Bendway Weirs Research Articles

Performance evaluation of bendway weirs with sloped and flat crest in mild and sharp bends

ABSTRACT The main aim of the present experimental study was to investigate the performance of bendway weirs (BWs) in large-scale channel bends with three different relative curvatures (i.e. the ratio of central-bend radius to top channel width). The geometry of BWs (such as inclination angle, length, and spacing) was chosen based on the state-of-the-art of design criteria. However, the significance of important hydraulic geometry (crest slope) was also examined in these different bends. The results indicate that the performance of sloped crest weirs in protecting the concave bend erosion was better and worse in moderate and sharp bends, respectively, compared to flat crest. Bendway weirs with horizontal crest and submergence ratio of 0.7 are found to be the most efficient in developing sediment deposition near the outer bank, even in sharp bends.

Large‐Scale Channel Response to Erosion‐Control Measures

AbstractErosion‐control measures in rivers aim to provide sufficient navigation width, reduce local erosion, or to protect neighboring communities from flooding. These measures are typically devised to solve a local problem. However, local channel modifications trigger a large‐scale channel response in the form of migrating bed level and sediment sorting waves. Our objective is to investigate the large‐scale channel response to such measures. We consider the lower Rhine River from Bonn (Germany) to Gorinchem (the Netherlands), where numerous erosion‐control measures have been implemented since the 1980s. We analyze measured bed level data (1999–2020) around four erosion‐control measures, comprising scour filling, bendway weirs, and two fixed beds. To get further insight on the physics behind the observed behavior, we set up an idealized one‐dimensional numerical model. Finally, we study how the geometry and spacing of the measures affect channel response. We show that erosion‐control measures reduce the sediment flux due to (a) lack of erosion over the measure and (b) sediment trapping upstream of the measure, resulting in downstream‐migrating incision waves that travel tens of kilometers at decadal timescales. When the measures are in close proximity, their downstream effects may be amplified. We conclude that, despite fulfilling erosion‐control goals at the local scale, erosion‐control measures may worsen large‐scale channel‐bed incision.

Illustrating land cover change associated with erosion management of the Little Blue River, Kansas, USA

AbstractErosion is a concern due to environmental degradation, loss of valuable cropland, increased sediment loads in aquatic systems, and reduced reservoir capacity. To manage erosion, riparian forest buffers and bendway weirs were installed in the Little Blue River, Kansas, during years 2002–2010. To illustrate land cover changes associated with management for upland and streambank erosion, indicated through terrestrial land gains, particularly of permanent tree cover relative to short‐lived crop cover, we digitized 1 m orthoimagery. For the pretreatment interval of 1991–2002, we digitized streambank edges to approximate change in river area. Due to river dynamics, for the 2002 and 2014 interval before and after treatment, we used land ownership parcels as fixed locations to assess changes in land area and land classes for 24 treated and 24 untreated parcels, each parcel group totaling 1575 ha. We appraised two extents of the unified streambank for both years and all land in parcels rather than isolating the river from the surrounding watershed. During 1991 to 2002, treated land parcels lost terrestrial land, whereas untreated land parcels gained land. During 2002–2014, treated land had greater river and crop cover, and less tree cover than untreated land. For the entire extent, with similar trends for the unified streambank extent, by 2014, treated parcels gained 27.3 ha of terrestrial land compared to 4.6 ha gained by untreated parcels. Gains occurred in tree cover and losses in river water and sediments cover. In treated parcels crop cover decreased, whereas in untreated parcels crop cover increased. Streamflows decreased over time, likely contributing to streambank stability. Despite lack of documented cost‐share agreements, in untreated parcels, landowners managed land by increasing tree cover to protect soil from erosion. All measurements were consistent with erosion followed by management for erosion through terrestrial land gains of tree cover.

Effects of relative curvature radius on erosion and sedimentation patterns in a 90o bend with and without bendway weirs

Bendway weirs are low-level transverse structures constructed on the outer bank of a channel bend to improve navigation, create aquatic habitat and control bend migration. This experimental study was aimed at investigating the erosion and sedimentation patterns in a 90° bend by varying the relative curvature radius (i.e. normalized to the channel width), in the absence and in the presence of bendway weirs, under different flow conditions. A series of flat weirs with relative heights of 30% and 70% were considered. The results indicate that as the relative curvature increases, the erosion near the weirs decreases while the point-bar at the inner bend wall extends. Regardless of the weir height, the increase in the relative curvature leads to a deeper scour depth at the tip of the weirs. Bendway weirs with submergence ratio of 70% are found to be the most efficient in protecting the outer bank from erosion.

Hydrodynamics of a tidal inlet under gray to green coastal protection interventions

Structural coastal protection interventions have been implemented worldwide to stabilize channels and shorelines in tidal inlets. Although these conventional “gray” interventions typically attain their goals, there is an increasing interest and need to consider nature-based or “green” interventions that also address system resilience and environmental impacts. For a better implementation of gray to green interventions in tidal inlets, their effectiveness and their effects on the velocity of these dynamic coastal systems need to be understood. The effects on flow velocity of gray to green coastal protection interventions for tidal inlets are assessed here by exploring six interventions intended to protect against erosion on the estuarine-side shoreline near the inlet. A field-calibrated numerical model with Willmott Skill scores rated as excellent for Oregon Inlet located in North Carolina, United States, is used to simulate tidal currents under present conditions and after implementing a seawall, a set of bendway weirs, a terminal groin extension, a dual-jetty system, a flood channel relocation, and an island restoration project. Comparisons of time series of flow velocities in the flood channel along an eroding, estuarine shoreline are used to identify the effectiveness of each coastal protection alternative at reducing erosive velocities. Geospatial difference maps are used to determine velocity changes caused by each alternative throughout the inlet system. With no coastal protection interventions, the velocities along the eroding shoreline exceeded an erosive threshold velocity (defined as 0.2 m/s) during 50% of the simulated period. Alternatives closer to the green side of the coastal protection intervention spectrum, such as channel relocation and island restoration, tend to display the most effectiveness at reducing flow velocities at the eroding shoreline while resulting in minimal inlet-wide hydrodynamic changes. On the other hand, gray alternatives either cause minimal (seawall and bendway weirs) or extreme (jetties) changes in velocities throughout the inlet system. This comparison of gray and green coastal protection interventions in tidal inlets serves as an example to contrast the effectiveness of different coastal protection alternatives at reducing erosive flow velocities and to inform alternative selection at other inlet systems.

Effect of Clearwater Scour on the Flow Field at a Single Bendway Weir: Two-Dimensional Numerical Modeling Supported by Flume Data

This study used two-dimensional (2D or depth-average) numerical modeling to examine the effects of clearwater scour on the flow field at a single bendway weir (BW) set at angles of 30°, 60°, and 90° pointing into the approach flow. The flow field was characterized in terms of depth-average velocity (location and magnitude) through the contraction formed around the BW and was supported by data and observations obtained from large and small flumes. In addition, a three-dimensional (3D) model was used to assess when a 2D model is of design use and when it is not. The 2D modeling showed that, when the ratio of flow depth (about four BW lengths upstream) to BW height (Y∶H) equaled 2.0, the differences (pre- and post-scour) in depth-average velocity magnitude and location of flow around the BW were fairly negligible for all orientations of the BW. However, when Y∶H equaled 1.25, the effects of the scour on the flow field were more prominent. Similar results were obtained for all three orientations of the BW. The value of Vmax obtained with the 2D model was closer to that measured in the large flume when Y∶H decreased from 2.0 to 1.25. This was due to the shallower flow’s greater effect in directing the flow around the BW than occurred for the deeper flow, in which flow had a larger vertical component of velocity when passing over the top of the BW. The practical implications of using a 2D model versus a 3D model were briefly assessed, and 2D models were found to be suitable for designing BWs to manage thalweg position but inadequate for estimating near-bank velocities.

Optimal Configuration of Rock Vanes and Bendway Weirs for River Bends: Numerical-Model Insights

AbstractPresented are insights from three-dimensional numerical models used to identify optimal performance configurations of rock vanes and bendway weirs placed in midsize rivers, such as the Midd...

Two-Dimensional Numerical Modeling of Flow in Physical Models of Rock Vane and Bendway Weir Configurations

While there are a wide range of design recommendations for using rock vanes and bendway weirs as streambank protection measures, no comprehensive, standard approach is currently available for design engineers to evaluate their hydraulic performance before construction. This study investigates using 2D numerical modeling as an option for predicting the hydraulic performance of rock vane and bendway weir structure designs for streambank protection. We used the Sedimentation and River Hydraulics (SRH)-2D depth-averaged numerical model to simulate flows around rock vane and bendway weir installations that were previously examined as part of a physical model study and that had water surface elevation and velocity observations. Overall, SRH-2D predicted the same general flow patterns as the physical model, but over- and underpredicted the flow velocity in some areas. These over- and underpredictions could be primarily attributed to the assumption of negligible vertical velocities. Nonetheless, the point differences between the predicted and observed velocities generally ranged from 15 to 25%, with some exceptions. The results showed that 2D numerical models could provide adequate insight into the hydraulic performance of rock vanes and bendway weirs. Accordingly, design guidance and implications of the study results are presented for design engineers.

Mean flow and turbulence characteristics around single-arm instream structures

ABSTRACT Flow patterns around three widely used instream rock structures in an open-channel flume were investigated using high-resolution large-eddy simulations. Specifically, rock vanes at two different installation angles, a J-hook vane, and bendway weirs at two different angles, with realistic geometries, were studied. Velocity fields in the wake of these instream structures were measured using acoustic Doppler velocimetry, and the measurements were compared with large-eddy simulation results. The computed three-dimensional flow fields were subsequently analysed to investigate and compare the flow patterns for five different cases. The analysis revealed important flow features with regard to three-dimensional flow structure, secondary flow patterns, potential vulnerability to scour, and energy dissipation mechanisms for different cases. The newly discovered flow features provided useful insights into understanding the flow mechanisms associated with instream structures.

Large-Flume Tests on Flow Dislodgment of Rocks Forming Bendway Weirs

AbstractThis paper presents findings from large-flume tests conducted to determine the velocity magnitudes at which water flow dislodges rocks from bendway weirs (BWs), rock structures used primari...

Erosion and sedimentation patterns associated with restoration structures of bendway weirs

Bendway weirs (B.W.), known as new and innovative river training structures, are positioned from the outer bank of the river bend and angled upstream toward the flow. In order to investigate the effect of bendway weirs on the sediment process, experiments were conducted in a 90° bend with a relative curvature of 3.3 with and without bendway weirs. Following the installation of bendway weirs with different crest slopes, the scour and sedimentation patterns were evaluated under clear water conditions. To this end, a series of B.W. (7 weirs) with three crest slopes (0, 5, and 10 percent) were installed at the outer bank side in different hydraulic conditions. Erodible bed topography measurements revealed that the maximum scour depth occurred at the tip of the first and last weirs. Moreover, the findings of this study clearly demonstrates the potential of bendway weirs in modifying morphology of river bends and their efficiency in reducing erosion and sedimentation. In general, compared to non-weirs condition, the eroded sediment volume increased by 53, 36, and 15 percent due to the installation of weirs with crest slopes of 0, 5, and 10 percent, respectively. Finally, this study revealed that the maximum scour depth decreased with an increase in the crest slope.

Implementing streambank erosion control measures in meandering streams: design procedure enhanced with numerical modelling

ABSTRACTDike structures have been used in many streams to control streambank erosion by redirecting the flow away from the banks towards the centre of the channel. In this study, the 2D finite-element surface water modelling system (FESWMS) was used to enhance the design of dike structures in two meandering stream reaches along the Raccoon River, Iowa, USA. FESWMS was used to find the optimal number and spacing between the dikes and to access their overall performance in controlling river-bank erosion in the study reaches. The study included also field measurements for model calibration and verification. The model results showed that the proposed dike structures, consisting of alternate five bendway weirs and four spurs at each site with a spacing of about 50 m, successfully reduced the flow velocity along the outside bank and increased the conveyance of the flow to the stream centre. The average lengths of the spurs and bendway weirs were about 16 and 32 m, respectively. The estimated velocity and Froude number values along the outside riverbank where the dikes were constructed were less than 1.0 m/s and 0.3, respectively, which are within the recommended values for erodible channel stability design. Site visits one year after the dike construction showed that the dikes were successful in controlling streambank erosion by allowing incoming sediment to deposit between the dikes and along the bank line. Sediment transport measurements at the study sites showed that the maximum deposition was about 0.8 m, which is much higher than a typical averaged deposition value of about 0.3 m/year. Further, recent Google Earth pictures for the sites showed that the bank slope lines almost recovered back in their original profiles and that the dikes became part of the restored banks, a strong indication of the success of the proposed dikes design.

Simulation-based optimization of in–stream structures design: bendway weirs

Bendway weirs are one of the most practical in–stream rock structures utilized to protect the outer bend of meandering streams and rivers from erosion. We present development of a simulation-based paradigm for effective design of bendway weir structures to enhance meandering stream bank stability and control lateral migration. To do so, we employ the St. Anthony Falls Laboratory Virtual StreamLab (VSL3D) code to elucidate the flow and sediment transport phenomena induced by interaction of flow, mobile bed, and in–stream structures in large rivers under prototype conditions. We carried out numerous numerical experiments to systematically simulate various arrangements of bendway weir in two river test-beds and gaining insights into the physical mechanisms via which such bendway weirs modify turbulent flow, sediment transport and scour processes. The so-gained physical insights are then taken into account to develop a set of practical physics-based design criteria for optimal placement of bendway weirs in large rivers.

Laboratory Study of Bendway Weirs as a Bank Erosion Countermeasure

AbstractBendway weirs are linear structures similar to spurs but designed to be overtopped by typical streamflows, and have recently attracted attention as an environmentally sensitive means of protecting the outer banks of stream bends. A laboratory study was conducted to examine the effectiveness of these type of structures, constructed following FHWA (U.S. Federal Highway Administration) design-manual guidelines, in protecting the outer bank of a 90° bend in a narrow channel with both erodible bed and banks. Experiments were performed at a single nominal (initial) depth, and so characterized by a single (initial) ratio of radius of curvature to top width of 3.3, with and without a field of five weirs. Measurements of erodible boundary elevations as well as the magnitudes of the time-averaged point horizontal velocities at various cross sections were taken. Although a definite overall deflective effect by the weirs was observed, the outer bank remained susceptible to significant erosion even in the pres...

Scour depth around flat and sloped crest bendway weirs: a laboratory study

ABSTRACTBendway weirs are low stone sills designed to control erosion at the outer bank of river bends, redirecting the current from the outer to the inner bank. Laboratory experiments were conducted in a movable bed channel with three consecutive meandering bends (relative curvature equal to 2) and continuous sediment supply from upstream. The aim of these tests was to study the effects of weir crest slope on maximum scour depth at the weir nose, on point-bar height and on cross-sectional eroded surface area in a meander live-bed. Concrete trapezoidal bendway weirs were constructed at the outer bank of the central bend. The investigated configurations consisted of series (3–5) of weirs characterized by prescribed: inclination angles (60°, 75° and 90°), length ratio (0.2, 0.3 and 0.4), crest slope (0%, 5%, 10% and 20%) discharge ratio (0.8, 1 and 1.2), the same height and the same distance between weir axes. Results showed that the maximum and minimum scour depths at the nose of sloping crest weirs were attained for α = 75° and RL = 0.3, respectively. By increasing the crest slope from 0% to 20%, the scour depth decreased more than three times. In addition, flat weirs appear to be more effective in capturing and redirecting the flow from the outer bank towards the inner bank. Finally, the data set collected during the experimental tests was used to derive an empirical relationship for predicting of maximum scour depth at the weir nose.

Analysis of the Impacts of Dikes on Flood Stages in the Middle Mississippi River

AbstractThe US Army Corps of Engineers has constructed numerous river engineering structures in and along the Middle Mississippi River. River training and bank stabilization measures include pile dikes, stone dikes, bendway weirs, chevrons, and revetments. Concerns have long been voiced about the effects of these structures on flood stages. Recent debate concerning the scientific basis for these concerns has been invigorated by specific gauge analysis that appears to show a rising trend in flood stages over the past 150 years. This paper attempts to advance the debate by providing an objective review of the specific gauge analysis technique that explains how the method should be performed and the results interpreted; identifies strengths and limitations; examines the uncertainties associated with application to the Middle Mississippi River given the available data; and reassesses the conclusions that can and cannot reasonably be drawn regarding the impacts of dikes and levees on flood stages, based on spe...

Evaluation of bendway weir bank stabilization techniques for stream restoration

In this study, velocity distribution patterns in a stream in the presence of bendway weirs were assessed for their effectiveness in bank stabilization in a demonstration site in the Las Vegas Wash, Nevada, USA. Streambanks of the target stream had been stabilized previously by longitudinal stone toe protection (LSTP) and stone spur dikes. Stone toes are meant to protect the toe of streambanks. Similarly, stone spur dikes consist of a series of dikes (of a crest relatively higher than that of the bendway weirs) protruding at right angles from the convex side of the streambanks. Velocity measurements were taken to evaluate the velocity field in the channel in the following three different cases: Case 1, channel with LSTP and the pre-existing spur dikes; Case 2, channel with LSTP with no deflecting structures (spur dikes removed); and Case 3, channel with LSTP and bendway weirs. The results suggested that bendway weirs caused a reduction in the uncontrolled high-velocity flow near the outer bank (right bank), allowing only controlled flow. Thus, bendway weirs were found to be more effective than spur dikes in reducing the velocity of flow at the vulnerable weir zones near the outer banks, thus providing a means to stabilize the streambanks.

Special Issue on River Flow Hydrodynamics: Physical and Ecological Aspects

As part of the celebration in Minneapolis on May 19–21, 2008, of the 70th year since the St. Anthony Falls Laboratory was first dedicated on November 17, 1938, a minisymposium on the topic of stream restoration fluid mechanics was organized within the inaugural conference of ASCE’s Engineering Mechanics Institute. Ten papers are included in this special issue representing contributions that were made to this minisymposium as well as manuscripts that were solicited from other authors in an effort to expand coverage of the subject. This issue provides the opportunity to review the progress that has been made in some areas of modeling river flow hydrodynamics, with emphasis on physical and ecological aspects. The issue begins with two papers dealing with in-stream structures, including those used for flow training and those that serve as bridge foundations, and their potential effects on river behavior. The first paper, “River Training and Ecological Enhancement Potential Using In-Stream Structures” by Radspinner et al., provides a review of the available literature and engineering design experience. Cross vanes, J-hook vanes, rock vanes, W-weirs, submerged vanes, stream barbs, bendway weirs, spurs, and constructed riffles, which represent the most commonly used instream flow control structures, are considered. Survey results obtained from federal and state agency personnel indicate that in terms of cost, performance, maintenance, and environmental enhancement, in-stream structures are preferable to riprap revetment, dredging, and concrete. Results from case studies and discussions with practitioners highlight the need for better-defined guidelines for the design of these structures. In their paper “Coherent Structure Dynamics in Turbulent Flows Past In-Stream Structures: Some Insights Gained via Numerical Simulation,” Paik et al. review the latest developments in the use of hybrid unsteady Reynolds-averaged Navier-Stokes/large-eddy simulation URANS/LES modeling strategies for simulating turbulent flows around in-stream structures. These computational fluid dynamics CFD techniques are capable of resolving coherent structures, such as the turbulent horseshoe vortex past bed-mounted piers, for a wide range of Reynolds number flows and thus of capturing some intricate aspects of the flow field and associated scour in the vicinity of hydraulic structures. Furthermore, the capability of modeling such structures at ecologically relevant scales is crucial for the development of improved stream restoration designs. The issue continues with a group of three papers that focus on the flow through vegetation hydrodynamics and nutrient uptake in streams. The first of these papers, “Effects of Added Vegetation on Sand Bar Stability and Stream Hydrodynamics” by Rominger

Calibration and Verification of a 2D Hydrodynamic Model for Simulating Flow around Emergent Bendway Weir Structures

The predictive capability of a two-dimensional (2D)-hydrodynamic model, the finite-element surface water modeling system (FESWMS), to describe adequately the flow characteristics around emergent bendway weir structures was evaluated. To examine FESWMS predictive capability, a sensitivity analysis was performed to identify the flow conditions and locations within the modeled reach, where FESWMS inputs for Manning’s n and eddy viscosity must be spatially distributed for to better represent the river bed flow roughness characteristics and regions where the flow is highly turbulent in nature. The sensitivity analysis showed that high flow conditions masked the impact of Manning’s n and eddy viscosity on the model outputs. Therefore, the model was calibrated under low flow conditions when the structures were emergent and had the largest impact on the flow pattern and model inputs. Detailed field measurements were performed under low flow conditions at the Raccoon River, Iowa for model calibration and verificat...

Bank-Attached Vanes for Bank Erosion Control and Restoration of River Meanders

The effectiveness of a novel approach of using vanes, installed at a low angle and attached to the bank, for bank protection and for the restoration of river meanders has been investigated in a laboratory study. Experiments were carried out in a large-scale meandering mobile-bed channel with graded sediment. The bed topography, three-dimensional flow pattern, and turbulence characteristics in the meandering channel with or without structures are analyzed. When a single or an array of such vanes is installed, the scour hole at the base of the outer bank is infilled and the thalweg is relocated toward the center of the river. The structures induce a secondary flow cell near the outer bank which counteracts the main spiral flow in the bend. In contrast to common spurs and bendway weirs, large-scale horizontal vortices are not generated behind the structures. Vanes which grade to the bed from bankfull level at the bank show better performance than low level ones, whereas multiple structures show positive effects as far downstream as the crossover section.