EFFECT OF COLLAR LENGTH ON TIME DEVELOPMENT OF SCOURING AROUND RECTANGULAR BRIDGE PIERS

The process of local scouring at bridge piers is time dependent. Using of a collar around a pier for scour protection reduces the rate of scouring considerably. Collars are horizontal plates which are attached to a pier. Collars acts as a barrier to the down flow and prevent its direct impingement into the streambed. In the present work collars with 3 different lengths (25, 35 and 45 cm) and constant width equal to 3 times the pier width were installed around a rectangular pier 5 cm wide and 15 cm long. Pier model had a circular nose and tail and was made from clear Perspex. Collars were installed at the streambed level and all tests were conducted at the threshold of bed material motion where maximum depth of scour hole is expected. Long time tests were carried out to study the development of the scour hole around the pier. Results of the present study showed that the collar length is an important factor that affects the rate of scouring. In experiments with 25 cm and 35 cm collars, the flow undermined the collar and penetrated below its upstream edge after 8 and 24 hours respectively. After this time, development of scouring was accelerated due to the action of horseshoe vortex. In the experiment with 45 cm collar, flow could not penetrate below the upstream edge of the collar after 185 hours. Therefore, scouring at the upstream region of the pier did not occur.