CLEAR-WATER SCOUR AROUND BRIDGE PIER GROUP

Recent attempts have been made to reduce scour around solid circular p iers. Such attempts use riprap, a circular collar around, a rectangula r slot through, and a delta wing-like fin ahead of piers. Providing an other scour reduction device, the present paper replaces the solid pie r by a group of three smaller piers. A pier group starting from its fo undation and extending fully into the flow depth is termed a ''full pi er group'' and one extending partially into flow depth a ''partial pie r group.'' The scour due to a pier group in its best orientation is co mpared with that due to a solid pier of diameter equal to the circumsc ribing circle diameter of the pier group. It was observed that the sco ur reduction due to a full pier group is about 40%, and 75% of this re duction can be realized by a partial pier group extending into half th e flow depth only. Also, the full pier group is seen to be more effect ive than a solid cylinder with a full slot of width equal to half its diameter and as effective as a solid cylinder with a collar 3.5 times its diameter. A collar on a pier group is much more effective than on a solid pier.