Designing Spillway Crests for High-Head Operation

A standard-shaped spillway, when operating at greater than design head, experiences a subatmospheric crest pressure. Quantitative data obtained from laboratory measurements of minimum pressure and discharge occurring during flow over spillways are presented in dimensionless form. Minimum crest pressures as well as discharge coefficients are shown to be functions of the spillway design-head-to-height and head-to-design-head ratios. A dimensionless parameter involving discharge and minimum pressure, but not head, is formulated and evaluated. The results are used in a numerical design example to proportion a standard-shaped spillway to pass a particular discharge with a predetermined pressure occurring on the crest. The resulting design is compared with an actual design for the same spillway.