R. D. Townsend’s research while affiliated with University of Ottawa and other places

Submerged groynes (low-profile linear rock structures, also called stream barbs) were installed in a reach of Sawmill Creek in Ottawa, Canada in order to mitigate outer bank erosion in two consecutive bends. The presence of these structures has had an effect on channel bathymetry and thusly on flow structure in the area. Changes in both the flow field and bathymetry have been recorded over five years (including two years prior to implementation of the groynes) using an Acoustic Doppler Current Profiler (ADCP), and a total station, respectively. Additionally, three-dimensional flow field data were collected using Acoustic Doppler Velocimeters (ADV) near the apex of the first bend. Measurements were collected prior to implementation of the groynes as well as at the end of the study term in order to compare the long term effects on flow structure and turbulence in the area. Over the study period, it was observed that the thalweg shifted toward the center of the channel (away from the outer bank) along with the higher velocity flows. This change in flow, as well as the change in apparent turbulence at the bend apex, can be attributed to the presence of the submerged groynes.

A series of seven stream barbs were installed in two consecutive channel bends of Sawmill Creek (Ottawa, Canada), a semi-alluvial stream with bed and banks composed of consolidated clay. Stream barbs (also known as submerged groynes) are low-profile linear rock structures that project out from the bank (in an upstream direction) to redirect flow and prevent erosion of the bank. As well as providing bank protection, barbs promote vegetated stream banks, create scour hole resting pools for fish habitat and can increase aquatic species diversity. Flow conditions (discharge and water levels), water velocity distribution using acoustic Doppler velocimeters and an acoustic Doppler current profiler, and bathymetry using a Total Station have been measured over a period of 5 years, both before (2 years) and after (2 years) the construction of the barbs, providing valuable data for understanding stream barb performance in a semi-alluvial channel. Pre-barb velocity measurements indicate that the flow field in the first bend is dominated by strong secondary circulation and high cross-stream bed stresses that comprise a substantial proportion of the total near-bed Reynolds stress. Post-barb results indicate that velocity magnitude along the outer bank was reduced. Scour occurred in the channel centre near barb tips, but in general, little to no bathymetry changes were measured between seasonal surveys, suggesting that the barbs had limited impact on bed topography and/or that semi-alluvial channel bends are resistant to change. Recommendations for future design and implementation of stream barbs are also included. Copyright © 2013 John Wiley & Sons, Ltd.

Most rivers exhibit regions of strong channel curvature that are characterized by more complex and variable flow and erosion patterns, compared to regions of lower curvature. Studies investigating high-curvature bends using eddy-resolving techniques have been limited, and the effect of bend angle on flow and erosion has rarely been investigated. This study investigates flow in a 135° nonerodible bank open channel bend of high curvature: ratio of radius of curvature, R, to channel width, B, is 1.5. The bathymetry is obtained during the final stages of a clear water scour experiment. Large Eddy Simulation is used to investigate the effect of secondary flow on the redistribution of streamwise momentum, the details of coherent structures, and mechanisms leading to erosion within the bend. Results are compared with those from a similar numerical study of a 193° sharply curved open channel bend with R/B = 1.35. The angle of the 135° bend is representative of typical regular meander geometry, while the larger angle of the 193° bend is representative of a tortuous meander geometry. The different bathymetries induced important quantitative and qualitative differences in the vortical and turbulence structure within the open channel for the two cases. Inner bank streamwise-oriented vortical (SOV) cells formed in both cases, but the position and extent of shear layers forming between regions of fast and slow moving fluid differed, and flow did not separate at the inner bank in the 135° bend. An outer-bank cell was observed in the 135° bend, but not in the 193° bend. Distributions of predicted boundary shear stresses indicated the capacity of the flow to erode the outer bank of a sharply curved bend under two representative regimes found in the field.

Submerged groynes (low-profile linear rock structures, also called stream barbs) were installed in a reach of Sawmill Creek in Ottawa, Canada in order to mitigate outer bank erosion in two consecutive bends. These structures have had an effect on both the aquatic life and habitat in the area. Changes in both the flow field and bathymetry have been recorded over 4 years (including 2 years prior to implementation of the groynes) using acoustic Doppler velocimeters, an acoustic Doppler current profiler, and a total station, respectively. The City of Ottawa’s Water Environment Protection Program (WEPP) has also provided fish abundance and habitat data for the study site as well as a control site located approximately 200 m downstream. The velocity and bathymetry data collected, combined with the fish abundance data, were used to demonstrate a causal relationship between the implementation of submerged groynes and the recorded change in fish abundance and habitat. There has been a consistent increase in fish abundance at the study site compared to that of the control site, indicating that the presence of the groynes is beneficial to the aquatic environment. This could be attributed to select features provided by the groynes which were not inherently available at the study site, namely: (1) velocity refuge and (2) visual isolation from predators. Keywords: barb, submerged groyne, urban creek, rehabilitation, fish abundance.

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 τ vw 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.

A series of laboratory flume experiments were performed to study the effect of stream barbs on flow field dynamics and sediment erosion in a 135° mobile-bed channel bend. Stream barbs (also known as spur dikes or submerged groynes) are low-profile linear rock features that redirect high velocity flow away from the outer bank of channel bends. Unlike emergent groynes, the submerged nature of these structures creates a unique combination of horizontal shear (plunging type flow) and vertical shear (at the groyne tip). Spatially dense, high frequency velocity data were collected and analyzed to describe the pattern and magnitude of three-dimensional (3D) velocity throughout the bend and in the vicinity of the stream barbs. This paper demonstrates that the outer bank region (particularly between barbs) may still be at risk of erosion (or even increased erosion greater than the same case without barbs) if stream barbs generate excessive secondary velocities (because of their size and layout) that are opposing the primary secondary flow naturally occurring in channel bends. Characterizing the role of flow field dynamics on the pattern of deposition and erosion through experimental measurements provided valuable data about how such flow features contribute to local scour, and about the performance of these structures.

Flow within an alluvial channel bend is significantly affected by channel geometry, including curvature ratio (bend radius/channel width, R=B) and aspect ratio (channel width/flow depth, B=H). High curvature bends (R=B ≤ 3) can experience substantially more erosion than milder curvature bends. This study employs a three-dimensional Reynolds-Averaged Navier-Stokes (RANS) model to investigate the effects of curvature ratio and aspect ratio on bend flow with respect to a high curvature (R=B ¼ 1.5) base case in a 135° bend. Experimental data are used to validate the RANS model predictions for the high curvature base case with a flat bed (FB) and an equilibrium deformed bed (DB). Five curvature ratios (1.5, 3, 5, 8, and 10) and four aspect ratios (5.00, 6.67, 9.09, and 12.50) are investigated. Results show that a decrease in R=B or B=H for the FB cases results in a strong increase in total circulation of the regions associated with the primary cell of cross-stream circulation (Γþ), an increase in maximum bed shear stress, and an increase in the contribution of the cross-stream component tothe total magnitude of bed shear stress. The values of R=B and B=H also affect the structure of the cross-stream flow. The primary cell of cross-stream circulation splits into two clockwise-rotating cells at low R=B values and the cell situated closer to the inner wall induces strong ejections of vorticity. At high R=B values, a secondary counter-clockwise rotating cell forms at the outer bank. At lower B=H values, the primary cell splits into two clockwise-rotating cells. This study shows that the position and size of regions of high bed hear stress, and thus the capacity of the flow to entrain sediment, depend strongly on bend curvature.

Detailed mapping of bathymetry and apparent bed load velocity using a boat-mounted acoustic Doppler current profiler (ADCP) was carried out along a 388-m section of the lower Missouri River near Columbia, Missouri. Sampling transects (moving boat) were completed at 5- and 20-m spacing along the study section. Stationary (fixed-boat) measurements were made by maintaining constant boat position over a target point where the position of the boat did not deviate more than 3 m in any direction. For each transect and stationary measurement, apparent bed load velocity (nu(b)) was estimated using ADCP bottom tracking data and high precision real-time kinematic (RTK) global positioning system (GPS). The principal objectives of this research are to (1) determine whether boat motion introduces a bias in apparent bed load velocity measurements; and (2) evaluate the reliability of ADCP bed velocity measurements for a range of sediment transport environments. Results indicate that both high transport ((nu) over bar (b)> 0.6 m/s) and moving-boat conditions (for both high and low transport environments) increase the relative variability in estimates of mean bed velocity. Despite this, the spatially dense single-transect measurements were capable of producing detailed bed velocity maps that correspond closely with the expected pattern of sediment transport over large dunes. DOI: 10.1061/(ASCE)HY.1943-7900.0000373. (C) 2011 American Society of Civil Engineers.

Detailed mapping of bathymetry and three-dimensional water velocities using a boat-mounted single-beam sonar and acoustic Doppler current profiler (ADCP) was carried out in the vicinity of two submerged wing dikes located in the Lower Missouri River near Columbia, Missouri. During high spring flows the wing dikes become submerged, creating a unique combination of vertical flow separation and overtopping (plunging) flow conditions, causing large-scale three-dimensional turbulent flow structures to form. On three different days and for a range of discharges, sampling transects at 5 and 20 m spacing were completed, covering the area adjacent to and upstream and downstream from two different wing dikes. The objectives of this research are to evaluate whether an ADCP can identify and measure large-scale flow features such as recirculating flow and vortex shedding that develop in the vicinity of a submerged wing dike; and whether or not moving-boat (single-transect) data are sufficient for resolving complex three-dimensional flow fields. Results indicate that spatial averaging from multiple nearby single transects may be more representative of an inherently complex (temporally and spatially variable) three-dimensional flow field than repeated single transects. Results also indicate a correspondence between the location of calculated vortex cores (resolved from the interpolated three-dimensional flow field) and the nearby scour holes, providing new insight into the connections between vertically oriented coherent structures and local scour, with the unique perspective of flow and morphology in a large river.

A series of seven stream barbs were installed at two consecutive channel bends in Sawmill Creek, a semi-alluvial stream located in Ottawa, Canada. Stream barbs (also known as submerged groynes) are low-profile linear rock structures that extend from the outside bank regions of channel bends in an upstream direction, to redirect the attacking currents and prevent erosion of the bank. As well as providing bank protection, these structures promote vegetated stream banks, create resting pools and scour holes for fish habitat, and increase bio- diversity for aquatic species. Despite these benefits, because of their relative novelty as river training structures, stream barbs are not a common means of stream bank protection in Canada and are possibly non-existent for semi-alluvial or clay channels. Three years of monitoring and measurement of flow conditions (discharge, water velocity and depth) and bathymetry, before (2 years) and after (1 year) the construction of the barbs, have been collected at the Sawmill Creek study site, providing valuable data for understanding their performance in a semi-alluvial channel. Sawmill Creek has a predominately clay bed and banks, presenting a rare opportunity to study the unique dynamics between flow and sediment transport within a clay channel. This paper reports on (i) the unique site conditions and monitoring methodology; (ii) preliminary results of the 3 year monitoring program; and (iii) recommendations for future design and implementation of these structures.