The main cause of failure of bridges is local scour around piers. Due to the geotechnical and economical reasons, group piers used in bridge design, so controlling the scour around these structures should be considered. Flow-altering countermeasures such as bed sill reduce the strength of horseshoe and wake vortices. In this study the efficiency of bed sill was investigated for two circular piers which are located normal to the main flow (side-by-side piers) with different spaces between piers. The bed sill was located at two positions downstream of the piers. The experiments were conducted in a rectangular flume under clear-water conditions. The results showed that the maximum scour depth occurred at downstream and far away from piers and bed sill and bed sill is effective structure for reducing the scour depth around side-by-side piers. exposes sill, scour develops rapidly and attains about the same depth without bed sill or gabion presence, especially at larger tested debris accumulation. Tafarojnoruz et al.(2012) tested the efficiency of some countermeasures (submerged vanes, bed sill, transverse sacrificial piles, collar, threading, and pier slot) for a circular pier. Their result for bed-sill shows that the scour depth in front of the pier decreases about 17.2% by using bed sill close to the pier (L= 0). About using countermeasures in group piers, Zarrati et al.(2006) examined the use of independent and continuous collars on group of two piers, transverse to the flow and in line with the flow. Their results indicated that collars are not so effective in reduction of scouring around two transverse piers. Heidarpour et al.(2010) studied the efficiency of collars on two and three piers in line. The efficiency of collars increased when the space between piers completely covered with collars. The influenced of collar in reduction of scour depth in rear piers is more remarkable than the first pier.

Due to the geotechnical and economical reason, group piers used in bridge design. Literatures show that a few studies available on the effect of of bed sill on piers especially in group piers, therefore the aim of this study is to determine the efficiency of bed sill in two piers transverse to the flow (side-byside group piers).