Flow over a “Killer” Weir Design Project

Flow over a sharp-crested weir can create dangerous countercurrents downstream under high tailwater conditions. A comprehensive experimental design project in open-channel hydraulics used in a senior/graduate course and in a freshman exploratory experience is illustrated. This applied problem allows the instructor to demonstrate hydraulic jumps, aerated versus nonaerated nappes, similitude and modeling, and the need for experimentation in hydraulics. A series of experiments requires students to compare their predictions of several parameters to those measured, and asks them to design a supplemental structure for the weir to reduce the power of the frontal vortex such that people are ejected and carried downstream to calmer water. The student designs are then tested in the flume during class. During the course of the project, spontaneous discussions occur about the extent to which engineers should be held liable for designs that may increase the likelihood of death. The experiments have been well received and have even saved the life of one student who was swept over a dam and caught in the roller downstream.