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Abstract Employment of protective structures is vital for reducing the effect of the local scour created surrounding the bridge piers. One of the most effective protective structures used for controlling and reducing scour surrounding the bridge piers is the collars. Hence, research on the mechanism of scouring surrounding the combination of pier and collar as well as the effect of collar on reduction of scouring surrounding the pier is of great significance. In this study, the effect of various collar width-to-pier width ratios on reduction of scouring parameters surrounding the rectangular piers has been investigated experimentally in various 180-degree sharp bend positions. The experiments were conducted under clear water and threshold of sediment motion in the upstream straight path. Results indicated that collars with collar width-to-pier width ratios equal to 3 to 4 perform the best at reducing the maximum depth of scouring in front of the nose of piers. Increasing the width of collar surrounding the piers reduces the maximum depth of scouring. Increasing the length of pier reduces the efficiency and effectiveness of the collar in delaying the scour process at the pier nose as well as the amount of the maximum scour reduction. The highest amount of scour depth reduction occurred in the vicinity of the pier and the collar at the 90-degree section with the collar installed with a ratio of collar width to pier width equal to 4 surrounding the piers with a ratio of length to width equal to 2 and 3 by approximately 75 and 70%, respectively.

In this paper, the effect of collar width ratio on the flow pattern around an oblong pier in a 180-degree channel bend was experimentally studied. This channel has a rectangular cross section. It is 1 m in width and 0.7 m in height. The upstream and downstream paths are respectively 6.5 and 5 m long. The ratio of the bend's central curvature radius to the channel width is 2; hence, it qualifies as a sharp bend. Experiments were carried out under clear water approach flow conditions. The results showed that the presence of collars around an oblong pier creates vortices in the opposite direction of the longitudinal flow, causes the distortion and disturbance of the streamlines toward the pier downstream, and decreases downflow strength in front of the pier nose. Furthermore, doubling the collar width results in 0.93 and 0.68 times the vorticity and the power of the secondary flow on the pier upstream, respectively. It also reduced the maximum values of the Reynolds stresses perpendicular to the y-plane in x direction and perpendicular to the z-plane in y direction by respectively 45 and 60%, and increased the Reynolds stress perpendicular to the z-plane in x direction by 25%. HIGHLIGHTS Experiments were carried under clear water conditions.; Doubling the collar width resulted in 0.93 times the vorticity.; Doubling the collar width has increased the Reynolds stress on zx by 25%.; The maximum vertical velocity has decreased by about 15% by doubling the collar width.; Increasing the collar width around the pier increases the maximum turbulence kinetic energy values by 50%.;

Base-level fall in river beds occurs due to varying natural or unnatural causes. Base-level fall causes the change in the behavior of flow at the location of drop in base-level. In such situations, most of scour occur at the foot of the slope, and slope wall retreats in the upstream direction. This phenomenon widens the wall of the river bank, thus leading to its destruction. The amount of bed topography variations and scour around a rectangular bridge pier with an oblong nose located in the 90 degree angle of a 180 degree sharp bend was studied in this work by generating base-level fall at the beginning of the 180 degree sharp bend, and it was compared with a case without a base-level fall. The results indicated that in the case of base-level fall at the upstream side of the bridge pier, increase in flow depth, as well as reduction in velocity at the area around the pier, is observed, and the maximum depth of scour hole and the volume of scour hole around the pier respectively reduce by 73 and 97% in comparison with those in the case where no base-level fall occurs.