عنوان مقاله [English]
The failure of the bridge caused by local scour, the importance and necessity of studying the prediction of scour depth and ways of reducing it. One of the important method to reduce the effects of erosion factors is to use a thin plate around the bridge pier called collar. Collar dimensions and the position have great effect on reducing local scour around bridge pier. In this research, collar with four forehead length (lca) equal to B, 2B, 4B and 9B (B is width of the pier) are examined. The collars were installed in four levels equal to: on the bed, 0.12B under the bed, 0.5B and B above the bed. The experiments have been conducted with three Froude Numbers equal to 0.13, 0.16 and 0.19 in clear water condition. The experiments have been done on a flume with the length of 6 m, width of 73 cm, and a depth of 60 cm with a slope near zero in Hydraulic Laboratory of Shahid Chamran University of Ahvaz. The results showed that increasing dimensions of collar will increase the efficiency of performance and the best dimension lca (length of oblong collar in the front of the pier) were B and 2B. The best position to install the collar were on the bed and 0.12B under the bed. In these situations the collar had 100% and 88% efficiency in respectively reducing the scour depth.