The Effect of Collar on the Size of Proper Riprap Cover around the Cylindrical Bridge Piers in the Rivers Bend

Riprap of bridge piers is placed to eliminate local scour and to secure the pier from failure. However, riprap surfaces frequently rupture to protect bridges pier during deluges despite arrangement of riprap particles around its foundations. Improper design of proper extent for riprap cover greatly reduces the riprap efficiency in local scour control. Therefore, in this research in order to determine the proper size of riprap cover at the angle of 60° of river bend, some experiments were carried out under clean water conditions in a flume of 180°. The effect of collar presence around the pier and also different size of collar on proper size of riprap were examined in different experiments. The results showed that the presence of collar around the cylindrical bridge pier decreased the proper size of riprap as 26% compared to the pier without collar. Moreover, as the collar diameter increased, the size of riprap cover decreased which is economically affordable.