محمد واقفی

Flow pattern in rivers, particularly at the bends or in proximity of hydraulic structures, is of a certain complexity, which results in formation of scour holes and destruction of the walls. Research on flow and scour patterns is hence of great significance. Shape of bridge piers is one of the parameters influencing and altering the flow pattern. Reduction of scour around bridge piers with the help of protective structures has always been taken into consideration by engineers. Collars are one of those protective structures which effectively help reduce scour by diverting the flows. This study has therefore investigated the flow pattern around rectangular and oblong piers with collars, located in the 90 degree angle of a 180 degree sharp bend. Furthermore, the scour around bridge piers of varied shapes, with or without collars, located in different angles of the 180 degree sharp bend was examined. The results of flow pattern experiments indicated that diversion of streamlines is higher around the rectangular pier as a result of the collision of the flow with the pier. Thus, with installation of the oblong pier, the maximum average inclination angle of the streamlines at the level close to water surface has been reduced by approximately 64% in comparison with the case of installing a rectangular pier. With placement of the oblong pier, the maximum secondary flow power and vorticity have respectively decreased by 35 and 45% compared to the case of installing a rectangular pier. However, the maximum kinetic energy of fluctuations was not significantly different around the two piers. The results of scour experiments also found that using the collar reduced the average maximum scour depth, the maximum sedimentation height, the scour hole volume, and the sediment reach downstream in every three angle of pier and collar installation. The maximum effect of the collar on scour occurred in installation of an elliptical pier in the 120 degree angle, where using a collar reduced