Spillway for Oahe Dam Missouri River, South Dakota.

The spillway, a low, broad-crested weir surmounted by 8 gates, will be operated when the pool reaches top-of-gate elevation thus initial flows will discharge into a dry exit channel under a 23.5-ft head. A 150-scale model was used to study spillway performance when this structure is the control and supercritical flow predominates. A shallow 6.5-ft-deep, rectangular stilling basin was developed which dispersed small flows and eliminated surface waves at large flows. Also a gate operation schedule was developed which, together with the shallow basin, comprised the optimum method of controlling flows on the apron downstream until submergence occurred. Tests of the exit channel a paved transition section and a riprap-protected section leading to the unlined channel showed that sector walls could replace the concrete transitions below the rectangular basin, and that the paved apron and riprap section could be reduced in length. Author