Scour Zone Research Articles

Numerical analysis of the effect of momentum ratio on the dynamics and sediment‐entrainment capacity of coherent flow structures at a stream confluence

The flow and turbulence structure at stream confluences are characterized by the formation of a mixing interface (MI) and, in some cases, of streamwise‐oriented vortical (SOV) cells flanking the MI. Depending on the junction angle and planform symmetry, as well as the velocity ratio across the MI, the MI can be in the Kelvin‐Helmholtz (KH) mode or in the wake mode. In the former case, the MI contains predominantly co‐rotating large‐scale quasi two‐dimensional (2‐D) eddies whose growth is driven by the KH instability and vortex pairing. In the latter case, the MI is populated by quasi 2‐D eddies with opposing senses of rotation. This study uses eddy resolving simulations to predict details of flow structure for KH‐ and wake‐mode conditions at a confluence for which field measurements are available. Results indicate that SOV cells at this confluence, which occur in both modes, redistribute momentum and mass, enhancing the potential for entrainment of bed material beneath the cells and for extraction of fluid and suspended sediment from the MI. The simulations predict that the cores of some of the primary SOV cells are subject to large‐scale bimodal oscillations toward and away from the MI that contribute to amplification of the turbulence close to the MI and enhance the capacity of the SOV cells to entrain sediment. At this confluence, which has a concordant bed and a large angle between the incoming streams ‐ conditions that generate strong adverse lateral pressure gradients adjacent to the MI ‐ the oscillating SOV cells interact with MI eddies to generate large bed friction velocities in the zone of scour immediately downstream of the confluence.

Aeolian sediment transport and morphologic change on a managed and an unmanaged foredune

ABSTRACTSediment transport and short‐term morphologic change were evaluated at a site where sand fences are deployed and the beach is raked (Managed Site) and a site where these human adjustments are not practiced (Unmanaged Site). Data were gathered across the seaward portion of a low foredune when winds blew nearly shore‐normal at mean speeds 8.9 to 9.3 m s‐1. Data from traps revealed sediment transport rates at unvegetated portions of the foredune crest (40.2 to 43.5 kg m‐1 h‐1) were greater than on the backshore (4.9 to 11.2 kg m‐1 h‐1) due to onshore decreases in surface moisture and speed‐up of the wind passing over the foredune. Data from erosion pins indicate sediment input to the dune was 1.48 m3 m‐1 alongshore at the Managed Site and 1.25 m3 m‐1 at the Unmanaged Site. The Unmanaged Site had deposition at the dune toe, erosion at mid‐slope, and deposition at the crest. Deposition occurred at mid‐slope on the Managed Site near a partially buried (0.58 m high) fence with a porosity of about 65%. Deposition at partially buried wrack on the upper backshore and dune toe at the Unmanaged Site was about twice as great as deposition in this zone at the Managed Site. Results indicate that: (1) the seaward slope of the foredune can be a more important source of sand to the lee of the crest than the beach; (2) wrack near the toe can decrease transport into the foredune; (3) a scour zone can occur on the foredune slope above the wrack line; (4) a fence placed in this location can promote deposition and offset scour, but fences can restrict delivery of sediment farther inland. Evaluation of alternative configurations of fences and strategies for managing wrack is required to better determine the ways that humans modify foredunes. Copyright © 2012 John Wiley & Sons, Ltd.

Turbulence and Vorticity in a Laboratory Channel Bend at Equilibrium Clear-Water Scour with and without Stream Barbs

Laboratory flume experiments were conducted to study the flow field and sediment dynamics in a mobile-bed channel bend with and without the presence of stream barbs (upstream-angled submerged groynes). This paper presents spatial distributions of the vorticity field and the turbulent Reynolds shear stresses, kinetic energy, and integral scales. The objectives of the present paper were to (1) advance understanding of the role that vorticity and turbulence play in the erosion process in channel bends with and without barbs, and (2) optimize barb design for reducing erosion along the outer bank regions of channel bends through consideration of flow-field turbulence and vorticity. Results indicate that (1) increased z-vorticity occurred in the outer bank scour zone for all runs with and without barbs; (2) for the runs without barbs and with moderately sized barbs, increased streamwise-cross-stream Reynolds stress (τuv) was found to coincide with locations of scour; and (3) for the run with large barbs, all three Reynolds shear stresses increased, but τvw was most consistently associated with scour locations. In general, local scour near the barbs was associated with increased z-vorticity, tke, and nonprincipal Reynolds stresses (τuv and τvw). Furthermore, it was observed that the outer bank region (particularly between adjacent barbs) may be susceptible to increased erosion if stream barbs, due to their size and layout, generate excessive turbulence and secondary velocities.

Deflector-jets affected by pre-aerated approach flow

Jets generated at chute ends move a potential scour zone away from the dam, and dissipate flow energy to reduce the scour. Usually, so-called flip buckets or ski jumps are provided to generate jets, yet they are rarely issued by deflectors. The application of a theoretical mass point parabola is limited to predict the jet features, because effects related to the take-off angle and the disintegration process affect the jet air transport and trajectories. This research discusses the effect of pre-aeration of the approach chute flow on the jet features. A comparison with a former study indicates that mainly the upper jet portion is affected, that the black water jet core becomes shorter, and that the air transport increases. Although the lower trajectory is less affected and thus remains more coherent, the small effect of pre-aeration is sufficient to reduce the total jet throw length, so that pre-aerated jets tend to be shorter as compared with black-water approach flow jets, in agreement with prototype observations.

Turbulent burst around experimental spur dike

Three-dimensional turbulent flow field was measured around an experimental spur dike by using a micro-acoustic Doppler velocimeter. Time fractions of turbulent burst events including outward interaction, ejection, inward interaction, and sweep were analyzed in each quadrant at the neighbor of the dike before and after the formation of scour hole. Over 80% of burst events near the bed have lower-order of magnitudes for both flat and scoured bed surfaces. Ejections and sweeps are prevalent before the local scour was initiated, and then outward interactions are dominant after the scour hole was formed. Conditional Reynolds stresses and high-order moments of turbulent velocities were analyzed along the thalweg. The magnitudes of u′– w′ pair were much larger than those of v′– w′ pair in the scour zone. Among four burst events, ejections and sweeps are the higher order events contributing to the Reynolds stresses. Since sediment entrainment and transport are closely associated with turbulent bursts near bed, the development of scour hole greatly depends on the higher order event near the bed.

Reply to the discussion on “Three-dimensional numerical study of flow structure in channel confluences”Appears in the Canadian Journal of Civil Engineering, 37(5): 772–781.

The authors would like to thank the discussers for their interest, comments, and suggestions regarding the article. The discussers have raised an interesting concern regarding the presence of sediment transport and its effect on the morphodynamic features in the channel confluences. As the discussers also commented, despite the number of works on this important issue, it still has not been completely understood. Some experimental works tried to provide a better understanding of the morphological features. Best (1988) characterized confluence bed morphology by scour zone in center of the confluence, avalanche face that forms in the mouth of upstream channels, and deposition zones. This study and some other works (e.g., Mosley 1976; Ashmore and Parker 1983) have investigated the depth and location of scour hole and shape of avalanche face at different conditions. Borghei and Sahebari (2010) have recently conducted some experiments on the morphological features such as the shape and size of the scouring hole at different geometries and flow conditions. The interesting recent work of the discussers, experimentally investigated the hydro–morpho–sedimentary processes in channel confluences in the laboratory using a 908 confluence flume (Leite Ribeiro et al. 2009, 2010). Five main morphological features, associated to six main flow features have been described in their study. In their work the discussers used a non-uniform and coarse sediment input to the main channel that caused sediment bars (gravel bars) to form, which changed the flow characteristic and the secondary cells. The important issue of the morphology of the confluence and its effect on the flow structure has also been considered by the authors. The authors previously (Shakibaeinia et al. 2007) investigated the morphodynamics of channel confluences and its effect on the flow structure using a 3D numerical model. The model was validated and evaluated using some experimental and field measurements. The model then was applied to predict the morphology and flow structure of channel confluences with different conditions (four confluence angles and three discharge ratios). The model was applied to a flat (but moveable) bed and it predicted the new morphology of the confluence. In addition to the main flow features (e.g., secondary flow and flow separation) three main morphological features, addressed by Best (1988), were also investigated by Shakibaeinia et al. (2007). The features include the scour hole in the beginning of the post confluence channel, the avalanche face at the mouth of the upstream channels, and the deposition zone associated with the flow separation area (Fig. 1). Four confluence angles (a = 15, 45, 90, and 105 degrees) and three discharge ratios (Qr = 0.25, 0.50, and 0.75) were considered for the modeling. The size and shape of each of the morphological features and their relation to the flow condition was investigated. A sample of the results can be seen in Fig. 2. The hydrodynamic features (after the bed morphology reached its steady state condition) were also investigated at different flow conditions. Some sample results can be found in Fig. 3 and Fig. 4. Detailed results can be found in Shakibaeinia et al. (2007). The advantages of this work compared to other studies on the morphodynamics of channel confluences is that it considers a wider range of flow conditions. The suggestion for further studies is to investigate the effect of the erodibility of the channel banks, which has a great effect on the morphodynamics of the confluences. In natural channels the flow condition in a confluence changes the channel bed configuration, as well as it deforms the channel banks. This important fact is ignored by most of the numerical and experimental studies (e.g., the works done by the authors and discussers). The outer bank of the post-confluence channel is affected by the tributary channel flow and is eroded while the inner bank is affected by the flow separation, which causes the sediment deposition in this area. The erosion and deposition eventually reshapes (curves) the post-secondary channels (as can usually be Received 10 November 2010. Revision accepted 16 November 2010. Published on the NRC Research Press Web site at cjce.nrc.ca on 8 December 2010.

The localized role of base level lowering on channel adjustment of tributary streams in the Trinity River basin downstream of Livingston Dam, Texas, USA

The purpose of this study is to describe and explain river channel cross-sectional change in tributary streams within the 60-km scour zone of the Trinity River, Texas, downstream of Livingston Dam. This study investigates changes within the tributary systems to base level changes triggered by incision, associated with scour and channel migration within the dammed trunk stream. The dam represents a precise moment and place of a system perturbation and therefore represents an opportunistic geomorphic experiment. Geomorphological effects of the lower Trinity River tributaries were investigated through three different types of data: resurveys of bridge cross sections, planform change as measured from aerial photographs, and field mapping of changes in channel geometry using fallen trees, undercut banks, and other geomorphic indicators of change. While field evidence of channel change was found at sites well upstream of tributary confluences with the Trinity, these were not consistent with responses to lower base level. Downcutting and lateral changes in response to lowered base level were confined to lower tributary reaches. This suggests distance decay and lag times in transmission of responses upstream and/or masking of dam-induced effects by other processes and controls. The most dynamic locations of geomorphic change are near the confluences of the tributaries with the Trinity River, where the response to dam-induced incision is strongly influenced by local trunk stream morphology, indicating a high degree of historical and geographical contingency.

Turbulence, displacement, death and worms: a day in the life of a fluvial Carboniferous bivalve

Kane, I.A. 2010: Turbulence, displacement, death and worms: a day in the life of a fluvial Carboniferous bivalve. Lethaia, Vol. 43, pp. 381–395. In the Pennsylvanian Rough Rock Flags and Rough Rock of northern England trace fossils attributed to the non-marine bivalve Carbonicola are found. Carbonicola, recorded by Lockeia and associated trace fossils, lived a semi-infaunal lifestyle and thus were influenced by both the sediment in which they were hosted, and the currents which supplied their nutrients and oxygen. A number of palaeocurrent indictors are commonly associated with Lockeia: (1) downstream inclination of vertical burrows; (2) palaeoflow-parallel orientation of long axes; (3) steeper scouring and higher sediment surface on the upstream side; and (4) diffuse lamination downstream of the trace or more widespread downstream erosion. Semi-infaunal, Carbonicola bivalves were partly exposed to the prevailing flow and acted as bed defects; flow separation and acceleration enhanced flow turbulence around the bivalve leading to erosion and the development of a variably developed fan-shaped zone of scour immediately downstream. Disturbance and destabilization of sediment in this way may have affected bivalves immediately downstream, plausibly explaining the relatively regular spacing pattern of individual or clustered Lockeia, exposed on bedding planes and revealed by nearest neighbour analyses. Bivalves that did not survive high-energy flow events were either trapped within the sediment, or transported downstream and deposited in lower energy environments within the otherwise high-energy deposits of the Rough Rock.

Response of bed morphology and bed material texture to hydrological conditions at an asymmetrical stream confluence

Changes in bed morphology and sedimentology at stream confluences in relation to specific hydrological events have not been documented extensively. Moreover, few, if any, studies have investigated the effect of net hydrological conditions on channel change at a confluence over a period of many years. Repeat cross section surveys and sampling of bed material at a small, asymmetrical stream confluence reveal the short-term effects of hydrological variability on bed morphology and spatial patterns of bed material texture. Survey data over a 15-year period illustrate the long-term response of channel morphology at the confluence to net hydrological conditions. Results confirm that this confluence is a dynamic fluvial environment, involving shifts in the zone of scour and repeated deposition and erosion of a bar complex at the downstream junction corner as the discharge ratio changes in response to hydrological variability in the tributary streams. Persistent faster runoff-response times of the lateral tributary compared to the main stem have resulted in long-term net erosion of the channel bank opposite this tributary. The documented morphological and sedimentological changes are consistent with and can be explained by results of experimental studies and by past investigations at this confluence that have documented changes in patterns of mean flow structure and turbulence in response to variation in the discharge ratio of contributing streams.

Buffered, lagged, or cooled? Disentangling hyporheic influences on temperature cycles in stream channels

We monitored summertime base flow water temperatures of hyporheic discharge to surface water in main, side, and spring channels located within the bank‐full scour zone of the gravel‐ and cobble‐bedded Umatilla River, Oregon, USA. Diel temperature cycles in hyporheic discharge were common, but spatially variable. Relative to the main channel's diel cycle, hyporheic discharge locations typically had similar daily mean temperatures, but smaller diel ranges (compressed by 2 to 6°C) and desynchronized phases (offset by 0 to 6 h). In spring channels (which received only hyporheic discharge), surface water diel cycles were also compressed (by 2 to 6°C) and desynchronized (by −4 to 6 h) relative to the main channel, creating diverse daytime and nighttime mosaics of surface water temperatures across main, side, and spring channels, despite only minor differences (<1°C) in daily mean temperatures among the channels. The river's hyporheic zone received and stored heat from the channel, yet hyporheic return flows carried heat back to the channel minutes to months after removal. Associated surface water temperature dynamics were therefore complex. Hyporheic discharge was not simply “cooler” or “warmer” than main channel water. Instead, instantaneous temperature differences between channel water and hyporheic discharge typically arose from diel temperature cycles in hyporheic discharge that were buffered and lagged relative to diel cycles in the main channel.

Opposite hysteresis of sand and gravel transport upstream and downstream of a bifurcation during a flood in the River Rhine, the Netherlands

AbstractAt river bifurcations water and sediment is divided among the downstream branches. Prediction of the sediment transport rate and division thereof at bifurcations is of utmost importance for understanding the evolution of the bifurcates for short-term management purposes and for long-term fluvial plain development. However, measured sediment transports in rivers rarely show a uniquely determined relation with hydrodynamic parameters. Commonly a hysteresis is observed of transport rate as a function of discharge or shear stress which cannot be explained with the standard sediment transport predictor approach. The aim of this paper is to investigate the causes of hysteresis at a bifurcation of the lower Rhine river, a meandering river with stable banks, large dunes during flood, and poorly sorted bed sediment. The hydrodynamics and bed sediment transport were measured in detail during a discharge wave with a recurrence interval larger than 10 years. Surprisingly, the hysteresis in bedload against discharge was in the opposite direction upstream and downstream of the bifurcation. The upstream clockwise hysteresis is caused by the lagging development of dunes during the flood. The counter-clockwise hysteresis downstream of the bifurcation is caused by a combination of processes in addition to dune lagging, namely 1) formation of a scour zone upstream of the bifurcation, causing a migrating fine sediment wave, and 2) vertical bed sorting of the bed sediment by dunes with avalanching lee-sides, together leading to surface-sediment fining and increased transport during and after the flood. These findings lead to challenges for future morphological models, particularly for bifurcations, which will have to deal with varying discharge, sediment sorting in the channel bed, lagging dunes and related hydraulic roughness.

Channel adjustments of the lower Trinity River, Texas, downstream of Livingston Dam

AbstractChannel cross‐sectional changes since construction of Livingston Dam and Lake Livingston in 1968 were studied in the lower Trinity River, Texas, to test theoretical models of channel adjustment, and to determine controls on the spatial extent of channel response. High and average flows were not significantly modified by the dam, but sediment transport is greatly reduced. The study is treated as an opportunistic experiment to examine the effects of a reduction in sediment supply when discharge regime is unchanged. Channel scour is evident for about 60 km downstream, and the general phenomena of incision, widening, coarsening of channel sediment and a decrease in channel slope are successfully predicted, in a qualitative sense, by standard models of channel response. However, there is no consistent channel response within this reach, as various qualitatively different combinations of increases, decreases or no change in width, depth, slope and roughness occur. These multiple modes of adjustment are predicted by the unstable hydraulic geometry model. Between about 60 km and the Trinity delta 175 km downstream of the dam, no morphological response to the dam is observed. Rather than a diminution of the dam's effects on fluvial processes, this is due to a fundamental change in controls of the fluvial system. The downstream end of the scour zone corresponds to the upstream extent of channel response to Holocene sea level rise. Beyond 60 km downstream, the Trinity River is characterized by extensive sediment storage and reduced conveyance capacity, so that even after dam construction sediment supply still exceeds transport capacity. The channel bed of much of this reach is near or below sea level, so that sea level rise and backwater effects from the estuary are more important controls on the fluvial system than upstream inputs. Copyright © 2005 John Wiley & Sons, Ltd.

Bedload path length and point bar development in gravel‐bed river models

AbstractLow‐sinuosity meandering gravel‐bed flume experiments were employed to investigate spatial patterns of deposition, which point to patterns of channel development related to the pool and bar morphology. At channel‐forming discharges, fluorescent bedload tracers indicate that deposition is typically focused around the point bar apex, downstream of the apex (contributing to downstream bar migration), and at the bar head/riffle surface. Seven flume experimental runs illustrate a sequence of point bar development related to the spatial patterns of tracer deposition, and the related path length distribution. At early stages of bar formation, transport is from the scour zone across the point bar head to the bar apex and bar margin downstream of the apex. As the point bar develops, bedload transport across the bar decreases, as transport along the channel thalweg increases and sediment is deposited along the bar margin. Deposition cells appear to move from downstream to upstream of the bar apex as this sequence of bar formation progresses. At low (non‐channel‐forming) discharges, transport occurs to the bar head/riffle surface with very little material being transported to the apex region or point bar interior. The implication is that there is an inherent connection between the loci of particle deposition and point bar formation, largely controlled by the morphology of the channel.

Testing a blowing snow model against distributed snow measurements at Upper Sheep Creek, Idaho, United States of America

In this paper a physically based snow transport model (SnowTran‐3D) was used to simulate snow drifting over a 30 m grid and was compared to detailed snow water equivalence (SWE) surveys on three dates within a small 0.25 km2 subwatershed, Upper Sheep Creek. Two precipitation scenarios and two vegetation scenarios were used to carry out four snow transport model runs in order to (1) evaluate the blowing snow model, (2) evaluate the sensitivity of the snow transport model to precipitation and vegetation inputs, and (3) evaluate the linearity of snow accumulation patterns and the utility of the drift factor concept in distributed snow modeling. Spatial comparison methods consisted of (1) pointwise comparisons of measured and modeled SWE, (2) visual comparisons of the spatial maps, (3) comparisons of the basin‐wide average SWE, (4) comparisons of zonal average SWE in accumulation and scour zones, and (5) comparisons of distribution functions. We found that the basin average modeled SWE was in reasonable agreement with observations and that visually the spatial pattern of snow accumulation was well represented except for a pattern shift. Pointwise comparisons between the modeled and observed SWE maps displayed significant errors. The distribution functions of SnowTran‐3D‐modeled drift factors from two precipitation scenarios on three dates were compared with the distribution function of observation‐based drift factors obtained previously by calibration to evaluate the assumption of linearity. We found only a 14% reduction in explained variance in the distribution function of drift factors for a 69% increase in precipitation, suggesting that the simplification provided by the use of drift factor distributions will result in errors that are tolerable in many cases.

Blowing sand and dust hazard, Tabuk, Saudi Arabia

Rationale Blowing sand and dust are increasingly common hazards associated with dryland economic development, largely because of disruption of desert soils by human activity (Cooke et al. 1982; Cooke & Doornkamp 1990). Blowing sand has long been recognized as a problem; it has clearly identifiable sources (blowouts, scour zones) and sinks (drifts, dunes) and is amenable to a range of management techniques once the problem has been assessed objectively (Cooke et al . 1982, 1993). The ‘dustification’ problem, by contrast, has come to prominence morev recently. It is a more complex process involving greater transport distances, and results in a very wide range of adverse impacts including abrasion to paintwork, damage to machinery, contamination of houses, food and water, electrical short-circuits, disruption to radio signals, killing of plants, suffocation of livestock, the spreading of disease, disruption to transport, and general reduction in environmental quality and amenity. The different modes of transport - saltation for sand and suspension for dust - means that while defensive measures may be employed against sand, they are less effective in the case of dust where the emphasis must be placed on curbing the production of fine-grained sediment from identified source areas.

Geometry of Scour Holes Associated with 90° Spur Dikes

The stable pools formed by spur dikes can be a significant factor for improving aquatic habitat in unstable degraded streams. Prediction of the volume and geometry of the scour hole associated with a spur dike yields information that is valuable for assessing the potential benefit to the aquatic habitat. Volumes of the scour hole in the vicinity of model spur dikes were measured in a laboratory flume under clear-water overtopping flows. Spur dike length, flow depth, and shear velocity ratio were varied in the experiments and found to significantly influence the volume of the scour hole. For overtopping flows, the ratio of the flow depth to the spur dike height was found to be an important control on the geometry of the resulting scour hole. At higher flow depth to spur dike height ratios, the location of the maximum depth of scour changed from the upstream point of the dike toward the channel bank and caused a secondary scour zone to form downstream of the spur dike. A preliminary technique is proposed to predict the volume of scour for spur dikes perpendicular to the bank.

Reinforced Soil for Bridge Support Applications on Low-Volume Roads

A quick and simple method of bridge substructure construction using geosynthetic reinforced soil (GRS) is illustrated. GRS is used to build abutments and pier foundations for simple bridges. It is a refinement of existing reinforced soil technology used during the last 20 years. The interaction of a closely spaced geosynthetic reinforced soil system and the reasons why conventional design methods are not appropriate for these closely spaced systems are explained. This method is not recommended for all bridge building assignments; for example, it is not suitable for construction of permanent bridges in scour zones. The technique is ideal for remote locations, inaccessible to use of concrete and other traditional materials. A generic style of GRS construction is explained to ensure performance and internal stability. Construction is rapid with conventional equipment. The materials are common, inexpensive, and generally available. An overview of recent full-scale research is provided. The results of two full-scale prototype tests are presented to demonstrate performance and limitations and to confirm the design of such systems. A case history is presented that shows the versatility of the technology in a bridge support application. A procedure for prestraining or preloading the reinforced soil to enhance performance is provided. For bridge support applications, preloading of the GRS has the benefit of limiting postconstruction creep settlement. Preloading also proof-tests the structure and verifies the quality of construction. Additional sketches are included to show its potential for common applications. A brief discussion about design considerations to limit potential problems is offered.

Physical Model Studies of a Real Time Riverbed Scour Measurement System

Riverbed scour around bridge foundations has led to numerous bridge failures due to the undermining of piers or abutments during floods. Remote monitoring of scour processes becomes very difficult during flood periods, at which time monitoring is most needed. The objective of the present study was to show that seismic methods could be used as the basis for a real time scour monitor system. These methods are based on using travel times of directly transmitted and refracted waves propagating through and in the vicinity of the bridge foundations. A series of experiments designed to test these concepts was carried out using a physical model. The physical model consisted of a rectangular or solid wall pier partly submerged in water and mounted in a sand layer. The wave generation and detection of the seismic energy was carried out with acoustic transducers. Initial tests showed that refracted waves and direct waves could be generated, propagated, and detected in this simplified model of a bridge pier. The results from the refraction wave tests and the direct wave tests indicate that both these methods could be useful in detecting the presence of scour zones. The experiments also suggested that these techniques could be used in an on-line scour monitor system.

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.

Turbulent flow structure at concordant and discordant open-channel confluences

Models of flow at river-channel confluences that consist of two concordant confluent channels with avalanche faces dipping into a scour zone are limited because this morphology may be the exception rather than the rule in nature. In this paper the mean and turbulent flow structure in the streamwise and vertical directions at both concordant and discordant laboratory confluences were examined in order to determine the effect of bed discordance on the flow field, and to assess its influence on sediment transport. Instantaneous velocities were measured with a laser Doppler anemometer using a dense spatial sampling grid. The spatial distribution of normal stress varies with bed geometry as bed discordance generates a distortion of the mixing layer between the confluent streams. Turbulent shear stress is larger in the discordant bed case and its peak is associated with the position of the mixing layer whereas for concordant beds the zone of mixing is characterised by a decrease in the Reynolds shear stress. Quadrant analysis also revealed differential dominating quadrants between the two bed geometries which will influence sediment transport routing and, consequently, the resulting bed morphology. These results highlight the need for significant modifications to current models of confluence flow dynamics in order to account for the bed configuration.