Control of Sedimentation at a Canal Intake Using a Sediment Trap

S. Mansour¹, S. Abdel-Fattah¹ and M.B.A. Saad²

¹ Researcher                  ² Director

Hydraulics Research Institute, Delta Barrage 13621, Egypt

Fax: 202-2189539 – E-mail: draulics@intouch.com

Abstract: El Nasr irrigation canal takes its water from El Nobaria canal and irrigates an area of about 40,000 Hectares. The canal has a right angle intake, which suffers from a sedimentation problem. 

A physical model of an undistorted scale 1:25 was constructed at the Hydraulics Research Institute, Egypt, to test the validity of the sediment trap. The sediment transport was measured in the field using a mechanical sampler called “Delft-Nile Sampler”. According to the results of the field measurements, a design of the sediment trap was proposed and tested in the model. Sediment transport was studied using a physical model by applying the flow velocity measurements in some of sediment transport prediction equations. The behaviour of the sediment trap, which is located in the Nobaria canal (200 m upstream of El Nasr intake structure), was tested using light plastic material. These model measurements were also used to check the approach flow pattern in addition to track the surface current by floats and dye.

The tests have shown that the use of a sediment trap will minimise the sediment entering El Nasr canal and thus will protect the mechanical parts of the pumping station.

1    Introduction

El Nasr canal irrigates the new reclaimed land in the north west part of Egypt.  The intake structure of this canal is located on the left bank of El Nobaria canal (km 58.50) at a right angle orientation.  Five pumping stations were constructed in series to lift the irrigation water along the canal according to the irrigation requirements. Each Pumping station lifts 10.5 m. The first pumping station is situated 1000 meter downstream of the intake structure of the canal. Figure 1 shows a schematic definition of the area under consideration. As the sediment load of El Nobaria canal is rather high, a severe sedimentation problem is characterised at the intake structure and at the approach channel of El Nasr canal.

In this paper, a full description of the problem definition, scale model, field measure-ments, sediment trap function and performance is presented.

1.1    Problem definition

Due to the orientation of the intake structure an unequal flow distribution problem was detected at the approach channel of El Nasr canal. As the in-curve vents of the intake regulator attract less water than the outer vents, sedimentation is expected to deposit in the inner curve.

A solution for the unequal flow distribution problem along the intake structure’s vents was given in [5].

1.2    Methodology

In order to reduce the maintenance costs and the operational problems of the pumping station No.1 and the intake structure of El Nasr canal, a detailed field and hydraulic study was performed for this area. The study comprised a field survey including sediment transport measurements using the well-calibrated Delft-Nile sampler. A physical model was constructed to carry out the needed tests.

2    The Prototype and Physical Model

A detailed hydrographic survey was carried out for 2 km of El Nobaria Canal at the intake of El Nasr canal area, and covers 1 km of the intake channel.  All relevant hydrological data were collected, such as maximum and minimum water levels and discharges in both canals.  Bed and suspended sediment loads were measured [3]. Based on the results of the field observations, an undistorted physical model of a scale 1:25 was constructed.  The scale was chosen to fit the available space and to ensure sufficient water depth for hydraulic measurements.

Based on Froud similarity conditions, the different flow characteristics are presented in Table (1). To calibrate the model, the water surface slope was controlled in the model according to that in the prototype.  The velocity distribution at three cross-sections was measured in the prototype and in the model for a given discharge.  Comparison between the model and the prototype measurements revealed an agreement in the range of ±2.5%.  Therefore, it was possible to say that the model simulates the prototype.

3    Field Measurements

The sediment discharge, (Q_s), was measured at three cross sections using the Delft-Nile Sampler. This sampler is designed to measure the bed load, suspended load and the flow velocity simultaneously [1]. The locations of the cross sections were selected in a stable reach to avoid non-steady bed conditions during the measurements, as shown in Figure 3. Each cross section was divided into three sub stations. The field measurements revealed that 64% of the total sediment load of El Nobaria canal is entering to El Nasr canal. Whereas, 36% of the total sediment load is passing downstream reach of El Nobaria canal. It was found that 67% of the sediment transport rate could be classified as suspended load while the remaining 33% could be considered bed load [3].

The total load transport rates were computed, as well, using the prediction methods of Bagnold (1966), Engelund-Hansen (1967), Ackers & White (1973), and Van Rijn (1984). These formulae were selected because they were developed under hydraulic and sediment conditions suitable for sand bed rivers and channels. Table (2) shows the measured and predicted values of the total sediment transport rates at the three cross sections. The prediction methods of Ackers & White (1973), and Van Rijn (1984) showed that 65% of the total sediment load of El Nobaria canal is entering to El Nasr canal. This result is consistent with the results of field measurements. Whereas, the prediction methods of Bagnold (1966) and Engelund-Hansen (1967) showed that 55% of the total sediment load of El Nobaria canal is entering to El Nasr canal. According to the field measurements, a sediment trap was designed.

4    Results

The suggested sediment trap was located, as shown in Figure 4 in El Nobaria canal 200m upstream of the intake structure centreline. It had a depth of 2m and a bottom width of 25m and a bottom length of 50m. The side slopes were chosen to be 5H:1V to ensure their stability under water.

The volume of the trap was to be sufficient to accommodate 3500 m³. According to field measurements, the bed load was about 34 m³/day. As nearly all of this rate was going to be settled in the trap, together with a part of the suspended load then the sediment trap was to be dredged each three months, in order to maintain its efficiency on trapping the sediment load of El Nobaria canal.

The sediment trap was tested in the physical model using light plastic material. This material simulated the movement of sediment particles. The plastic material was laid in the model for 48 hours and then the model was gradually and slowly started up. It was left working for 10 hours. This period represented a working period of around 2 days in the prototype. The test results showed that nearly most of the plastic material (85%) settled in the sediment trap.

5    Conclusions

Due to the orientation of the intake channel of El Nasr canal an unequal flow velocity distribution was observed at its intake regulator. Another problem existed at the intake location, which was the deposition of sediment at the intake regulator and at the approach channel of El Nasr canal. The deposition problem was due to the high sediment load of El Nobaria canal. The construction of a sediment trap had proved to be a good solution to attract the high sediment load of El Nobaria canal. The dimensioning of the trap was reached according to the field measurements. These measurements were carried out using the Delft-Nile Sampler and resulted in less value than the predicted values using the well-known sediment transport equations.

To maintain the efficiency of the sediment trap regular dredging was required. The performance of the trap was tested in a scale model using light plastic materials. The model was calibrated according to the field measurements. As a result, clear water was observed to enter El Nasr canal then there was no deposition problem at its intake structure.

References

[1]    Gaweesh, M.T.K., and Van Rijn, L.C. Bed Load Sampling in Sand Bed Rivers, Journal of Hydraulic Engineering, ASCE, Vol. 120, No. 12, Hydraulics Division, USA, 1994.

[2]    Hydraulics Research Institute, Hydraulic Study of Sediment Transport at the Intake Structure of El Nasr Canal of the Nobaria Canal, Technical Report, Delta Barrage, Egypt, 1987.

[3]    Hydraulics Research Institute, Field measurements of sedimentation at the intake of El Nasr canal, Technical Report (65/1999), Delta Barrage, Egypt, 1999.

[4]    Iowa Institute of Hydraulic Research, Enhanced Performance and Reliability of Water Intakes for Generating Stations, Iowa, USA, 1992.

[5]    Mansour, S., and Saad, M.B.A. Use of Submerged Wall in Controlling Sedimentation at El Nasr Canal, ISSH’2000, Beijing, China, 2000.

[6]    Vanoni, V.A. Sedimentation Engineering, ASCE, New York, USA, 1975.  

                    Table 1    Model scale

        Table 2    Measured and predicted total sediment transport rates

Fig. 1    Schematic description of the considered area

Fig. 2    Schematic plan of the physical model

Fig. 3    Measured sediment load at the three cross sections

Fig. 4    Layout of the sediment trap