Plunge Basins

Plunge Basins for Cantilever Pipe Outlets
-Reducing the risk of dam failure

Water released through a cantilever pipe outlet such as a principal spillway pipe of a dam has a significant amount of energy associated with it. Water energy can result in serious erosion of the downstream channel if not dissipated appropriately. If not addressed, the erosion created at the principal spillway outlet can increase the risk of a dam incident and/or failure.
In the early design of USDA-assisted watershed flood control dams, the principal spillway outlet basin was designed with bare soil. Observations noted in field investigations indicated significant erosion of the basin, which increased the risk for dam failure. To address the problem, a research program at the USDA-ARS Hydraulic Engineering Research Unit (HERU) was developed by scientist Fred Blaisdell. Small-scale study results provided an estimation on the scour depth and dimensions of the expected scour hole developed from a free-flowing cantilever outlet pipe. Eventually, the design concepts developed by Blaisdell would published in Agricultural Handbook ARS-76 Scour at Cantilever Pipe Outlets. USDA-NRCS adopted the design procedures in Design Note 6 of the National Engineering Handbook, Engineering Field Manual. As many research programs go, scientific discovery brings about more scientific questions. As such, HERU scientist, Charles Rice, continued research on the effects of submergence of cantilever pipe outlets and protection of the plunge basin. To learn more about this research, visit our publication index and request publications on related topics.

Development of an economical design for protection of plunge basins for cantilever outlets were adopted as a standard design by the USDA-NRCS.