Abbas Safaei - Water Science and Environmental Research Center, Shoushtar Branch, Islamic Azad University, Shoushtar, Iran.
Mohsen Solimani babarsad - Department of Water Sciences, Water Science and Environmental Research Center, Shoushtar Branch, Islamic Azad University, Shoushtar, Iran.
Roozbeh Aghamajidi - Assistant Professor Civil Department, Islamic Azad University, Sepidan Branch, Sepidan, Iran.
Payam Eftekhar - Department of Water Engineering, Faculty of Civil and Environment Engineering, University of Amir Kabir (Poly Technic), Tehran, Iran.

Bridge pier's local scouring is known to be a destructive factor in river engineering science. This phenomenon is widespread in river intersecting structures such as bridge piers, spur dykes, and downstream river structures. Extensive research has been conducted to reduce and control destructive phenomena, and many solutions have been proposed. These solutions are divided into two parts, namely, direct and indirect protection. In this study, the direct method was studied by defining scenarios. Since many bridges are affected by scouring during the operation, in the present study, the collar method, which is known as a direct protection method, in the case of flexible and permeable, is suggested. The technique is presented an adjustable chain collar, three times bigger than the pier's width (w/d=۳), and its effect is investigated in clear water conditions. In the defined scenarios, three different diameters of the chain as CI=۵ mm, CII=۱۰ mm, and CIII=۱۵ mm were used to control chain shapes' effect, and three dimensionless flow parameters (U/Uc ۰.۷۳, ۰.۸۵, and ۰.۹۶) were selected to investigate the effect of flow conditions. According to the results, the scour depth is related to changes in the diameter of the collar chain, as the final scour depth decreases by increasing the diameter of the chain from CI to CIII. Therefore, in the best conditions, for CIII, the dimensionless ratio of scouring reduction ( ) is equal to ۷۱% near to inception motion parameter (U/Uc=۰.۹۶).

Bridge pier, Flexible Collar, Scour depth, Local Scouring, Horseshoe Vortex