream during a dam‐controlled flood in May of 2015 was monitored with bedload transport measurements, an array of seismometers, and repeated topographic surveys. Half of the augmented gravel was injected into the flow with front‐end loaders on the rising limb of the flood and the other half was injected on the first day of the peak. Virtually all of the gravel transported past the injection point was deposited within about 7 to 10 channel widths of the injection point. Most of the injected gravel deposited along the left bank of the river whereas the right half of the channel bed was dominated by scour. The downstream third of the depositional area consisted of a small dune field that developed prior to the second gravel injection and subsequently migrated about one channel width downstream. A second depositional front was observed upstream from the gravel injection point, where a delta‐like wedge of bed material developed in the first hours of the flow release and changed little over the remainder of the release. These two depositional areas represent small‐scale bed‐material storage reservoirs with the potential to accumulate and periodically release packets of bed material. Interactions with such storage reservoirs are hypothesized to cause large bed‐material pulses to disperse by fragmenting into multiple smaller pulses. As a refinement to the conceptual model that views sediment pulse evolution in terms of dispersion and translation, the concept of pulse fragmentation has practical implications for gravel management. It implies that gravel augmentations can produce morphologic changes at locations that are separated from the augmentation point by arbitrarily long reaches, and it highlights the dependence of pulse propagation rates on the nature and distribution of the bed‐material storage reservoirs in the channel system. Copyright © 2017 John Wiley & Sons, Ltd.     

Effects of lateral confinement in natural and leveed reaches of a gravel‐bed river: Snake River,Wyoming, USA

This study examined the effects of natural and anthropogenic changes in confining margin width by applying remote sensing techniques – fusing LiDAR topography with image‐derived bathymetry – over a large spatial extent: 58 km of the Snake River, Wyoming, USA. Fused digital elevation models from 2007 and 2012 were differenced to quantify changes in the volume of stored sediment, develop morphological sediment budgets, and infer spatial gradients in bed material transport. Our study spanned two similar reaches that were subject to different controls on confining margin width: natural terraces versus artificial levees. Channel planform in reaches with similar slope and confining margin width differed depending on whether the margins were natural or anthropogenic. The effects of tributaries also differed between the two reaches. Generally, the natural reach featured greater confining margin widths and was depositional, whereas artificial lateral constriction in the leveed reach produced a sediment budget that was closer to balanced. Although our remote sensing methods provided topographic data over a large area, net volumetric changes were not statistically significant due to the uncertainty associated with bed elevation estimates. We therefore focused on along‐channel spatial differences in bed material transport rather than absolute volumes of sediment. To complement indirect estimates of sediment transport derived by morphological sediment budgeting, we collected field data on bed mobility through a tracer study. Surface and subsurface grain size measurements were combined with bed mobility observations to calculate armoring and dimensionless sediment transport ratios, which indicated that sediment supply exceeded transport capacity in the natural reach and vice versa in the leveed reach. We hypothesize that constriction by levees induced an initial phase of incision and bed armoring. Because levees prevented bank erosion, the channel excavated sediment by migrating rapidly across the restricted braidplain and eroding bars and islands. Copyright © 2017 John Wiley & Sons, Ltd.     

EXPERIMENTAL INVESTIGATION AND ANALYSIS OF HEC-6 RIVER MORPHOLOGICAL MODEL

Only comparatively few experimental studies have been carried out to investigate the performance of the HEC-6 river morphological model. The model was developed by the Hydrologic Engineering Center of the US Army Corps of Engineers. In this study, experiments were carried out in a 20 m long concrete flume 0.6 m wide with varying rectangular cross-sections. The channel bed is paved with uniform sand of D₅₀ = 0.9 mm and D₉₀ = 1.2 mm within the test reach of 12 m. Two types of experiments were carried out with sediment transport, one under steady uniform flow and another under steady non-uniform flow conditions. Nine steady uniform flow experiments were carried out to compare the measured equilibrium relationship of flow and sediment transport rate with two bedload formulae, namely, Du Boys and Meyer–Peter and Muller, and with three total load formulae, namely, Toffaleti, Laursen and Yang. It was found that even though the sediment transport consists of a certain portion of bedload, the total load formulae give satisfactory results and better agreement than the two bedload formulae. Five steady non-uniform flow experiments were carried out under various conditions of varying bed profile and channel width and also with sediment addition and withdrawal. The measured transient water surface and bed profiles are compared with the computed results from the HEC-6 model. It was found that the Toffaleti and Yang total load formulae used in the HEC-6 model give the most satisfactory prediction of actual bed profiles under various conditions of non-uniform flow and sediment transport. The effects of Manning's n, variations of sediment inflow, various sediment transport formulae, sediment grain size and the model numerical parameters, i.e. distance interval Δx and numerical weighting factor, on the computed water surface and bed profiles were determined. It was found that the selection of the sediment transport formulae has the most significant effect on the computed results. It can be concluded that the HEC-6 model can predict satisfactorily a long-term average pattern of local scour and deposition along a channel with either a small abrupt change in geometry or gradually varying cross-sections. However, the accuracy of the model prediction is reduced in the regions where highly non-uniform flow occurs.     

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.     

Turbulent flow structures and geomorphic characteristics of a mining affected alluvial channel

Sediment mining in rivers may have a major impact on river geomorphology and research is required to quantify these impacts. In this research, experimental studies were conducted to analyse the morphological changes of channel bed and the turbulent characteristics of flow in the presence of mining. The channel bed profile shows erosion at the bank of the pit and that the erosion expands to the whole width of the channel and propagates downstream with time. The deposition of sediment occurs along the upstream edge of the pit and the depth of the pit decreases with time. Velocity reversal occurs at the central bottom of the pit related to a recirculation zone. Reynolds shear stress and the turbulent intensities become higher in the mining pit region and downstream of it as compared to the upstream section, causing a more rapid movement of bed particles. Analysis of the bursting phenomenon shows that the contribution of sweep and ejection events to the total Reynolds shear stress is more dominant over outward and inward interaction events. The dominance of the sweep event over ejection is observed at the near‐bed region for all the sections, but the depth range of dominance of sweep events in the pit and downstream of the pit is found to be more than the upstream. The increase in thickness is responsible for the increase in bed material transport. The increased sediment transport capacity at the mining pit and downstream of it caused the deformation and lowering of channel bed downstream. An empirical formulation of bedload transport for mining induced channels is derived from two different sized uniform bed materials. Copyright © 2018 John Wiley & Sons, Ltd.     

Coarse bedload transport in a mountain river

Coarse bedload transport dynamics are investigated utilizing hydrodynamic and sediment transport data obtained in an extensively instrumented study reach located in Squaw Creek, Montana, USA. During 1991 and 1992, a number of discrete bedload transport events associated with the daily rise and fall in stream discharge were investigated. Data show that initiation of sediment transport was accompanied by a reduction in bed roughness and by changes in bulk hydraulic parameters. For larger discharges, coarser fractions of the bed material mobilized, and bedload transport rates and average hydraulic parameters stabilized. As discharge reduced, mobile coarse particles became less frequent and deposited fine particles were removed, resulting in an increase in bed roughness. These observations are attributed to the downstream translation of bar sediments during the passage of a hydrograph. Bedload pulses were aperiodic but spatially variable. Flow turbulence and velocity profile data obtained during low flows allowed comparison between average bed shear stress and apparent bed roughness estimates obtained using different approaches. © 1998 John Wiley & Sons, Ltd.