Secondary flow in anabranch confluences of a braided,gravel-bed stream

Measurements of the primary and secondary velocity components were máde in two, active, braided river anabranch confluences with a simple Y-shaped plan form, in the gravelly Sunwapta River (D₅₀ of approximately 30 mm). Flow velocity was measured at regularly-spaced intervals using a bidirectional electromagnetic current meter and the measured downstream and cross-stream velocities were converted to primary and secondary velocities to yield the secondary circulation. The primary (downstream) velocity field shows two high velocity streams from the two tributaries which merge (and, in some cases, accelerate) into a single high velocity core over the thalweg. Primary flow velocity declines as the flow expands and diverges at the downstream end of the confluence. The secondary circulation is dominated by two helical cells, back-to-back, plunging over the thalweg and diverging at the bed. This is the first confirmation of this flow structure in confluences, based on field measurements. The strength of the secondary cells declines downstream through each confluence, and laterally away from the thalweg area in cross-section. There is also a tendency for one cell, from the larger of the tributaries, to override the other. The secondary and primary flow structure and strength differs slightly between the two confluences and this is reflected in differences in scour hole form.     

Reduction of local scour at river conf luences using a collar

River confluences (RCs) are important features within river systems where the three dimensional (3D) flow structures and the downstream mixing of flows can cause deep scour holes. Despite this, few methods have been proposed to control scouring at RCs. In this study, application of a collar was experimentally examined for local scour control at the point where two rivers flow together. In parti-cular, experimental tests were done with and without collar application at three different locations. The results reveal that the scour depth is directly proportional to the discharge ratio, i.e. the ratio of lateral discharge to that in the channel downstream of the confluence, and the densimetric Froude number (Frg). In addition, installation of a collar at RCs can decrease the scour depth up to 100%, thus completely avoiding the scour process. The results also show that by increasing the Frg the optimal installation location for a collar changes and moves towards the river bed level. Using a collar can also reduce the height of the point bar formed downstream of the confluence. The outcomes of the study allow deri-vation of an equation for predicting scour depth when a collar is applied as a countermeasure. The analysis of this equation shows that the estimates are mostly affected by the Frg.     

Time-averaged flow structure in the central region of a stream confluence

Previous process-oriented field studies of stream confluences have focused mainly on fluvial dynamics at or immediately downstream of the location where the confluent flows enter the downstream channel. This study examines in detail the spatial evolution of the time-averaged downstream velocity, cross-stream velocity, and temperature fields between the junction apex, where the flows initially meet, and the entrance to the downstream channel. A well-defined, vertically oriented mixing interface exists within this portion of the confluence, suggesting that lateral mixing of the incoming flows is limited. The downstream velocity field near the junction apex is characterized by two high-velocity cores separated by an intervening region of low-velocity or recirculating fluid. In the downstream direction, the high-velocity cores move inwards towards the mixing interface and high-velocity fluid progressively extends downwards into a zone of scour, resulting in an increase in flow velocity in the centre of the confluence. The cross-stream velocity field is dominated by flow convergence, but also includes a component associated with a consistent pattern of secondary circulation. This pattern is characterized by two surface-convergent helical cells, one on each side of the mixing interface. The helical cells appear to be the mechanism by which high-momentum fluid near the surface is advected downwards into the zone of scour. For transport-ineffective flows, the dimensions and intensities of the cells are controlled by the momentum ratio of the confluent streams and by the extant bed morphology within the confluence. Although the flow structure of formative events was not measured directly in this study, documented patterns of erosion and deposition within the central region of the confluence suggest that these events are dynamically similar to the measured flows, except for the fact that formative flows are not constrained by, but can reshape, the bed morphology. The results of this investigation are consistent with and augment previous findings on time-averaged flow structure in the downstream portion of the confluence. © 1998 John Wiley & Sons, Ltd.     

Experimental study on flow pattern and sediment transportation at a 90°open-channel confluence

In this paper,the evolutions of flow pattern and sediment transportation at a 90° open-channel confluence with different discharge ratios (q*) of the tributary flow to the total flow were studied.The e...     

Spatial variability of three‐dimensional Reynolds stresses in a developing channel bend

Experimental results of the mean flow field and turbulence characteristics for flow in a model channel bend with a mobile sand bed are presented. Acoustic Doppler velocimeters (ADVs) were used to measure the three components of instantaneous velocities at multiple cross sections in a 135° channel bend for two separate experiments at different stages of clear water scour conditions. With measurements at multiple cross sections through the bend it was possible to map the changes in both the spatial distribution of the mean velocity field and the three Reynolds shear stresses. Turbulent stresses are known to contribute to sediment transport and the three‐dimensionality inherent to flow in open channel bends presents a useful case for determining specific relations between three‐dimensional turbulence and sediment entrainment and transport. These measurements will also provide the necessary data for validating numerical simulations of turbulent flow and sediment transport. The results show that the magnitude and distribution of three‐dimensional Reynolds stresses increase through the bend, with streamwise‐cross stream and cross stream‐vertical components exceeding the maximum principal Reynolds stress through the bend. The most intriguing observation is that near‐bed maximum positive streamwise‐cross stream Reynolds stress coincides with the leading edge of the outer bank scour hole (or thalweg), while maximum cross stream‐vertical Reynolds stress (in combination with high negative streamwise‐cross stream Reynolds stress near the bend apex) coincides with the leading edge of the inner bank bar. Maximum Reynolds stress and average turbulent kinetic energy appear to be greater and more localized over the scour hole before final equilibrium scour is reached. This suggests that the turbulent energy in the flow is higher while the channel bed is developing, and both lower turbulent energy and a broader distribution of turbulent stresses near the bed are required for cessation of particle mobilization and transport. Copyright © 2010 John Wiley & Sons, Ltd.     

Local scour around two side-by-side cylindrical bridge piers under ice-covered conditions

In the current study, 108 flume experiments with non-uniform, cohesionless sediments have been done to investigate the local scour process around four pairs of side-by-side bridge piers under both open channel and ice-covered flow conditions. Similar to local scour around bridge piers under open channel conditions and a single bridge pier, it was observed in the experiments that the maximum scour depth always occurred at the upstream face of the pier under ice-covered conditions. Further, the smaller the pier size and the greater the spacing distance between the bridge piers, the weaker the horseshoe vortices around the bridge piers, and, thus, the shallower the scour holes around them. Finally, empirical equations were developed to estimate the maximum scour depth around two side-by-side bridge piers under both open channel and ice-covered flow conditions.     

Formation of stationary alternate bars in a steep channel with mixed-size sediment: A flume experiment

Alternate bars were formed by sediment transport in a flume with Froude-modelled flow and relative roughness characteristic of gravel-boulder channels with steep slopes. The flume (0.3 m wide × 7.5 m long) was filled with a sand-gravel mixture, which was also fed into the top of the flume at a constant rate under constant discharge. Channel slope was set at 0.03. Initially, coarse particles accumulated on incipient bar heads near one side of the flume and diverted flow and bedload transport across the flume toward a pool scoured against the opposite flume wall downstream. Sorting in the pool directed coarse particles onto the next bar head downstream. Alternate sequences of pools and coarse bar heads were thereby linked down the entire flume by interactions of sediment sorting, flow, and channel morphology. During episodes of bar construction, unsorted bedload invaded interior bar surfaces and was deposited. Persistent deposition of coarse particles on bar heads prevented downstream migration of bars by inhibiting bar-head erosion and bedload transport over bars. Likely factors leading to bar-head stabilization in modelled gravel-bed channels are coarse mixed-size sediment, steep channel gradients, and shallow depths.     

EFFECTS OF FLOW INTENSITY, OBSTACLE ALIGNMENT AND CROSS-SECTION GEOMETRY ON SCOUR AT BRIDGE ABUTMENTS

1 INTRODUCTION Local scour at bridge piers and abutments has long concerned engineers. Scour depth estimation has attracted considerable research interest and activity, and a number of prediction methods exist at present (see, e.g., comprehensive lists of predictors given by Melville and Colleman, 2000). Several studies have been completed since the 1950s for the particular case of scour at bridge abutments. Since the beginning of the 1980s, the University of Auckland, New Zealand, …     

Influence of junction angle on three‐dimensional flow structure and bed morphology at confluent meander bends during different hydrological conditions

Recent field and modeling investigations have examined the fluvial dynamics of confluent meander bends where a straight tributary channel enters a meandering river at the apex of a bend with a 90° junction angle. Past work on confluences with asymmetrical and symmetrical planforms has shown that the angle of tributary entry has a strong influence on mutual deflection of confluent flows and the spatial extent of confluence hydrodynamic and morphodynamic features. This paper examines three‐dimensional flow structure and bed morphology for incoming flows with high and low momentum‐flux ratios at two large, natural confluent meander bends that have different tributary entry angles. At the high‐angle (90°) confluent meander bend, mutual deflection of converging flows abruptly turns fluid from the lateral tributary into the downstream channel and flow in the main river is deflected away from the outer bank of the bend by a bar that extends downstream of the junction corner along the inner bank of the tributary. Two counter‐rotating helical cells inherited from upstream flow curvature flank the mixing interface, which overlies a central pool. A large influx of sediment to the confluence from a meander cutoff immediately upstream has produced substantial morphologic change during large, tributary‐dominant discharge events, resulting in displacement of the pool inward and substantial erosion of the point bar in the main channel. In contrast, flow deflection is less pronounced at the low‐angle (36°) confluent meander bend, where the converging flows are nearly parallel to one another upon entering the confluence. A large helical cell imparted from upstream flow curvature in the main river occupies most of the downstream channel for prevailing low momentum‐flux ratio conditions and a weak counter‐rotating cell forms during infrequent tributary‐dominant flow events. Bed morphology remains relatively stable and does not exhibit extensive scour that often occurs at confluences with concordant beds. Copyright © 2014 John Wiley & Sons, Ltd.     

FLOW FIELD IN SCOURED ZONE OF CHANNEL CONTRACTIONS

Experiments were conducted in a laboratory flume to measure the two-dimensional turbulent flow field in the scoured zone of channel contractions under a clear-water scour condition. The Acoustic Doppler Velocimeter (ADV) was used to detect the flow field at different vertical lines along the centerline of uncontracted (main channel) and contracted zones of the channel. The distributions of time-averaged velocity components, turbulent intensity, turbulent kinetic energy, and Reynolds stresses are presented in nondimensional graphical form. The bed shear stresses are computed from the measured Reynolds stresses being in threshold condition within the zone of contraction where bed was scoured. The data presented in this paper would be useful to the investigators for the development of kinematic flow model and morphological model of scour at a channel or river contraction.     

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.     

Hydro‐morphodynamic evolution in a 90° movable bed discordant confluence with low discharge ratio

Confluences with low discharge and momentum ratios, where narrow steep tributaries with high sediment load join a wide low‐gradient main channel that provides the main discharge, are often observed in high mountain regions such as in the upper‐Rhone river catchment in Switzerland. Few existing studies have examined the hydro‐morphodynamics of this type of river confluence while considering sediment discharge in both confluent channels. This paper presents the evolution of the bed morphology and hydrodynamics as observed in an experimental facility with a movable bed. For that purpose, one experiment was carried out in a laboratory confluence with low discharge and momentum ratios, where constant sediment rates were supplied to both flumes. During the experiment, bed topography and water surface elevations were systematically recorded. When the bed topography reached a steady state (so‐called equilibrium) and the outgoing sediment rate approximated the incoming rate, flow velocity was measured at 12 different points distributed throughout the confluence, and the grain size distribution of the bed surface was analyzed. Typical morphodynamic features of discordant confluences such as a bank‐attached bar and a flow deflection zone are identified in this study. Nevertheless, the presence of a marked scour hole in the discordant confluence and distinct flow regimes for the tributary and main channel, differ from results obtained in previous studies. Strong acceleration of the flow along the outer bank of the main channel is responsible for the scour hole. This erosion is facilitated by the sediment discharge into the confluence from the main channel which inhibits bed armoring in this region. The supercritical flow regime observed in the tributary is the hydrodynamic response to the imposed sediment rate in the tributary. Copyright © 2015 John Wiley & Sons, Ltd.     

A combined field,laboratory and numerical study of the forces applied to,and the potential for removal of,bar top vegetation in a braided river

Vegetation can have an important role in controlling channel planform, through its effects on channel roughness, and root‐reinforcement of bank and bar materials. Along the Platte River in central Nebraska, USA, The Platte River Recovery Implementation Program (PRRIP) has been tasked with managing the planform of the river to benefit endangered species. To investigate the potential use of planned short duration high flow (SDHF) events to manage bar vegetation, this study combined several approaches to determine whether flows of up to 227 m³s^?1 through the central Platte River, could remove cottonwood, Phragmites and reed canarygrass stands of various ages and densities from in‐channel bars. First, fieldwork was carried out to measure the uprooting resistance, and resistance to bending for each species. Second, a set of flume experiments was carried out to measure the forces exerted on the three species of interest under different flow conditions. Finally, a numerical study comparing drag forces (driving) measured in the flume study, with uprooting forces (resisting) measured in the field, was carried out for each species to determine the likelihood of plant removal by SDHF events. Results showed that plants with more than a year of root growth, likely cannot be removed through drag and local scour alone, even at the 100‐year recurrence interval discharge. At most, a few cottonwood seedlings could be removed from bars through drag, scour and undercutting, where rooting depths are still small. The results presented here help us further understand the positive feedbacks that lead to the creation of permanent, vegetated bars rather than mobile braided channels. As such, the findings could help inform management decisions for other braided rivers, and the combined field, flume and modeling techniques used in this study could be applied to other fluvial systems where vegetation and planform dynamics are of interest. Copyright © 2016 John Wiley & Sons, Ltd.     

Changes in channel size at river confluences with coarse bed material

A field survey of thirty stream junctions from a small watershed, together with data collected by Miller (1958), allowed us to investigate morphometric adjustments occurring at confluences. The model proposed by Roy and Woldenberg (1986) was slightly modified and used as a tool for morphometric analysis. Two parameters are necessary in order to evaluate the rate of change in channel size at a confluence: the area ratio (channel capacity above the confluence: channel capacity below the confluence) and the discharge ratio (discharge of the minor tributary: discharge of the major tributary). Our data show that total channel capacity tends to decline below most confluences. A reduction in cross-sectional area implies an increase in average flow velocity. This interpretation is consistent with Lyell's observations and with results from recent flume experiments (Best and Reid, 1984).     

Influence of large woody debris on sediment scour at bridge piers

Large woody debris(LWD) reduces the flow area,deviate the flow and increases the velocity in correspondence of the bridge pier,therefore increases the maximum scour hole depth and accelerates sediment removal.Logs and drifts accumulated on bridge piers are of different dimensions.According to logs characteristics and river morphology,drift accumulations can either extend downstream the bridge pier or they can accumulate totally upstream.This paper aims to analyze the effect of drift accumulation planimetry on bridge pier scour.The experimental investigation has been carried out at the PITLAB hydraulic centre of Civil Engineering Department,University of Pisa,Italy.Drift accumulation was characterized by different relative longitudinal lengths,flow area occlusions,length of longitudinal drift and downstream planimetrical positions relative to the pier center.The experimental investigation has been carried out in clear-water conditions.Several pier sizes,channel widths and sediment materials have been tested.Maximum scour hole in presence of drift accumulation have been compared to the maximum scour hole for an isolated pier.Finally,data were compared with previous literature findings,which highlight the effect of the downstream extension of drift accumulation on bridge pier scour.New relationships have been proposed to predict the effect of drift accumulation on bridge pier scour,both in terms of relative maximum scour and temporal scour evolution.     

VORTEX FLOW FIELD IN A SCOUR HOLE AROUND ABUTMENTS

The three-dimensional flow field in a scour hole around different abutments under a clear water regime was experimentally measured in a laboratory flume, using the Acoustic Doppler Velocimeter (ADV). Three types of abutments used in the experiments were vertical-wall (rectangular section), 45° wing-wall (45° polygonal section) and semicircular. The three-dimensional time-averaged velocity components were detected at different vertical planes for vertical-wall abutment and azimuthal planes for wing-wall and semicircular abutments. The velocity components were also measured at different horizontal planes. In the upstream, presentations of flow field through vector plots at vertical / azimuthal and horizontal planes show the existence of a primary vortex associated with the downflow inside the scour hole. On the other hand, in the downstream, the flow field shows a reversed flow near the abutments having a subsequent recovery with a passage of flow as a part of the main flow. The data presented in this p     

SCOUR DEPTH AND FLOW PATTERN OF ERODING PLANE JETS

Local scour may occur when a hydraulic structure is positioned in a channel with an erodible bed. Herewith investigated experimentally are the erosion and flow pattern due to a water jet passing over a channel bed at the asymptotic (final) state. The development of the scour hole, its maximum scour depth and length, are recorded and compared with available scour-depth relations. Two sets of experiments (see Table 1) were carded out. Set 1 (3 runs) was concerned with measuring the vertical instantaneous velocity distribution in the scour hole. The scour hole at the asymptotic (final) state, t=100 [h] was investigated. Set 2 (5 runs) was concerned with studying the physics of scouring. Thescour hole at about mid-state, t≈0.5 [h], was investigated; subsequently the scour-hole depth was linearly extended on the semi-log scale to 72 [h]; no velocity measurements were performed. The present data are put in context with some (popular) existing relations; recommended is a modification of some of these relations。     

Laboratory modelling of gravel braided stream morphology

Hydraulic modelling principles, together with a knowledge of channel pattern thresholds, allow the development of a small scale model of a gravel braided stream with flow characteristics and equivalent dimensions of a natural river. The forms and processes of natural gravel braided rivers are reproduced by imposing a constant flume discharge and slope, and maintaining approximate equilibrium with an adjustable sediment feed. Beginning from a straight trough, braiding is initiated by development of a series of alternating bars and scour pools which produce bends of increasing amplitude, leading finally to channel division. These lobate bars accrete downstream by deposition of bed material at their margins, often in the form of avalanche faces. Together with the scour pools with which they are necessarily closely associated, these bars are the fundamental elements of the channel pattern. Channel migration and division is a response to the development of bars, and these adjustments leave portions of the originally active bars in the form of exposed and eroded remnants. Complex flats built from these lobate forms show varying degrees of preservation of the original depositional units, but the model allows observation of the systematic construction of some flats. Sorting of sediment on active bars with avalanche faces shows a distinct fining downstream. This may be the result of the accretion of fining upwards avalanche faces along the bar margins rather than a ‘winnowing out’ of fine material. The processes and forms observed in the model appear to be very similar to those occurring in natural gravel braided streams during peak flows.     

Morphologic adjustments of actively evolving highly curved neck cutoffs

Neck cutoffs and their resultant oxbow lakes are important and prominent features of riverine landscapes. Detailed field-based research focusing on the morphologic evolution of neck cutoffs is currently insufficient to fully characterize cutoff evolution. High-resolution bathymetric data were collected over 3 years for the purpose of determining channel morphology and morphologic change on three actively evolving neck cutoffs. Results indicate the following general trends in morphologic adjustment: (1) a longitudinal bar in the upstream meander limb that develops near the entrance to the abandoned bend; (2) a deep scour hole in the downstream meander limb immediately downstream of the cutoff channel; (3) erosion of the bank opposite the cutoff in the downstream meander limb; (4) a cutoff bar in the downstream meander limb at the junction corner of the cutoff channel and the downstream meander limb; and (5) perching of the exit of the abandoned bend above the cutoff channel due to channel bed incision. The results presented herein were used to develop a conceptual model that depicts the morphologic evolution of highly curving neck cutoffs. The findings of this research are combined with recent analyses of the three-dimensional flow structure through neck cutoffs to provide a mechanistic explanation for the morphodynamics of neck cutoffs. © 2019 John Wiley & Sons, Ltd.