Experiment Study on Bed Topography around Wedged Bandal-like, Bandal-like and Impermeable Spur dike Structures at 180 Degree Bend in Non-Submerged Conditions

Due to scouring problems in impermeable spur dikes, which compromise structural strength and stability and as a result repair and maintenance costs increase, an alternative method of scouring is needed to protect the river bank. In this area, bandal-like structure can be used. This structure is a combination of an impermeable spur dike and a permeable spur dike. The upper part is impermeable causes the upper half of the flow run towards the center and the inner wall and the lower part is permeable enabling the lower half of the stream passes through it (Rahman et al., 2003a, b). The lower part causes the flow to slow down and reduces speed, and because the flow near the bed has a higher concentration, it leads to settling sediments in the downstream of the river near the coast. On the other hand, because of the flow through the lower half, the power of downstream flows are reduced compared to the impermeable spur dike and the horseshoe vortices are vanished. Therefore, the scouring rate in the bandal-like structure is less than the impermeable spur dike (Zhang et al., 2010; Teraguchi et al., 2011b)Regarding that the scouring rate, the bandal-like structure is less than an impermeable spur dike therefore, its use has been suggested by some researchers. Since the sedimentation rate behind the bandal-like structure is less than the impermeable spur dike, in the present study, a new structure called the wedge bandal-like is proposed to enhance the sedimentation of the bandal-like. Wedge bandal-like structure is constructed of a bandal-like structure and a repellent impermeable triangular spur dike. The purpose of using this structure is to use both the advantages of the bandal-like structure as well as the benefits of repellent impermeable triangular spur dike, for deviation of flow and sediment transport to the back of the structure and sedimentation. Both structures cause the secondary flow of the surface to move towards the outer arc before moving to the outer shore down the bottom and practically shore the coastal tail into middle areas.