Three-dimensional flow dynamics around deflectors

RIVER RESEARCH AND APPLICATIONS

River Res. Applic. 21: 961–975 (2005)

Published online in Wiley InterScience(www.interscience.wiley.com). DOI: 10.1002/rra.852

THREE-DIMENSIONAL FLOW DYNAMICS AROUND DEFLECTORS

PASCALE M. BIRON,a* COLLEEN ROBSON,b MICHEL F. LAPOINTEb and SUSAN J. GASKINc

a Department of Geography, Planning and Environment, Concordia University, 1455 De Maisonneuve Boulevard W,

Montreal, Quebec, H3G 1M8, Canadab Department of Geography, McGill University, 805 Sherbrooke Street W, Montreal, Quebec, H3A 2K6, Canada

c Department of Civil Engineering, McGill University, 805 Sherbrooke Street W, Montreal, Quebec, H3A 2K6, Canada

ABSTRACT

Many river rehabilitation projects to enhance the aquatic habitat focus on the creation of pool and riffle habitat by theimplementation of flow deflectors, with various degrees of successes and failures. A more comprehensive understanding ofthe complex three-dimensional flow dynamics that induces scour around instream structures is required for a more effectivedesign. The objective of this study is to examine the three-dimensional mean and turbulent flow characteristics around pairedflow deflectors for various types of deflector design in a laboratory flume. Three deflector angles (45�, 90� and 135�) and twodeflector heights (with flow under and over the deflector height) were tested over a smooth (plexiglas) bed and a sand bed.Three-dimensional velocity measurements were taken with an acoustic Doppler velocimeter at several planform positions attwo heights above the bed. Results show that the 90� deflectors create the most important disturbance in the mean flow field,in turbulence intensity and bed shear stress. There is, however, a marked difference in the spatial distribution of the mean andturbulent parameters over a mobile bed and over a smooth, fixed bed. This stresses the importance of understanding thefeedback between bed topography and flow dynamics and limits the applicability of conclusions drawn from plane bed experi-ments to natural rivers. Copyright # 2005 John Wiley & Sons, Ltd.

key words: river rehabilitation; flow deflectors; deflector design; turbulent flow; bed shear stress; three-dimensional flow dynamics;recirculation zone; acoustic Doppler velocimeter

INTRODUCTION

Many rivers in the last few decades have been severely affected by human activities. As a consequence, problems

such as stream bank erosion, silted gravel streams, lack of instream and overhanging cover, as well as the general

loss of bed heterogeneity have occurred (Mitchell et al., 1998; Pretty et al., 2003). Spatial and temporal hetero-

geneity in rivers is essential for most fish species as they require different habitats for spawning and for juvenile

survival (Copp, 1997). Thus, river restoration projects to enhance the aquatic habitat often focus on the creation of

pool and riffle habitat by the implementation of instream structures such as flow deflectors (Thompson, 2002).

These structures are designed to form and/or maintain pools in the centre of the channel through scouring action

(Hunter, 1991; Hey, 1996; Thompson, 2002).

Most river rehabilitation projects are still carried out using a trial-and-error approach (Gippel, 1995; Brookes

and Shields, 1996; Ralph and Poole, 2002). Various angles, heights and contraction ratios of deflectors have been

used in the past with different degrees of success and failure (Thompson, 2002). A more comprehensive under-

standing of the complex three-dimensional flow dynamics that induces scour around instream structures would

contribute to a more effective design (Kondolf, 1998; Shamloo et al., 2001; Chrisohoides et al., 2003). So far, only

rough guidelines exist for the design of structures (Shamloo et al., 2001; Thompson and Stull, 2002), for example

indicating flow around stream deflectors (Figure 1).

To our knowledge, there has not been any detailed study on flow dynamics around paired deflectors, such as those

often encountered in fish habitat rehabilitation schemes (Hey, 1996). Most of the literature on flow dynamics around

Received 16 October 2003

Revised 5 July 2004

Copyright # 2005 John Wiley & Sons, Ltd. Accepted 20 September 2004

*Correspondence to: Pascale M. Biron, Department of Geography, Planning and Environment, Concordia University, 1455 De MaisonneuveBoulevard W, Montreal, Quebec, H3G 1M8, Canada. E-mail: [email protected]

instream structures has focused on bridge piers (Ahmed and Rajaratnam, 1998; Sarker, 1998; Graf and Istiarto, 2002)

and abutments (Molinas et al., 1998; Ahmed and Rajaratnam, 2000; Molinas and Hafez, 2000; Chrisohoides et al.,

2003). Although there are similarities between these structures and deflectors, from an engineering point of view, the

objective is to limit scouring whereas the objective of deflectors is to promote it. Also, contrary to the above instream

structures, the combined effect of each deflector may produce a more complex flow field in the centre of the channel

than would be obtained from a single deflector, particularly for long deflectors with respect to the width of the river.

Furthermore, with the exception of Graf and Istiarto (2000), turbulence was not taken into account in these studies.

To quantify accurately scour around instream structures, the intensity of turbulence in the vicinity of the structure

needs to be understood, as well as the three-dimensional mean flow field (Chrisohoides et al., 2003).

The objective of this study is to examine the three-dimensional mean and turbulent flow characteristics around

paired flow deflectors over a plane and mobile bed for various types of deflector design in a laboratory flume. This

aims at providing useful guidelines for the implementation of deflectors to restore fish habitat in rivers, as well as a

database to validate three-dimensional numerical models which are increasingly being used to simulate the flow

field around instream structures (Ouillon and Dartus, 1997; Sinha and Marelius, 2000; Chrisohoides et al., 2003).

METHODOLOGY

The experiments were carried out in a 0.60m wide, 4m long, recirculating flume, located in the Hydraulics

Laboratory at McGill University (Canada). A Plexiglas channel was inserted into the flume, reducing the flume

width (W) to 0.40m. The bed slope used for all runs was 0.0005 and the flow depth (Z) was 0.095m. Baffles were

used just downstream of the inlet tank in order to remove any large-scale disturbances in the flow.

Experiments were carried out over a plane (Plexiglas) bed and over a mobile bed. Mobile bed experiments used

uniform-sized sand (D50¼ 1.1mm) starting from a flat bed situation. The average discharge for the plane bed runs

was 7.8� 10�3m3 s�1 and for the mobile bed runs was 9.1� 10�3m3 s�1. The Froude number was 0.21 and 0.24

over Plexiglas and sand, respectively. The Reynolds number was 19 403 over Plexiglas and 22 636 over sand. The

Figure 1. River habitat improvement using structural measures: (a) weirs/dams; (b) deflectors; (c) vanes (from Hey, 1996). Reproduced bypermission of Blackwell Publishing Ltd

962 P. M. BIRON ET AL.

Copyright # 2005 John Wiley & Sons, Ltd. River Res. Applic. 21: 961–975 (2005)

upstream flow conditions for the mobile bed runs were chosen to obtain a clear water scour with a ratio of shear

velocity to the critical shear velocity of 0.75 in order to compare the scour dimensions with existing models (see

Biron et al., 2004a).

Three-dimensional velocity measurements were obtained using an acoustic Doppler velocimeter (ADV) with a

measuring volume located 0.05m from the receiver at a sampling frequency of 25Hz for one minute. The down-

stream component of velocity was parallel to the flume wall and no attempt was made to align the flow components

to the local streamline direction. Data are sometimes rotated to limit the error in Reynolds shear stress (McLelland

and Nicholas, 2000). However, in complex flow fields where the local streamline may vary markedly from one

point of measurement to the other, applying such a rotation may complicate the analysis more than anything

else (Roy et al., 1996). ADV data with signal correlations greater than 70% were retained (Lane et al., 1998).

The phase-space threshold method was used to remove suspected spikes (Goring and Nikora, 2002). On average,

around 4% of the sample points were removed.

Bed topography was measured using a manual height gauge. The accuracy of the measurements is� 2mm in

both the vertical and the horizontal. On average, 304 points were recorded for each mobile bed run (point

density¼ 214/m2). Maps of bed topography and flow field variables were created from a spline interpolation of

the point measurements using GIS software (ArcView 8.2).

Sharp-edged Plexiglas deflectors of 0.013m in thickness and 0.05m in length were positioned 2.1m from the

inlet. The contraction ratio (length of deflector/width of channel) was 0.25. Experiments were carried out for three

deflector angles: downstream-oriented (45�), perpendicular (90�) and upstream-oriented (135�). Additionally, twodeflector heights were characterized, corresponding to a situation where flow is under and over the deflector height.

In the latter case, the overtopping ratio (Zr ¼ Z=Zd, where Z is flow depth and Zd is the deflector’s height) was 1.27.

Table I summarizes the experimental runs and the associated dataset in each case. The detailed velocity surveys

had an average of 61 velocity measurements close to the bed and at 0.025m from the bed at 14 cross-sections, with

a higher density around the deflectors (Figure 2). The latter corresponds to a non-dimensional height (zþ ¼ z=Z,where z is the height above the bed) of 0.26. The near-bed non-dimensional height was 0.05 (0.005m) and 0.07

(0.007m), respectively, for the plane and mobile bed runs. Note that points very close to the deflectors could not be

sampled with the downward-looking ADV as at least one beam was going through the Plexiglas deflector.

Accurate bed shear stress estimates are difficult to obtain in such a complex flow field. A comparison of different

techniques revealed that because of the local streamline variations close to the deflector, Reynolds shear stress was

not the most appropriate method in this case (Biron et al., 2004b). The turbulent kinetic energy method is used

instead, where bed shear stress (�0) is:

�0 ¼ C1 0:5 � <u02> þ <v02> þ <w02>� �� ð1Þ

Table I. Summary of the experimental data

Run no. Type of Deflector Flow height/ Data collectedbed angle (degree) deflector

Velocity Bed topography

1 plane 45 under x2 plane 45 over x3 plane 90 under x4 plane 90 over x5 plane 135 under x6 plane 135 over x7 mobile 45 under x x8 mobile 45 over x9 mobile 90 under x x10 mobile 90 over x11 mobile 135 under x x12 mobile 135 over x

FLOW DYNAMICS AROUND DEFLECTORS 963


where u0, v0 and w0 represent longitudinal, lateral and vertical fluctuations of velocity, respectively, < > denotes a

time-average, � is mass density and C1 is a proportionality constant (c. 0.19) (Soulsby, 1983; Kim et al., 2000;

Pope, 2000).

Turbulence intensities (TI) for the longitudinal component of velocity were calculated as:

TI ¼ RMSu= ð2Þ

where RMSu is the root mean square value of the longitudinal component of velocity and represents the

average velocity value upstream of the deflectors.

RESULTS

Plane bed experiments

Velocity distribution. The presence of the deflectors alters the velocity distribution in the channel. There are

clear zones of reduced flow velocities close to the deflectors while the flow is accelerated in the mid-channel down-

stream of the deflectors in all experimental set-ups (Figures 3 and 4). The areal extent of the disturbance caused by

the deflectors was quantified by calculating the two-dimensional surface areas of two zones: the recirculation zone

and the high-speed region. The recirculation zone is defined as the region with combined lateral and downstream

velocities that are less than or equal to 0.05m s�1 (20% of the upstream average velocity). Note that the error in the

interpolation algorithm close to the wall increases as it was not possible to collect data in the first 5 cm from the

wall with the downward-looking ADV. The high-speed region is defined by velocities that are greater than

1.5 times the upstream average velocity (>0.35m s� 1).

For measurements close to the bed when flow is under the deflector height, the area of the recirculation zone

caused by the 90� deflectors is 1.7 and 4.3 times larger than the area influenced by the 45� and 135� deflectors,respectively (Table II). During overtopping conditions, the difference between the 90� and 45� deflectors is

less. However, the recirculation zone created by the 90� design is still substantially larger than that for 135�.

Figure 2. Position of the velocity measurements (þ ). The thick lines perpendicular to the flume walls are the deflectors



Figure 3. Average velocity distribution ð½2 þ <v>2 þ <w>2�0:5Þ for plane bed runs at zþ ¼ 0:05 for: (a) 45� set-up under the deflector’sheight; (b) 45� set-up overtopping; (c) 90� set-up under the deflector’s height; (d) 90� set-up overtopping; (e) 135� set-up under the deflector’s

height; (f) 135� set-up overtopping

Figure 4. Average velocity distribution ð½2 þ <v>2 þ <w>2�0:5Þ for plane bed runs at zþ ¼ 0.26 for: (a) 45� set-up under the deflector’sheight; (b) 45� set-up overtopping; (c) 90� set-up under the deflector’s height; (d) 90� set-up overtopping; (e) 135� set-up under the deflector’s

height; (f) 135� set-up overtopping



Furthermore, the recirculation zone near the 45� and 135� deflectors is shifted downstream during overtopping

conditions, whereas it remains more or less in the same position for the 90� deflectors (Figure 3). The length of

the recirculation zone non-dimensionalized by the length of the deflector close to the bed is on average 7.1 for the

90�, 5.7 for the 45� and 1.6 for the 135� deflectors (Table II).

The high-speed region resulting from the 90� deflectors is on average 6.6 times larger than that of the 45� orien-tation, and 2.9 times larger than the zone resulting from the 135� deflectors. There is not much difference between

the overtopping and under-the-deflector-height condition, except for the 135� design where the high-speed zone ismore than twice the area for overtopping flow (Table II).

Higher up in the flow, the differences in both the size of the recirculation and high-speed zones are not as impor-

tant (Figure 4). The 90� deflectors still generate the largest recirculation zone, but the 45� and 135� deflectors

are very similar both in area and length (Table II). The high-speed zone is also the largest for the 90� case withthe other two designs having similar areas, except for the 135� deflectors’ overtopping condition. As was the case

close to the bed, there is a marked difference for the upstream-oriented structure between conditions under and

over the height of the deflectors.

Close to the bed, small zones of negative vertical velocities are seen very close to the tips of the deflectors, par-

ticularly for overtopping conditions (Figure 5b,d). Velocity vectors are skewed as they approach and pass the

deflectors in all runs (Figure 5). In the zone directly downstream of the deflectors, velocities are reduced and tend

towards the bank. Higher up in the flow, negative velocities are observed in all cases close to the deflector tips but

the zones are larger for overtopping conditions (Figure 6).

Turbulence. For all experimental set-ups, the highest values of near-bed turbulence intensity occur in the

wake downstream of the deflectors at both heights above the bed (Figures 7 and 8). There are low levels of turbu-

lence intensities at the deflector tips, particularly when flow is under the height of the deflectors close to the bed

(Figure 7). Note that the larger interpolation errors in the near-wall zone due to a lack of measurement points

explain the very homogeneous spatial pattern in turbulence intensity from the tip of the deflector up to the wall.

There are no marked differences between the deflector angles in the pattern of turbulence intensity, although

turbulence intensities were highest for the 90� deflectors. Higher up in the flow, there is an overall decrease in

turbulence intensity. Larger turbulence intensity values were recorded near the tips of the deflectors than near

the bed (Figure 8). As was the case near the bed, overtopping flow generates larger turbulence intensity values.

The spatial distribution of the turbulent kinetic energy bed shear stress amplification follows a pattern similar to

that of turbulence intensity (Figure 9). The shear stress amplification near the deflectors is small, particularly where

flow is under the deflector height (Figure 9a,c,e). There are larger values of shear stress close to the deflectors for

overtopping conditions. In all cases, the peak shear stress values were recorded in the wake downstream of the

deflectors. The 90� design caused the largest increase in bed shear stress, with values up to 15 times the upstream

average, whereas the 45� deflectors generated the least increase.

Table II. Extent of disturbance resulting from deflectors for the plane bed runs

Deflector Non-dimensional Average recirculation Recirculation Non-dimensional High-speed zonetype height (zþ ) zone area (m2) zone length (m) recirculation length* area (m2)

45� under 0.05 0.010 0.29 5.8 0.01045� over 0.05 0.011 0.28 5.6 0.01290� under 0.05 0.017 0.38 7.6 0.07690� over 0.05 0.012 0.33 6.6 0.070135� under 0.05 0.004 0.16 1.2 0.015135� over 0.05 0.0035 0.10 2.0 0.03445� under 0.26 0.013 0.33 6.6 0.06245� over 0.26 0.010 0.28 5.6 0.05990� under 0.26 0.021 0.48 9.6 0.10790� over 0.26 0.014 0.35 7.0 0.099135� under 0.26 0.013 0.29 5.8 0.059135� over 0.26 0.010 0.27 5.4 0.084

*Recirculation zone length/length of the deflector.



Mobile bed experiments

Bed topography. The angle of the deflectors has a major effect on the resulting bed topography (Figure 10).

The 90� deflectors resulted in the greatest scour hole depth and volume when flow was both under and over the

height of the deflectors. The 45� deflectors had the smallest impact on bed topography, particularly for overtopping

flow. Sand bars also formed downstream of the deflectors for all runs.

Figure 5. Vertical velocity distribution and planform vectors for plane bed runs at zþ ¼ 0.05 for: (a) 45� set-up under the deflector’s height;(b) 45� set-up overtopping; (c) 90� set-up under the deflector’s height; (d) 90� set-up overtopping; (e) 135� set-up under the deflector’s height;

(f) 135� set-up overtopping

Figure 6. Vertical velocity distribution and planform vectors for plane bed runs at zþ ¼ 0.26 for: (a) 45� set-up under the deflector’s height;(b) 45� set-up overtopping; (c) 90� set-up under the deflector’s height; (d) 90� set-up overtopping; (e) 135� set-up under the deflector’s height;

(f) 135� set-up overtopping



Figure 7. Longitudinal turbulence intensity for plane bed runs at zþ ¼ 0.05 for: (a) 45� set-up under the deflector’s height; (b) 45� set-up over-topping; (c) 90� set-up under the deflector’s height; (d) 90� set-up overtopping; (e) 135� set-up under the deflector’s height; (f) 135� set-up

overtopping. Root mean square (RMS) u is divided by average near-bed upstream velocity (0.195m/s)

Figure 8. Longitudinal turbulence intensity for plane bed runs at zþ ¼ 0.26 for: (a) 45� set-up under the deflector’s height; (b) 45� set-up over-topping; (c) 90� set-up under the deflector’s height; (d) 90� set-up overtopping; (e) 135� set-up under the deflector’s height; (f) 135� set-up

overtopping. RMS u is divided by average upstream velocity at zþ ¼ 0.26 (0.245m/s)



Figure 9. Amplification in the turbulent kinetic energy bed shear stress for plane bed runs for: (a) 45� set-up under the deflector’s height; (b) 45�set-up overtopping; (c) 90� set-up under the deflector’s height; (d) 90� set-up overtopping; (e) 135� set-up under the deflector’s height; (f) 135�

set-up overtopping. The amplification is based on upstream average shear stress value (0.084N/m2)

Figure 10. Bed topography for: (a) 45� set-up under the deflector’s height; (b) 45� set-up overtopping; (c) 90� set-up under the deflector’sheight; (d) 90� set-up overtopping; (e) 135� set-up under the deflector’s height; (f) 135� set-up overtopping



Velocity distribution. Although flume slope and flow depth were the same, velocities are smaller near the

deflectors at both measurement heights over the mobile, rougher bed, than over the smooth Plexiglas bed

(Figure 11). For the 90� and 45� deflectors, there is a clear recirculating zone near the deflectors with velocities

lower than 0.05m s�1. This zone is slightly larger than over the plane bed and matches the scour dimensions.

For the 135� design, however, only a small recirculating zone is present (Figure 11). For all angles, the zone is

shorter than over the plane bed with non-dimensional lengths ranging from 0.8 (135�) to 3.6 (90�) (Table III). Thesize of the recirculation zone decreases with height above the bed, whereas it was increasing over a plane bed. Near

the tips of the 135� deflectors, velocities remained high (Figure 11f).

The maximum velocities downstream of the deflectors are between 0.25 and 0.30m s�1, which is considerably

slower than the values of 0.30 to 0.40m s�1 that were obtained over a plane bed (Figures 3 and 4). The so-called

high-speed fluid zone over sand forms a narrow band extending the width of the channel (Figure 11).

For both the 45� and 90� runs, regions of negative vertical velocities are present upstream of the deflectors close

to the bed (Figure 12a–c). The absence of such a zone for the 135� design is likely due to the limited number of

valid measurement points in this area. Positive vertical velocities are observed along the sand bars for all angles. In

the scour holes, flow velocity is greatly reduced and the recirculation is apparent in the planform vectors with

marked variations in the local streamlines. For all runs, flow vectors over the sand bars are oriented towards

the banks. Higher up in the flow, negative velocities were recorded nearer the deflectors while a zone of positive

Figure 11. Average velocity distribution ð½2 þ <v>2 þ <w>2�0:5Þ for mobile bed runs for the 45� set-up at (a) zþ ¼ 0.07; (b) zþ ¼ 0.26;the 90� set-up at (c) zþ ¼ 0.07; (d) zþ ¼ 0.26; and the 135� set-up at (e) zþ ¼ 0.07; (f) zþ ¼ 0.26

Table III. Extent of the recirculating zone resulting from deflectors for the mobile bed runs

Deflector angle Average Recirculation Recirculation Non-dimensional(degree) zone area (m2) zone length (m) recirculation length*

45 0.011 0.11 2.290 0.023 0.18 3.6135 0.005 0.04 0.8

*Recirculation zone length/length of the deflector.



velocities remained over the sand bars (Figure 12d–f). The velocity vectors in the scour holes are only slightly

skewed compared with the near-bed situation.

Turbulence. Turbulence intensities are overall lower over sand than they were over a plane bed (Figure 13).

High turbulence intensity values were recorded near the deflectors and along the sand bars. This is different from

Figure 12. Vertical velocity distribution and planform vectors for mobile bed runs for the 45� set-up at (a) zþ ¼ 0.07; (b) zþ ¼ 0.26; the 90�set-up at (c) zþ ¼ 0.07; (d) zþ ¼ 0.26; and the 135� set-up at (e) zþ ¼ 0.07; (f) zþ ¼ 0.26

Figure 13. Longitudinal turbulence intensity for mobile bed runs for the 45� set-up at (a) zþ ¼ 0.07; (b) zþ ¼ 0.26; the 90� set-up at (c)zþ ¼ 0.07; (d) zþ ¼ 0.26; and the 135� set-up at (e) zþ ¼ 0.07; (f) zþ ¼ 0.26. RMS u is divided by average upstream velocity near the bed

(0.159m/s) and at zþ ¼ 0.26 (0.234m/s)



the plane bed experiments where there was no apparent increase in turbulence intensity close to the deflectors

(Figure 7). Close to the bed, the 90� and 135� deflectors produce the greatest extent of downstream disturbance,

whereas turbulence around the 45� deflectors is much less important (Figure 13a–c). Higher up in the flow, turbu-

lence intensities are lower overall, with a maximum of 20% compared to the near-bed results where values in

excess of 35% were recorded (Figure 13d–f). At zþ ¼ 0.026, the highest values of turbulence intensity are located

close to the sand bars, at a position similar to that recorded for the plane bed runs (Figure 8). The 135� deflectorsgenerate the largest turbulence intensities.

As expected, the patterns of near-bed turbulence intensity and bed shear stress are similar (Figure 14). Bed shear

stress increases markedly close to the deflectors for the 90� and 135� orientations and values up to five times the

upstream average are observed for the 135� case. The 45� design does not generate a large increase in bed shear

stress, except on the right side of the channel. A second shear stress peak is observed for all runs immediately

downstream of the sand bars. Note that overall, the amplification of shear stress is much less than for the plane

bed runs (Figure 9). Furthermore, the peak values of amplification for the mobile bed are closer to the deflectors

than for the plane bed experiments.

DISCUSSION

Over a plane bed, the influence of the deflectors creates distinct zones of reduced flow velocities close to the deflec-

tors while at the mid-channel location, flow is accelerated at both measurement heights. For all runs, the highest

turbulence intensities and peak values of shear stress occurred in the wake downstream of the deflectors. The 90�

deflectors have the most important effect on the average velocity, turbulence intensity and shear stress distribution

compared to the 45� and 135� orientations.

Figure 14. Amplification in the turbulent kinetic energy bed shear stress for mobile bed runs for (a) 45� set-up; (b) 90� set-up; (c) 135� set-up.The amplification is based on upstream average shear stress value (0.084N/m2)



Over a mobile bed, the mean and turbulent flow structure is markedly different from the plane bed situation. This

is related both to the increased bed roughness and to the feedback between flow and bed topography, mainly with

the scour hole formed close to the deflectors. Sediment that is eroded from the scour zone is deposited further

downstream to create sand bars over which vertical velocities are positive. There is less acceleration over the sand

bed than over the fixed bed downstream of the deflectors, which is to be expected because of the increased friction

created by sand grains and bed forms. The sand bed recirculation zone is limited to the scour zone whereas it was

much more elongated over the plane bed. Higher turbulence intensities and shear stress values are present in the

scour region, particularly for the 90� and 135� orientations.For both types of bed, the 90� deflectors created the largest disturbance to the flow field. However, over a mobile

bed, the turbulence characteristics of the 135� and 90� deflectors are more alike, whereas the mean flow field

remains distinct. The important differences in the flow structure that were observed between the two types of

bed emphasize the importance of taking into account the feedback that occurs between the flow and the bed forms.

This suggests a potential limit to the conclusions that can be drawn from fixed bed experiments and applied to a

mobile bed situation (Chrisohoides et al., 2003).

There are not many detailed flow dynamics studies on flow deflectors to compare our results with. The schematic

streamlines close to the bed depicted in Figure 1 correspond to our flow vector measurements. However, because

the downward-oriented ADV measuring volume is located 0.05m lower than the receivers, it was not possible to

obtain velocity measurements close to the water surface to verify if streamlines are oriented towards the bank for

45� overtopping flow and towards the centre of the channel for 135� deflectors, as in Figure 1. The streamline

orientation is important as it will affect bank erosion and potential bank protection schemes when implementing

deflectors in rivers. More velocity measurement points closer to the deflectors and higher in the flow are needed to

investigate this question. In this study, vectors are oriented towards the banks further downstream of the deflectors

(Figures 5, 6 and 12) compared to Figure 1. Over sand, the bank-oriented vectors also correspond to high average

velocity (Figure 11) and may create problems of bank erosion downstream of the deflectors.

In the wake downstream of the deflectors, larger bed shear stress values were observed over the fixed smooth bed

compared to the mobile bed. Increased vorticity of the wake eddies is expected to intensify flow turbulence (Ettema

and Muste, 2004), although this has rarely been quantified in previous studies. Large turbulent kinetic energy

values have been observed over a fixed bed downstream of a groyne along the recirculation line (Ouillon and Dar-

tus, 1997). Chrisohoides et al. (2003) also revealed a massively separated wake consisting of large-scale eddies

that extended up to five to six abutment lengths downstream over a fixed bed.

Previous studies have shown that at the tips of the deflectors, turbulence, vortices (horseshoe vortex system),

downflows, and bed shear stress are intensified (Rajaratnam and Nwachukwu, 1983; Molinas et al., 1998). Ahmed

and Rajaratnam (1998) observed smaller shear stress amplification over a smooth bed compared to a rough bed

near an obstacle. Ahmed and Rajaratnam (2000) measured a shear stress amplification of 3.6 near an abutment with

a 0.08 contraction ratio. Shear stress amplification measured here was between two and four near the deflector tips

for the plane bed runs and around five for the rough bed in the scour region.

The lack of detailed measurements due to the physical constraint of the downward-looking ADV probe close to

the deflector hinders understanding of the flow dynamics in this critical zone. Three-dimensional numerical mod-

elling has been used more extensively recently (Ouillon and Dartus, 1997; Sinha and Marelius, 2000; Chrisohoides

et al., 2003) and provides a high density of points that can reveal more easily than with physical models the com-

plex flow dynamics around instream structures. For example, the role of recirculation flow and multiple vortices on

the scouring process may be more important than believed so far (Chrisohoides et al., 2003).

This experimental study is a first step in improving our understanding of the three-dimensional flow dynamics

around paired deflectors in the much more complex natural environment where grain size, bed topography and

planform geometry are highly variable. Furthermore, the effect of increasing the discharge during overtopping

conditions, such as that occurring during spring flood events, was not considered here. In our results, only the

135� oriented deflectors showed a marked difference between flow height under or over the height of deflectors.

However, overtopping flow conditions during higher discharge events will likely produce more substantial changes

for all angles in the mean and turbulent flow field that need to be assessed to limit bank erosion and structure failure

problems. These may also be aggravated by ice floe movement such as observed in Canadian rivers during the

spring flood.



CONCLUSION

Experimental work using three angles and two heights of deflectors over a plane and mobile bed reveals important

differences in the three-dimensional flow dynamics around these structures. The 90� deflectors create the most

important disturbance to both the mean and turbulent flow field, which results in the largest scour hole. Over a

plane bed, upstream- (135�) and downstream- (45�) oriented deflectors have similar patterns of mean velocity,

turbulence intensity and shear stress whereas over a mobile bed, the 135� orientation creates a disturbance of

the turbulent flow field similar to that of the 90� design.There is a marked difference in the spatial distribution of the mean and turbulent parameters over a plane bed

and a mobile bed, which stresses the importance of understanding the feedback between bed topography and flow

dynamics and limits the applicability of plane bed experiments to natural river studies. Over a sand bed, high tur-

bulence and bed shear stress values are observed close to the deflectors whereas this is seen further downstream in

the wake of the deflectors over a fixed bed.

Successful rehabilitation projects must have a clear understanding of the complex three-dimensional flow

dynamics created by flow deflectors. Laboratory research is necessarily limited to very simple planform geometry,

bed and bank topography. Fieldwork studies are required to understand the flow field around deflectors in complex

natural rivers. More research on the ‘optimal’ design is needed prior to the implementation of these structures,

ideally using three-dimensional numerical modelling which allows a high density of points to be obtained readily.

ACKNOWLEDGEMENTS

This research was funded by the Fonds quebecois pour la nature et les technologies. Thanks to the technicians in

the Hydraulics Laboratory at McGill University, John Bartczak and Damon Kiperchuk. This manuscript has bene-

fited from the constructive suggestions of two anonymous reviewers.

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Three-dimensional flow dynamics around deflectors