Laboratory Investigation of Submerged Vane Shapes Effect on River Banks Protection

Transcription

1 Australian Journal of Basic and Applied Sciences, 5(12): , 2011 ISSN Laboratory Investigation of Submerged Vane Shapes Effect on River Banks Protection Touraj Samimi Behbahan Department of Civil Engineering, Islamic Azad University - Behbahan Branch, Behbahan, Iran. Abstract: Sediment control in alluvial rivers is an important task in river engineering. Submerged vanes which are installed on the river bed at different angles of attack to the flow direction with different array can control river bank erosion, improve river cross-section and bed morphology. These vanes can be made of metal, wood etc. In present study, to investigate effect of vane-shapes on river banks protection, a series of experiments on a straight canal with 20-m length; 0.7-m width and 0.6-m depth were carried using galvanized vanes in different shapes. These vanes with 1-mm thickness, 15- cm width and 13-cm height were installed on the canal bed such that the vanes height above the river bed was 7-cm. three types of same size vanes were used in these experiments. The first type were flat, which were installed in an angle of attack of 20-degree to flow direction. The second type were angled vanes with an angle of 20-degree in the middle of vanes, which were installed such that the first- half of these vanes were in an angle of attack of 10-degree and the second half in 30- degree of flow direction. The third type include two parts flat and cured, these vanes were installed such that the flat part were in an angle of 20-degree with flow direction. Using measured data, the canal and related vanes were modeled by ANSYS and SURFER software. Results show that the curved and angled vanes compared to flat vanes are more effective in river-bank protection by 35% and 20% respectively. Key words: Submerged vanes, sediment transport, river bank protection and ANSYS software. INTRODUCTION Sediment management especially river bank scour and sediment deposition, is one of the most important task in river engineering. River banks scour, transport and deposition of scour material cause reduction of river flood- conveyance capacity. Many different techniques such as gabions and groins have been used to protect river banks. Application of submerged vanes along river banks modify sediment transport regime and may control scour and deposition places. These vanes can be designed and installed on the river bed in multi-rows and variety of angles of attack to the flow direction often between 10 to 30 degrees depend to the vanes installation angle of attack to the flow direction. They create secondary currents around themselves, and modify velocity distribution and sediment transport regime. Due to the pressure change around the vanes, the pressure increasing from bottom to top on the low-pressure side, decreasing from bottom to top on the high- pressure side. These pressure changes cause the fluid flowing along the high-pressure side to obtain an upward velocity component, while on the low-pressure side there is a downward velocity component, Fig. (1). Fig. 1: Vortex Configuration around a bottom vane. Odgaard and kenedy (1983) were attempted to design a system of vanes to stop or reduce bank erosion in river curves (Odgaard, A.J., 1983). Odgaard and spoljaric (1986) through laboratory tests were suggested that vanes were laid out to change the cross- sectional profile of the bed in a straight channel (Odgaard, A.J., 1986). Corresponding Author: Touraj Samimi Behbahan, Department of Civil Engineering, Islamic Azad University -Behbahan Branch, Behbahan, Iran. [samimi_touraj@yahoo.com] 1402

2 Significant changes in depth were achieved without causing significant changes in cross-sectional area, energy slope, and downstream sediment transport. Design, installation and performance of a system of submerged vanes for erosion protection in a bend of Fast- Nishnabotana river, Iowa, were investigated by Odgaard et al., The system was found to effectively reduce velocity and scour along the bank without changing the energy slope of the channel. Odgaard et al., 1986, developed a procedure for a rational design of a system of submerged vanes for depth control in alluvial- river channels. They suggested that the submerged vane structure may be an effective, economic, low- maintenance and environmentally acceptable sediment control structure with a wide range of applications. Wang, 1991, is developed a theory for calculating vane induced circulation and associated transverse velocity components, bed shear stresses and changes in depth distribution (Wang, Y., 1991). Odgaard and wang, 1991, were investigated scour and deposition control with submerged vanes and reported that the vanes function by generating secondary circulation in the flow (Odgaard, A.J., 1991). The circulation alters magnitude and direction of the bed shear stressed and causes a change in the distribution of velocity, depth and sediment transport in the area affected by the vanes. Marlius and sinhe, 1998, were investigated the physics of flow passing the vanes with a large angle attack with flow direction. The main objective of their study was to find an optimal angle of attack to flow direction which produces a strong secondary current (Marlius, F. and Sinhe, S.K, 1998). Voisin and Townsend, 2002, were established a curved rectangular channel to investigate effect of submerged vanes on the shore protection of bends (Voisin, A. and Townsend, R.D., 2002). The submerged vanes which have been used in different investigations include flat plates made of different materials. In the present study, to investigate effect of vane-shapes on the scour and sediment control, three types of vane shapes: flat, angled and curved types were used along a straight channel in a series of laboratory experiments. Hydraulic parameters of flow and depth of sediment were measured. Experiments: Experiments have been carried on a rectangular cemented canal of 20-m long, 70-cm wide, 60-cm deep and longitudinal slope of 0.005, Located in the civil engineering department hydraulic lab of the Shahid Bahonar University of Kerman, Iran (Fig. 2). Two storage tanks were located at the two ends of the canal. The first storage tank with 4*3*2 m 3 capacity and the second tank with 4*4*4 m 3 capacity were located at the upstream and downstream of the canal respectively. A centrifugal pump is located in the second storage tank to recirculate water through two 20-cm pipes to the upstream tank. Discharge control was obtained by gate valves which were installed on the pipes. First tank was divided in two parts which were related to each other from the bottom, pumped- water falls into the first part of the tank and enters the second part from the bottom to achieved uniformity of flow. The canal bottom consisted of a 6-cm thick layer of sand with median diameters of 1 to 4 mm. Fig. 2: Laboratory of Shahid Bahonar University. In these experiments a total of 40 galvanized plates of 15-cm long, 13-cm high and 1-mm thick in double and triple arrays along the bank was installed. These vanes were installed on the canal bed at an angle of attack of 20- degree with the flow, such that the heights of vanes above the sand level were 7-cm. The stream wise distance between arrays was 30-cm; the lateral spacing and the distance from bank were 12-cm and 8-cm respectively. Three types of same size vanes were used in three sets of experiments. The first type were flat and instated at an angle of attack of 20-degree with flow direction. The second type were angled vanes with an angle of 20- degree in the median of vanes, which were installed such that the first-half of these vans were in an angle of attack of 10-degree and the second part in 30-degree with flow direction. The third type includes two parts, flat and carved. This type was instated such that the flat part was in an angle of attack of 20-degree with flow direction. 1403

3 The vanes were arranged in two patterns: parallel to each other and zig zag. For each type of vanes four experiments were carried in double and triple arrays with parallel and zig zag patterns, Fig (3-6). Each experiment takes three hours to establish a stable position in canal bed morphology, after that, the pump was shutdown, The bed sediment elevations and the flow velocity were measured by the sand surface meter wh-406 instrument with the trade mark of KENEK, made in Japan, with accuracy of 0.5 mm for depth- meter and 1- mm/sec for velocity meter. Fig. 3: Bed changes in several channel cross sections for vanes layout in two parallel rows. Fig. 4: Bed changes in several channel cross sections for vanes layout in two zig zag rows. RESULTS AND DISCUSSIONS Results show that the first two or three rows of vanes do not affect bank protection. After third row of vanes, curved vans compare to angled and flat vanes show more affect on bank protection. Measurements and 1404

4 computations show 35% sediment supper elevation. In using curved vanes, 20% for angled vanes compare to flat ones. Bed sediment conditions can be considered in three ports of canal as follow: Fig. 5: Bed changes in several channel cross sections for vanes layout in three parallel rows. Fig. 6: Bed changes in several channel cross sections for vanes layout in three zig zag rows. The first part include the first three rows of vanes, experiments show that there is some scour along this part without sedimentation. This fact leads the engineers to install submerged vanes three rows before bank river protection place. The second part includes the remaining vanes, from the third row through the end. In this region, complete bank protection have been done. The third part includes a distance from last row of vanes toward downstream until 6 to 8 times of vanes height. 1405

5 River bank can be protected at this section, but the sediment elevation decrease gradually to reach the initial level. In Fig. (3-6), section 1 presents canal bed morphology at the first part, sections 6 and 10 at the second part and section 13 at the third part of canal vanes installation region. This constructed channel modeled with ANSYS software and flow speed on different points confirmed the obtained results that mentioned in previous section, Fig (7). Bed surface of channel modeled with SURFER software, Fig (8). Obtained results show that in experiments with curve vanes, the coast stabilized more integrated. Fluid velocity in middle of the channel is more. Then if these vanes are used in two sides of coasts, after a while coasts are stabilized and the middle of river is deeper. Another result that can be obtained from SURFER software is submerged vanes don`t have any effect on sediment transport in length of channel and their effect is only on sediment transport in width of channel. Fig. 7: Fluid velocities obtained from ANSYS software for angular vanes in two parallel rows. Fig. 8: Channel bed after experiment for curve vanes in three parallel rows, obtained from SURFER software. Conclusion: Coastal erosion is one of the main problems in the river engineering. Recently, submerged vanes are a technique that is used to solve this problem. These vanes will be installed with the angle between 10 to 30 degrees than the water flow to the ground floor and with the creation of spin currents cause changes in the sedimentation of rivers. In this paper using experiments on a channel and also the vanes with the new shapes of curve and the angular, the comparison between arrangements of these two types of has been done. The results indicated that curved vanes with about 15% more effective in stabilizing the shores of the angular vanes. 1406

6 REFERENCES Marlius, F. and S.K. Sinhe, "Experimental Investigation of Flow Past Submerged Vanes", Journal of hydraulic engineering, ASCE, 124(5): Odgaard, A. J., Wang, Y., 1991"Sediment control by Submerged Vanes", Journal of hydraulic engineering, ASCE, 117(3): Odgaard, A.J., A. Spoljaric, "Sediment Control by Submerged Vanes", Journal of hydraulic engineering, ASCE, 112(12): Odgaard, A.J., J.F. Kenndy, "River-Bend Bank by Protection Submerged Vanes", Journal of hydraulic engineering, ASCE., 109(8): Voisin, A. and R.D. Townsend, "Model Testing of Submerged Vanes in Strongly curved narrow channel bends", Can. J. civil. Eng., 29: Wang, Y., "Sediment Control by Submerged Vanes", Ph.D Thesis University of Iowa, Iowa City, Iowa. 1407