A.B.M. Faruquzzaman Bhuiyan and
Richard D. Hey
School of Environmental Sciences
University of East Anglia, Norwich NR47TJ, UK
Fax: 441 603 507719, E-mail: Faruk.Bhuiyan@uea.ac.uk
Abstract:
The effectiveness of instream J-vanes for river
restoration and bank protection in meander bends have been investigated in a
laboratory study. The experiments were carried out in a large scale meandering
mobile bed channel with a bed composed of graded materials. Detailed data on
three-dimensional velocities around the structures, bed load transport,
erosion-deposition and corresponding cross-sectional shape changes have been
collected. If a single or an array of such vanes are installed in the channel
the scour hole at the outer bank was infilled and a deeper channel was formed
with coarser substrate away from the outer bank. These influences extend further
upstream and downstream of the location of the structures. The structures modify
the flow pattern so that a counter rotating secondary flow cell is formed near
the outer bank. In contrast to common spurs, no large scale horizontal vortices
are generated behind the structures. The longitudinal slope of water surface
does not change significantly due to the structure and lateral water levels show
slight variations which are related to the generation of eddies. Structures
which grade to the bed from bankful level at the bank show better performance
than low level ones where multiple structures show positive effects through the
bend as far as the next crossover. Following installation of the structure bed
material is transported along the evolving thalweg but afterwards transport occurs mainly over the point bar.
The flow structure, downstream vortex pattern and erosion-deposition
characteristics induced by the vanes should be considered thoroughly before this
type of structure is recommended for habitat improvement and bank stabilisation.
Keywords: hydraulic structure, j-vane, river restoration, bank protection, mobile bed, scour, deposition
Maintenance and restoration of rivers for environment friendly natural ecosystem is one of the major issues in river management. The implication is to provide a suitable habitat for aquatic flora, fauna, organisms, vertibrates , invertibrates etc. To achieve this objective a realtively stable reach with natural physical characteristics of rivers in terms of wetted area, flow velocity, turbulence, substrate conditions should be maintained. From an ecological point of view, interaction between aquatic organisms and microscale phenomena like small wake regions in areas smaller than the reach-scale is important (ASCE 1992). The influence of habitat characteristics like substrate, flow and turbulence conditions on aquatic organisms (benthic invertibrates, fishes etc.) have been studied by different researchers (e.g., Statzner and Higler 1986; Milhous 1998; Robson et al. 1999). In the case of movable bed rivers the bottom boundary is subjected to spatial and temporal changes which modify habitat characteristics and dictates the existence of specialized species of benthos.
To maintain or restore a river's habitat diversity characteristics of a river different nonstructural and structural approaches have been proposed (Hey 1990). Structural measures include different types of weirs, deflectors and vanes. The design and implementation of these types of structures in gravel bed rivers have been explained by Rosgen (1996). They affect habitat characteristics by modifying the local substrate and flow conditions as well as riparian condition when bank stability is considered. Although these structures are recommended frequently, very few studies have been conducted to investigate the detail effects of these structures on flow and substrate condition and, in the case of mobile bed channels, on sediment transport.
In the present study, laboratory investigation have been carried out on instream W-weir and J-vanes to explore flow patterns, turbulence, scour-deposition and their effect on bedload. The later type of structure has a sloping crest extending from the riverbank and forming a low angle with the upstream bank. It can be used to create an asymmetric cross section with a deep scour hole to provide a substrate which is relatively free of fine materials. When used in arrays around a bend they afford considerable bank protection. This paper explains the effect of using J-vanes in a meandering mobile bed channel. Three-dimensional flow measurements have been carried out to identify the changes of mean flow velocity, secondary flow patterns and vortices downstream of the structures. Data have been collected to identify the erosion-depositional zones and bed load transport in the selected sections. Impact of single (both high and low levels) and multiple structures in the meander have been investigated.
The laboratory experiments were carried out in the Flood Channel Facility of HR Wallingford Ltd, UK. Four tests were carried out with J-vanes: full height single vane (pitching from bank top level to the bed), low height single vane (at half bankful level) and full height multiple vanes with two different orientations (Fig. 1).
A time period of 2 to 3 weeks was required to complete each of the above mentioned tests. To prepare the bed of the channel graded sediment of the size range 0.06mm to 5 mm were used. During experiments the outgoing sediments through the downstream boundary were recirculated with a slurry pump. The experimental setup is constructed on a 50m X 10m basin in which the main channel (sine generated channel with sinuosity 1.38 and wavelength 14.96 m) has trapezoidal cross sections with variable side slopes and top width of 1.6 m. To start a test, the existing sand bed was screeded to thoroughly mix the sediment. A plain bed of 150 mm deep was prepared. Following a period of a sub-bankful flow, the flow was increased to bankful flow, 97 l/s, for 2~3 days to create a self formed bed. This bed contained features similar to naturally observed meandering channels (e.g. point bar near the inner bank of meander, highly scoured outer bank toe, characteristic crossing bed features, ripples, dunes etc.). The structures were constructed in the drained self-formed bed with as little as possible disturbance of the surrounding bed.
With the
engineering works in place, the flow (97 l/s) was resumed.
The downstream boundary condition was set up by fixing the tailgate at
the level that produced uniform flow in the previous experiments. This allowed
direct comparisons of the results of the engineered channel with those of the
previous experiment. The specified flow was continued for consecutive 2-4 days
to obtain a quasi-steady or dynamic equilibrium of the bed. This was confirmed
from the frequent reading of water levels and visual observation of sediment
transport and movement of dunes in the bed. The channel was then fully drained
out before the bed was profiled. After that the main part of data collection was
done keeping the discharge same.
Different types of data were collected in a
systematic way during each test. Water levels were measured every 1-2 hr of the
first 2-3 days with electronic point gauge (0.01 mm precision) attached to a bridge which could be moved to any position;
upstream-downstream and left to right across the channel. Computer controlled
touch sensitive bed profilers were used to profile cross sections at a
longitudinal spacing of 1/64 to 1/8th of a full meander and measured over a half
to one and half wavelengths depending on the test. Three types of Acoustic
Doppler Velocity meter (ADV: 3D side facing, 3D down facing and 2D side facing)
were used to take three-dimensional and two-dimensional velocities covering the
points near the boundaries. The ADV was mounted on a traverser and the time
dependent movements of the traverser were controlled by software. Velocities
were recorded at points with a 10-40 mm vertical spacing and a 100-150 mm
lateral distances. A minimum of 60 seconds sampling time was used at each point.
Local bedload was sampled with a calibrated quarter size Helley-Smith sampler in
the same x-sections as the velocity measured at 100 mm centres for 5 min at each
location. At the inlet, recirculating sediment weight was measured at every 5
min for continuous 6 hr period in a selected day. Video recordings of flow
patterns were works were done by injecting dye into the flow to observe the
complex flow patterns (vortex shedding) around the engineering works and for
subsequent interpretation. In each test a number of overlapping photographs of
the bed were taken from vertically above to prepare a photo mosaic. In addition
for some tests oblique photographs and digital pictures were also taken to
prepare a Digital Elevation Model (DEM) and to determine the sediment sorting of
the bed. This will be useful for mathematical modeling and for characterization
of the local roughness of the bed.
The structures significantly modify the local flow field and turbulence pattern and this encourages scouring of the bed near the spur tip. The change in bed topography (scouring and deposition) occurs at very high rate initially and then diminishes. After a certain time period (3 to 5 days) a semi-equilibrium condition of the bed is reached when further local scour is negligible. As sediment transport continues, associated bedforms cause a fluctuation of the bed level up to the limit of the height of the moving features of the bed.
In each test, erosion-deposition patterns were identified by comparing the bed levels with and without structures. Fig. 2 shows such erosion-deposition patches for the single vane (full height). This shows that maximum scour has occurred near the tip of the vane parallel to the bank. The scoured section extended further upstream. Some of the bed changes in the figure (especially on the left side of the channel) would be accounted to the presence of bedforms at different locations during data collection. Due to the sloping crest of the spur and its low angle with bank, the deflection of the flow is such that the thalweg is relocated towards the middle of the channel and the point bar remaining substantially intact. As seen from the figure, deposited material has filled up of the old scour holes both upstream and downstream from the vane. Downstream deposition is of larger magnitude and occurs over a longer stretch. The nearbank siltation zone and the mid channel erosion zones are separated by a zero bed level change contour which indicates that the effect of the structure is reversed in the two areas. The same effects are also observed with the low structure but at to a lesser degree and the upstream deposition is not significant. When the high and low level single structures are compared, scouring of the bed with the high vane is as much as twice that with the low one. The maximum scour depth in low vane is 77% of the maximum depth without a structure while with the high vane it is much closer to the non-engineered case (91%). This indicates that the scouring strength of the flow with the high vane has not been modified significantly in the bend although the location has changed. This has an implication when less disturbance to the natural flow is expected with the structure. To check the overall effect of the structure on the channel reach, the total volume of the flow and the wetted surface area of the channel have been calculated from the upstream crossing to downstream one around the bend. The overall changes are less than 5% in both the low and high vane tests.
The effects of the structures on the flow pattern is local but this has an important implication for the overall effectiveness of the structure. After installing the vanes the longitudinal average slope (0.00133) does not change significantly but the lateral water surface are affected. In Fig. 3. mean values of the three components of velocities are presented where the isovels of streamwise velocity are overlayed by the cross-sectional velocity vectors at sections I and I2. These shows the marked difference of the flow pattern in a engineered channel compared with nonengineered channel.
Due to the affect of the structure (single high vane), just before the bend apex, the flow is guided both to the right and left of the structure. At the right side water flows over the crest towards mid-channel. This creates a counter clockwise rotation of the flow which opposes the clockwise secondary cell of the bend. A vertical shear layer in between two cells exists which is just to the right of the submerged crest of the vane. An additional zone of flow near the bed and to the left of the base of the vane exists which is the directed nearbed jet-like flow following the line of the vane in outward and oblique direction. Just downstream of the structure (e.g., section I2), two dominant zones of counter rotating flow cells are clear. A strong downwelling zone exists in between these two cells where unsteady nature of the water surface was observed. Smaller intermittent vortex structures with a vertical axis of rotations were also observed along that line (vortex street) which also indicates the existence of a shear layer there. This downwelling region moves gradually leftward towards mid channel in the downsream directions. A deeper section of the channel is formed in this highly turbulent flow region which ensures the protection of the outer bank from excessive scouring. From this discussion of the flow, it is obvious that there are significant differences between the flow structures and scour-deposition patterns generated by a rectangular large angled spur and low angled tapered spur (J-vane).
In Fig. 4,
normalised sediment transport for a full height single vane (JBF) and multiple
vanes (JMB) at sections I and J are shown. With the single structure bed load
was sampled earlier than in the multiple vane case. Consequently in this case
the equilibrium state was not reached. After installing the vanes sediment
transport was observed to occur at a high rate around the structure with
relatively reduced rate of transport (comparing to non-engineered channel) over
the point bar (e.g., JBF-I). After the bed had adjusted to the structure and
approached an equilibrium condition, the transport around the structure becomes
negligible (e.g. in JMB-I).
J-vanes
significantly modify flow patterns and vortex structures locally and on larger
scale when multiple structures are used in an array. There are marked
differences in the effects of this type of structures with the conventional
spurs or groynes. Following the initial stage after installation of the
structure, the bed load transport path in the bend apex does not change
considerably. Changes in flow velocities, turbulence, vortex pattern and
resulting substrate condition should be considered before recommending the
installation of vanes at a specific site.
References
ASCE Task Committee on Sediment Transport and Aquatic Habitats, Sedimentation Commiittee (1992) Sediment and Aquatic Habitat in River Systems, J. Hydraul. Eng., 118, 669-687.
Hey, R. D. (1990) Environmentally Sensitive River Engineering, in River Restoration edited by G. Petts and P. Callow, Blackwell Sciences Ltd.
Milhous, R. T. (1998) Modelling of Instream Flow Needs: The Link between Sediment and Aquatic Habitat, Regul. Rivers: Res. Mgmt., 14: 79-94.
Robson, B. J., Chester E. T. and Davis J. A. (1999) Manipulating the Intensity of Near-Bed Turbulence in Rivers: Effects on Benthic Invertebrate, Fresh. Biol. 42: 645-653.
Rosgen, D. L. (1996) Natural Channel and River Restoration Short Course Notes, Wildland Hydrology, Pagosa Springs, Colorado.
Statzner, B. and Higler, B. (1986) Stream Hydraulics as a Major Determinant of Benthic Invertebrate Zonation Pattern, Fresh. Biol. 16: 127-139.
Fig. 1 (a) Layout of experimental channel with sections of measurements (b) J-vane
Fig. 2 Erosion-deposition for single high level j-vane
Fig. 3 Contours of longitudinal velocity (cm/s) and lateral velocity vectors (a) sec i (b) sec i2
Fig. 4 Normalised sediment transport at different location of the sections i and j