Drownproofing of Low Overflow Structures

Abstract

Submergence of hydraulic jumps occurs frequently at low overflow structures, such as weirs and low‐head dams. In these cases the jump degenerates into a rotating current that produces a strong upstream‐directed surface velocity in an otherwise seemingly quiescent downstream pool. This vortex is commonly called the hydraulic by canoeists and water safety experts who fear and respect its potential to maim and kill, yet the phenomenon is virtually unknown in the hydraulic engineering literature. Using analysis and experimental evidence, the paper shows how the hydraulic may be eliminated by either increasing the height of the structure sufficiently to promote formation of a proper hydraulic jump, or by adopting the cascade concept of continuous energy dissipation on a baffled chute. In the former instance, the required height can easily become so great, however, that it may be deemed unacceptable. On the other hand, baffled‐chute spillways are environmentally attractive, known to function well over a wide range of operating conditions, and completely independent of tailwater depth.