عنوان مقاله [English]
Background and Objectives: One of the new methods for controlling erosion on the outer bank of river curve is the use of plates connected to the riverbank. Coastal vanes are the environmental structures which are used to control bank erosion, divert flow from bank to river center, improve sediment transport status, develop river for sailing, restorate and develop river's aquatic habitat. Despite the many advantages of coastal vanes, there is no detailed information about erosion and sedimentation and flow patterns around them under different hydraulic and geometric conditions and fewer research have been done. In the present study, the effect of different installation scenarios of triangular permeable vanes on bed sediment longitudinal and transverse profile changes in a laboratory channel with a 180-degree bend is evaluated.
Methods: In this research, the main purpose is to investigate sedimentation and erosion patterns in the rivers’ bend using coastal triangular vanes; in this regard, the impact of distance between triangular vanes, their effective length, and Froud number on the erosion control of the outer bank of the 180 degree bend is focused. Experiments in a laboratory flume with a mild 180-degree bend with a ratio and a rectangular cross section with a width of 0.6 meters is done. The angle of fixed triangular vanes is 60 degrees, the height of the permeable triangular vanes from the sediment surface is 10cm and the permeability of the triangular vanes is 12%. The experiments were carried out with limpid water mode. The effective lengths of the prepared vanes were 12, 15 and 20 cm with installation intervals of 60, 75 and 100 cm respectively and were run at two different inlet discharges.
Results: Investigating the effect of the distance between permeable triangular vanes on bed topography shows that in each Froud number, with increasing the distance of vanes from each other, the maximum scour depth will be increased. The results demonstrate the use of permeable triangular vanes causes the flow deviation from the outer bank to the center and then the inner bank of the flume, which leads to control the erosion in the river outer bank. By increasing the inlet flow discharge and increasing the effective length of triangular vanes and the distance of triangular vanes from each other in a 180-degree bend, the maximum scour depth and volume are increased. The installation of triangular vanes with an effective length of and a distance of 5L causes the flow deviation from the outer bank to the vanes cape and the middle of the flume, consequently results in a decrease in the shear stress in the middle of the flume and scour depth of the erosion channel and creates thalweg in the vanes cape.
Conclusion: The results illustrate that by installing the triangular vanes, the point-bars are created in the interval between them. Therefore, during the effective length of 12cm, the maximum height of the point-bar on the outer bank equals to 30 and 31% of water depth, between the 0 to 170 degrees and 68.7 to 115 degrees equal to 11 times and 3 times the distance between the vanes, for discharge values of 13.5 and 15.5 l/sec, respectively. Also, for the mentioned conditions the width of the point-bar on the outer bank reaches 85% and 75% of vanes’ effective length , respectively. Permeable triangular vanes with six pillars are recommended for meandering rivers like Karun in 90 and 180-degree arcs. These rivers have high torsion and curvature as well as high flow depth and low flow velocity in them, which contain high suspended sediments and the slope of this type of rivers is 0.001 or less and due to the low slope, their sediment carrying potential. Sediment loads are mostly fine.