محمد واقفی

Bridges are one of the most important and useful river structures which are destroyed under the influence of hydraulic factors. Occurrence of scour around bridge piers is one of the major causes of bridge destruction. Therefore, proposing methods of controlling and reducing this phenomenon is highly critical. Various methods have been proposed to control and reduce local scour around bridge piers. Application of collars as protective structures is amongst the methods which greatly attract researchers’ attention. Different parameters including the dimensions, thickness, and level of installation of the collar influence the function of such a protective structure. To this aim, this paper investigated the effect of thickness and level of the collar around rectangular and circular piers located in different positions of a 180 degree sharp bend on flow and scour patterns by using experimental simulations. The results of the present study indicated that installation of the collar around the piers prevents down flows, created by collision of flow with the pier, from colliding with the bed. Furthermore, increasing collar thickness increases deviation of the streamlines at the upstream side of the pier after collision with the edge of the collar towards bed, which in turn increases the depth of scour hole. Increasing collar thickness at the upstream side of the rectangular pier reduces the kinetic energy of the flow by approximately 60%. The results of flow pattern experiments demonstrated that the most optimum level of collar in the 180 degree sharp bend around rectangular and circular piers is at a distance from the initial bed level to 0.4 times the pier width lower than the initial bed level. Hence, the closer the level of collar to the initial bed level is, the more enhanced the performance of the collar will be in reduction of scour. Installation of the collar at the initial bed level, and 0.2, and 0.4 times the pier width lower than the initial base level around the c