AN EXPERIMENTAL INVESTIGATION ON SEDIMENT CONTROL IN INTAKES USING SUBMERGED VANES

Sadjedi Sabegh M., Habibi M., Rahmanian M.

Soil Conservation and Watershed Management Research Center,

P. O. Box: 13445-1136, Tehran, Iran

Abstract: Sediment deposition in the entrance of river intake structures is an important problem that reduces the system efficiently and increases the cost of dredging and system maintenance. Thus using a suitable method for preventing sedimentation is necessary. Submerged vanes are cheap and simple structures that can be used for this purpose without further protection. The correct design and application of submerged vanes need experimental investigation on the physical models.

In this project the goal is to study about the mechanism of sedimentation in the entrance of delivery channel and investigation on the role of submerged vanes in preventing sedimentation in the delivery channel of flood distribution system. The main variable parameters studied in this experimental research are discharge and submerged vanes array. Other parameters of vanes array include the longitudinal distance between two successive row of vanes, the across distance of vanes, number of vanes in each row, total number of vanes and height of vanes.

After primary experiments, thirty-eight experiments were done in two series with application of submerged vanes. The methodology of this research is as follow: visiting the prototype and field observation of the problem, studying different proposed solutions, information and data collection and processing, design of physical model based on mechanical similarity, constructing the model, model calibration, performing experiments, analysis of experimental results and making comments.

The following results were obtained based on this research:

l        Optimizing the submerged vanes orientation for guiding the sediments into the main channel and decreasing sediment inclusion and deposition in the entrance of delivery channel.

l        Finding the best design parameters for submerged vanes consisting the best orientation, the cross-distance of the array of vanes, and the angle of vanes.

l        Recommendations for using submerged vanes in practical projects. 

Keywords: submerged vane，intake，physical model，sedimentation.

1    INTRODUCTION

Odgaard and Kennedy proposed the technology of submerged vanes, at first in 1983. In primary applications only one submerged vane was used with an angle of 15-30 degree versus direction of flow and with a height of 0.2d-0.5d (d is the flow depth) and as a result of pressure gradient in both parts of vane, vortex current in downstream was created. When a number of vanes set in each row regularly, the created vortex current was stronger. This current created some changes in the direction and quality of bed shear stress and in turn in the topography of streambed. These changes in bed are shown in Figure 1.

It was suggested that in order to produce strong vortex currents in a straight stream, the transversal distance between the vanes must be less than 2h-3h (h is the vane height above the stream bed) and with longer distances the vanes will create single vortex. Also the longitudinal distance between vanes must be 15h-30h that depends on the location and geometry of the stream. The distance to wall mustn’t also be more than 4h. The first row in vane collections must be established in the streambed three rows upward of the protected place and three rows downward of it. Always submerged vanes (in-groups of 2-4 vanes) were located parallel to themselves in each row.

The main application of submerged vanes is erosion control of channel curve, changing the channel cross section, adjusting the stream direction and creating a new bed morphology. One of the other applications of submerged vanes is in water diversion and intakes that will cause adjusting the flow and sediment patterns in the entrance of intake.

2    GENERALITIES IN PHYSICAL MODEL

The proposed physical model in this research was based on the practical problem of sedimentation in the intake of Bishe-zard River (one of the Iranian rivers in Fars province). Using submerged vanes in front of the intake was selected as the most proper way to overcome the sediment problem. In this research after studying the flow patterns in the main channel and intake, a physical model was made and changes on the arrows of transversal vanes causing changes on bed morphology were investigated (assuming the width of delivery channel is half of the width of main channel).

On the basis of limitation on depth, available laboratory area, available discharge, flume dimensions, flume roughness, sediment size and height of vanes, the horizontal and vertical scales of the model were designed at 1/40 and 1/10, respectively. Using these scales and Froude law of similarity, hydraulic characteristics of flow in the model are calculated and introduced in Table 1. For calibration of roughness and threshold movement of sediments in different discharges, sediment diameter was designed about 3.4 mm. The model was made in a laboratory flume 2.5 m wide and 30 m long and with a diversion channel 1.3 m wide. The experimental set up is shown in Figure 2.

3    DESCRIPTION OF EXPERIMENTS

Experiment without using submerged vanes

In order to know how flow patterns act on the intake without application of submerges vanes a set of experiments was done with three discharges of 38, 114 and 137 lit/s. In all of these discharges the diverted current in the intake was about %20 of that of the main channel. In these experiments average hydraulic time in each run was about 2.5 hours until receiving the sustainable condition and the flow patterns in intake were similar to the flow patterns in the river curve. Therefore sediment deposition was observed in the inner side of intake and erosion was recorded in the outer side.

In fact the experimental observation showed that sedimentation in the inner side of intake has a conical shape with the head of cone in the entrance of intake that expands inside the diversion channel. Also the diverted flow has caused erosion of the bed in the outer side of the intake. Also with increasing discharge, the maximum height of the deposited sediment became closer to the inner wall of intake, and the sedimentation cone expanded in a greater area and also the erosion in outer side of division channel became more. This process was continued until the intake became full of sediment.

Experiment with applying submerged vanes

These experiments have been conducted in two following sets:

1- a group of submerges vanes oriented in three vanes in each row

In these experiments nine rows of vanes were used with a height of 2.5 cm; length of 7.5 cm and declination angle of 20 degree with respect to the flow direction (Figure 3). The experiments included 16 runs with discharges of 114, 137 and 81 lit/s (Table 2).

2- a group of submerged vanes applied in zigzag form

In this set, vanes were located in zigzag rows and in two columns. Here, the inner vanes with height of 3.5 cm were 1 cm higher than the outer vanes (Figure 4). The idea for zigzag orientation of vanes was propounded from the fact that the energy of diverted current is the main reason for sediment inclusion into the entrance of intake. In this case the longitudinal distance of vanes column was 12 cm and the other condition of rows were similar to the first series of experiments. In this set 22 runs were done (Table 3).

4    ANALYSES OF EXPERIMENTAL RESULTS

Since the main aim of the present research was modification of channel bed topography adjacent to the entrance of intake with the purpose of decreasing sediment inclusion into the diversion channel, thus the vane orientation that was able to give a deeper and more uniform groove in front of the intake to conduct sediments towards the main channel and to prevent sediment from entering into the diversion channel, was appointed as the best solution.

The best solution for the first series of experiments with three vanes orientation in each row was obtained when the distance of inner vanes from the channel wall was 3h and the distance between the two other vanes in each row was 2h (here h is equal to 2.5 cm). In this case, the maximum groove depth in the main channel was 2.8 cm (equal to 1.12h) for discharge of 81 lit/s. In discharge of 114 lit/s, maximum groove depth was 3.4 cm (equal to 1.36h), and in discharge of 137 lit/s, maximum groove depth was 3.6 cm (equal 1.44h). Here, with 70% increase in the discharge the maximum groove depth increased %30.

In these experiments the location and length of grooves remained constant, the maximum groove depth was happened in the first third of intake opening and the average groove width changed from 5h to 8h.

In this case the other important observation was sediment deposition at the end of vanes that caused sediment inclusion into the intake. The amount of sediment entering the intake reduced to %50 compared to the volume of sediments entering the intake without vanes. When the distance between two vanes increased from 2h to 3h, in discharge of 114 lit/s, the maximum groove depth became 3 cm (equal to 1.2h) that shows %13 decreasing. When the distance between the first vane and the channel wall decreased from 3h to 2h, the groove depth also decreased %36. The experimental results showed that when the across distance between vanes is more than 3h; the vanes act as singular vane and in the best-case sediment deposition in the intake decreases 55%.

Application of vanes in zigzag form was tested with discharges of 81, 114 and 137 lit/s. In these series, the height of inner vanes towards the intake was taken 3.5 cm and the height of external vanes was 2.5 cm, both in the range of 0.2d-0.5d. Here there was an unconcernedly vortex between the successive rows of vanes and therefore sediment transport between vanes was easier than that with three vanes in each row. The minimum sediment inclusion in this series was obtained when the distance of inner vanes from the channel wall was 1h (2.5cm) and the distance between the two other vanes in each row was 3h (7.5cm). Also in these series (zigzag form), the maximum groove depths for discharges of 81, 114 and 137 lit/s were 2.9 cm (equal to 1.16h), 3.9 cm (equal to 1.56h) and 4.1 cm (equal to 1.64h) respectively. Here with %70 increase in the discharge, the maximum groove depth increased %45.

In these experiments the groove development began from the first vane and the width of groove in the entrance of intake was 6h. The groove was continued throughout the rows of vanes.  Also the amount of sediment entering the intake decreased %75---%80 and the shape of sedimentation cone changed to a more expanded form.

5    CONCLUSION

Two series of laboratory experiments were done on a water intake applying submerged vanes in front of the intake. In the first series the vanes were oriented in three vanes in each row and in the next series vanes were used in zigzag form. In each series of experiments the lateral and longitudinal distance of vanes were changed to show the best orientation.

In the three vanes orientation the best result was obtained when the distance of inner vanes from the channel wall was 3h and the distance between the two other vanes in each row was 2h, but for vanes with zigzag form, when the inner distance was 1h and the across distance between vanes was 3h, the best solution was obtained.

Comparing the results of the two above series of experiment, it can be concluded that with using three vanes in each row, the sedimentation in the intake and delivery channel decreases %55. However with applying vanes in zigzag orientation the depth and shape of groove becomes more suitable and the sediment deposition decreases 75%.

References

[1]    Barkdoll, B. D., Hagen, B. L.& Odgaard A.J. (1995), Sediment Exclusion at Hydropower Intakes Using Submerged Vanes, Proc. Waterpower, ASCE.

[2]    Nakato, T., Kennedy, J. F. & Bauerly, D. (1990), Pump Station Intake Shoaling Control with Submerged Vanes, Journal of hydraulic engineering, ASCE, 116 (1), 119-128.

[3]    Odgaard, A. J. & Wang Y. (1991), Sediment Management with Submerged Vanes. 1:Theory, Journal of Hydraulic Engineering, ASCE, 117(3). 267-283.

[4]    Odgaard, A.J. & Wang J. (1991), Sediment Management With Submerged vanes. 2: Application, Journal of Hydraulic Engineering, ASCE, 117(3). 284-302.

[5]    Wang Y. & Odgaard A .J. & Melville, B. W. & Jain, S. C. (1996), Sediment Control at Water Intakes, Journal of Hydraulic Engineering, ASCE, 122(6), 353-356.

            Table 1    Hydraulic characteristics of flow in prototype and model

            Table 2    Experimental values in series A

               Table 3    Experimental values in series B

Notation

The following symbols are used in this paper:

D           flow depth            q_d        diverted discharge in the intake

d_m                      model depth         Q_m      model discharge

d_p                       prototype depth           Q_p    prototype discharge

dv _(max)                        maximum groove depth       s1    distance of inner vanes from the channel wall

h            vane height           S2, S3        distance between vanes

Q           discharge in main channel    T     return period

Fig.1    Sediment deposition pattern in intake without applying submerged vanes

(Q= 114 lit/s, Q_d= 0.2Q)

Fig. 2    Schematic plan of physical model in laboratory

Fig. 3    Plan location of submerged vanes near the intake (three vanes in each row)

Fig. 4    Plan location of submerged vanes near the intake (two vanes in each row, zigzag form)