Drop Height Influence on Outlet Scour

A study was conducted to determine how fall distance, or drop height, beneath a culvert outlet affects the dimensions of localized scour. Eighteen experiments were performed in which flow discharged from a circular, 4‐in.‐diameter pipe onto a near‐horizontal bed for 316 min. The bed material was relatively uniform sand with median grain diameter, d50=1.86mm. The scour hole dimensions, depth, width, length, and volume, were documented for drop heights of 0, 1, 2, and 4 times the culvert diameter above the bed. A series of expressions were developed correlating the dimensionless scour hold parameters of depth, width, length, and volume to the modified discharge intensity, QA-1g-0.5RH-0.5, and to the drop height. It was determined that culverts placed above the bed result in deeper, wider, and shorter scour holes than culverts placed adjacent to the bed. Criteria for adjusting the existing U.S. Department of Transportation scour hole prediction procedure for drop height is presented in this paper.