Sediment entering lateral intakes depends on the flow pattern at the intake entrance. Using a structure in front of the intake entrance can change this pattern and as a result the entering sediment. One of the effective method to change the pattern and manage sediment entering a lateral intake is to use a skimming wall. The removal of sediments from the intake entrance using a skimming wall led to reduction of sediment volume at the intake. To guide flow into the diversion canal and increase skimming wall performance a spur dike was utilized at the opposite side of the intake channel. In this study, the effect of the skimming wall's angle with the bank, a combination of spur dike and skimming wall and discharge changes on controlling sediments entering the intake, intake ratio and bed topography were investigated experimentally. The effect of a skimming wall with three angles (10°, 14°, and 18°) and a combination of skimming wall and spur dike on opposite sides of the intake were investigated. Conducting dimensional analysis, non-dimensional ratios were extracted and test variables were specified. Results showed that in the case of having a skimming wall combined with a spur dike, the amount of sediment entering the intake decreased by 81%, 78.5% and 76% on average for walls with angles of 10°, 14° and 18° respectively. Combining a skimming wall and spur dike has a higher effect on reducing sediments entering the intake compared with a skimming wall alone by about 15%.
Neary et al. (1999) developed a 3D numerical model of flow on a 90° branch in a channel with rectangular section and verified it using experimental results. According to their findings, as the flow diversion ratio increases, the width of vortex area and its length decreases and increases respectively. Ramamurthy et al. (2007) demonstrated that increasing the flow diversion ratio reduces the length and width of the flow separation zone in the intake channel. In addition, the width of the separation zone in the intake channel is less at the floor compared to the surface (Hager 1992). Marelius & Sinha (1998) and Kuhnle et al. (1999) showed that the intensity of bed sediments entering an intake can be negligible after installing submerged vanes only when the discharge ratio of the width unit of the intake to the width unit of the main channel (qr) is less than about 0.2. After experimental study, to increase qr and maintain the efficiency of the submerged vanes, two solutions proved appropriate; first, embedding the lateral intake next to the submerged vanes and second, widening the intake entrance (Barbhuiya & Dey 2004). Ahmad (1953) showed that spur dikes inclined towards the upstream have better performance in terms of flow strength and deposition. Results have shown that a spur dike has a more prominent effect in controlling sediments. Considering the literature, most studies have been conducted on submerged vanes, sill, spur dike or a combination of them in intakes. Therefore, more studies are required in this field especially when a skimming wall is employed in front of the intake. In addition, to represent the effect of skimming wall angle on the amount of sediment entering the intake, it is required to use a combination of wall and spur dike and conduct a comparison of this state with a no-structure state. Therefore, the aim of this study was to use a skimming wall in controlling the sediment entering a lateral intake with an angle of 60° from the rectangular channel.
MATERIALS AND METHODS
: ratio of discharge of sediment entering the intake channel to discharge of sediment entering the main channel; : ratio of discharge of intake channel to main channel discharge (intake ratio); : Froude number of flow in the upstream of the intake; : ratio of flow depth in the main channel to the height of the skimming wall plates; β1: angle of the skimming wall with the main channel bank.
RESULTS AND DISCUSSION
Ratio of sediment diversion into the intake
The relationship between H/d and Gr
Relation between skimming wall parameter β1 and ratio of sediment entering intake
Changes in cross-section at upper and lower entrances
Investigating the cross-section of the bed in the upper entrance (x = 10 m), it can be divided into six sections. The first section is from the intake bank to skimming wall, which is 0–13 cm, 0–18.14 cm and 0–23.18 cm, as shown in Figure 10(b), 10(d) and 10(f) respectively, for a wall with angles of 10°, 14° and 18° respectively. In this region, as the flow enters this section and flows over the skimming wall, it removes sediments behind the structure and moves them into the intake. After a while, this section deepens and degradation continues to the end of the experiment. In some experiments, degradation depth reaches the floor of the channel. In all experiments, the major part of the sediments delivered into the intake is related to the section behind the skimming wall (from the intake bank to the skimming wall). In the second section, starting from in front of the skimming wall to the transverse distance of 45 cm (13–45 cm), sediment accumulation occurs. Since the height of the skimming wall is higher than the bed layer height, it was expected that no sediment would enter the channel; however, because of flow turbulence in the inlet area of the intake as well as deposition of part of the sediments and the creation of a slope at the foot of the skimming wall, sediments slide on this slope and enter the intake channel (Figure 8(a) and 8(b)).
Although, the skimming wall does not control all sediments, a significant decrease is observed in sediments entering the intake. The third section is from the transverse distance of 45 to 70 cm. In this section, the longitudinal speed of the flow increases by installing the spur dike in the opposite bank and above the intake that enforces skimming wall performance and creates a strong secondary flow, and sediments are prevented from entering the intake. This secondary flow in the channel floor is against the intake flow on the surface towards the intake. The concurrent existence of intake and spur dike in the opposite bank results in transverse dislocation of maximum speed and therefore dislocation of the thalweg. Dislocation of maximum transverse velocity starts from upstream of the spur dike and reaches its maximum value in front of the intake because of the flow accelerating in the intake. This fast transverse dislocation of maximum velocity at the angle of 18° results in 70 mm degradation of the floor.
The fourth section is from the transverse distance of 70 to 90 cm. This area is strongly affected by the spur dike. Near the spur dike, as the power of the secondary flow increases, sediments are removed from that area and are transferred to the downstream of the spur dike. Near the nose of the spur dike, the average vertical component of the speed is enforced and washes the sediments and transfers them to the downstream of the spur dike. The maximum height of sediment accumulation behind the spur dike increases to 50 mm or 30% of the total height and sediments entering the intake are decreased by installing the spur dike against the intake channel. The reason for this could be the reduced width of the flow separation line from installing the spur dike, the width of the flow separation line being high in the floor and because of the high concentration of sediments on the floor, and a high volume of sediments entering the intake by intake suction. According to the results of a study conducted by Gohari et al. (2009), by installing a spur dike in front of the intake, the width of the flow separation line decreases on the floor and less sediment enters the intake; on the other hand, by extending the flow separation line on the surface, more flow enters the intake. The fifth section is from the transverse distance of 90 to 130 cm. The scouring effect of the spur dike nose continues in this section; however, it is not observed in the profile related to control degradation. The degradation depth for an angle of 10° is more than for other angles. The sixth section is from the transverse distance of 130 to 150 cm. No scouring occurs in this section and sediment accumulation is observed.
Comparing results of this study with results of other studies
The aim of this study was to use a skimming wall and combination of skimming wall and spur dike in controlling the sediment entering a lateral intake at an angle of 60° from the rectangular channel. Results showed that in the case of having a skimming wall combined with a spur dike, the amount of sediment entering the intake decreased by 81%, on average. Combining skimming wall and spur dike has a superior effect of 15% in reducing sediments entering the intake compared to the skimming wall alone. With increasing sediment discharge ratio, the diverted discharge ratio increases. In the intake opening behind the skimming wall, we have scouring in the upper section and sediment accumulation in the lower section of the lower opening. A skimming wall with an angle of 10° combined with 60° angle spur dike 2b distant from the centre of the intake entrance is more effective for increasing intake discharge and controlling sediment relative to 14° and 18° angles. As (H/d) increases 37.5%, 100%, and 100%, for 10°, 14°, and 18° respectively, for the skimming wall alone Gr increases 63.5%, 69%, and 88.5%, and for the combination of skimming wall and spur dike it increases 87.5%, 96.5%, and 57%.
The results also show that using a skimming wall and spur dike make it possible to direct the thalweg toward the intake port, and a trench is made toward the intake port.
With regard to the combination of skimming wall and spur dike being new, it is suggested that other experiments should be conducted with various Froude numbers and spur dikes with different dimensions and situations.