Evaluating Spillway Adequacy

by John K. Hawk, (M.ASCE), Chief; Project Supervision West Branch, Chicago Regional Office, Federal Energy Regulatory Commission, Chicago, IL,

Serial Information: Civil Engineering—ASCE, 1992, Vol. 62, Issue 5, Pg. 74-76

Document Type: Feature article


More than a third of all dam failures are caused by overtopping. Equipment malfunctions or operations errors are sometimes to blame, but the principal cause is inadequate spillway capacity. This has led the Federal Energy Regulatory Commission (FERC) to place special emphasis on evaluating spillway adequacy in dam safety regulations. For high- and significant-hazard potential dams, floods up to the probable maximum flood (PMF) must be considered. At first, this appears to establish the PMF as the rigid standard for all high- and significant-hazard dams. In fact, it provides a framework for a flexible approach to identifying the inflow design flood (IDF). If failure during a PMF-magnitude flood would significantly increase the hazard to life and property downstream, the inflow design flood must be the PMF. If not, a smaller IDF is acceptable. The article describes FERC's incremental hazard evaluation procedure for determining the IDF. A sidebar describes an application at Brule Dam in Wisconsin.

Subject Headings: Floods | Dam failures | Spillways | Failure analysis | Inflow | Hydraulic design | Wave overtopping | Equipment and machinery | Wisconsin | United States

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