Todays, scouring is one of the most significant issues in river engineering. Every year, many bridges around the world destroy by this phenomenon. In fact, scouring is a kind of erosion that occurs by the action of complex vortex flows around piers and causes to form a hole around them. Through the time, the dimensions of these holes increase and finally cause destruction. In hydraulic designing of bridges, estimating the scour hole depth is very important. The aggregates of the river bed usually have a nonuniform texture and this nonuniformity cause the formation of an armor layer that prevent scouring of lower layer. The study of single piers have done by many researchers, however, there are not sufficient studies about multiple piers in the presence of armor layer. Therefore studying this case is necessary. In this research the armoring effect on local scouring around cylindrical multiple piers has been carried out with double and triple piers in the presence of armor layer with distances 6b, 8b and 10b. (b=pier diameter) Also, comparison of the results corresponding to multiple and single piers, the temporal development of scour hole, and the effect of dictances between piers on the scouring depth have been studied. The experiments have been done in the hydraulic lab of Isfahan university of technology in a rectangular flume.(length=7m,urn:schemas-microsoft-com:office:office" / Key words : local scour, cylindrical group piers, armor layer, flow characteristics around pier, Time development of scour
