Submitted to: Watershed News
Publication Type: Popular Publication
Publication Acceptance Date: December 1, 1995
Publication Date: N/A
Interpretive Summary: Pipe spillways concentrate the flow and cause three-dimensional scour holes that may result in failure of the outlet structure itself and the larger structure it is designed to protect. Current design criteria do not address tailwater conditions with partially to fully submerged outlets. Studies were conducted and relationships developed to determine the geometry of riprap-lined plunge pools in this tailwater range. A popular article was prepared describing this research for the Watershed News, a publication of the National Watershed Coalition.
Technical Abstract: Predicting the development of local scour and the ultimate dimensions of a scour hole is an important engineering problem for many types of hydraulic structures, including pipe spillways. Pipe spillways have been used for relatively large upstream floodwater detention reservoirs since creation of the NRCS (formerly the SCS) in the 1930's. Current design criteria assume a afree outlet condition, i.e., the vertical distance from the pipe outlet invert to the tailwater surface is greater than one pipe diameter. The criteria do not address partially to fully submerged outlet conditions. Studies were conducted using laboratory-scaled physical models to extend the available design criteria to cover the complete range of tailwater elevations possible for cantilevered pipe spillway outlets. Tests were conducted by varying the pipe diameter, tailwater elevation, riprap size, downstream channel bed elevation, and flow rate.