Amirkabir University of Technology Amirkabir Journal of Civil Engineering 2588-297X 53 5 2021 07 23 Experimental Study on the Effect of Circular Collar on Cylindrical Bridge Pier Scour in Unsteady Flow Experimental Study on the Effect of Circular Collar on Cylindrical Bridge Pier Scour in Unsteady Flow 1721 1742 3705 10.22060/ceej.2019.17078.6460 FA Yousef Rajabizadeh Department of Water Engineering and Management, Tarbiat Modares University, Tehran, Iran Seyed Ali Ayyoubzadeh Department of Water Engineering and Management, Tarbiat Modares University, Tehran, Iran Kourosh Qaderi Department of Water Science and Engineering, Shahid Bahonar University, Kerman, Iran Journal Article 2019 09 22 <span style="letter-spacing: .05pt;">The bridge pier scour is one of the major causes of bridge destruction worldwide. Despite extensive research in this field, the study of this topic has always been of interest to researchers and it is necessary to study different methods to counter it. The collar is a simple and inexpensive structure for protection against bridge pier scour. In this study, the effect of collar size and height on bridge pier scour and determination of the best collar performance conditions in steady and unsteady flow conditions were investigated. The experiments used three different collar sizes and three different mounting heights relative to the substrate surface. After calculating the percentage of scour reduction in all test cases and comparing them, investigation of time evolution diagram of scour hole and comparison of maximum depth of scouring created in different states, It was found that increasing the size of the collar and decreasing the height of the collar installation relative to the substrate surface resulted in a further decrease in the depth of the bridge pier scour and this change was more evident in the unsteady flow state. Also by comparing the two variables of size and height of collar installation, it was found that the height of collar with dimensionless ratio <em>H</em>/<em>D</em>=0.5 in steady and unsteady flow had the best performance and the greatest effect in reduction scour around bridge pier.</span> <span style="letter-spacing: .05pt;">The bridge pier scour is one of the major causes of bridge destruction worldwide. Despite extensive research in this field, the study of this topic has always been of interest to researchers and it is necessary to study different methods to counter it. The collar is a simple and inexpensive structure for protection against bridge pier scour. In this study, the effect of collar size and height on bridge pier scour and determination of the best collar performance conditions in steady and unsteady flow conditions were investigated. The experiments used three different collar sizes and three different mounting heights relative to the substrate surface. After calculating the percentage of scour reduction in all test cases and comparing them, investigation of time evolution diagram of scour hole and comparison of maximum depth of scouring created in different states, It was found that increasing the size of the collar and decreasing the height of the collar installation relative to the substrate surface resulted in a further decrease in the depth of the bridge pier scour and this change was more evident in the unsteady flow state. Also by comparing the two variables of size and height of collar installation, it was found that the height of collar with dimensionless ratio <em>H</em>/<em>D</em>=0.5 in steady and unsteady flow had the best performance and the greatest effect in reduction scour around bridge pier.</span> https://ceej.aut.ac.ir/article_3705_2cfa3753d6a524711acb5fce38eeca1a.pdf