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. Ei ghteen experiments were performed in which flow discharged from a circ ular, 4-in.-diameter pipe onto a near-horizontal bed for 316 min. The bed material was relatively uniform sand with median Brain diameter, d (50) = 1.86 mm. The scour hole dimensions, depth, width, length, and v olume, were documented for drop heights of 0, 1, 2, and 4 times the cu lvert diameter above the bed. A series of expressions were developed c orrelating the dimensionless scour hold parameters of depth, width, le ngth, and volume to the modified discharge intensity, QA(-1)g(-0.5)R(H )(-0.5), and to the drop height. It was determined that culverts place d above the bed result in deeper, wider, and shorter scour holes than culverts placed adjacent to the bed. Criteria for adjusting the existi ng U.S. Department of Transportation scour hole prediction procedure f or drop height is presented in this paper.