A Look at Earth Spillway Design and Evaluation After More than 50 Years of Experience

More than 23,000 dams have been constructed in the United States which rely on auxiliary earth and rock spillways to safely pass flood flows around dams. In the absence of analytical tools to estimate spillway integrity, the design of these spillways was based on empirical methods and engineering judgment without any formal analysis of the spillways= performance during the design storm. The design philosophy recognized the fact that the auxiliary spillways are used infrequently and that some damage to the spillway was acceptable provided the spillway did not breach and the integrity of the dam was not compromised. Flood events throughout the United States have tested the integrity of some of these spillways. Many of the spillways suffered extensive damage and even complete breaching during storm events less severe than the design storm. As a result, the United States Department of Agriculture, Natural Resources Conservation Service (NRCS), and the Agricultural Research Service (ARS), completed a laboratory research and field data acquisition program to assess the performance of existing earth spillways, improve the prediction of the risk of spillway breach, and to develop procedures for spillway design and analysis. Their efforts led to the development of a state-of-the-art computer program released in 1998 called SITES, as well as updating procedures and requirements for earth spillway design. Since the release of these technical resources, more and more state dam safety regulatory agencies have begun requiring dam owners to perform stability and integrity analyses using the SITES program to evaluate the performance of both existing and new earth spillways. The purpose of this paper is to provide a brief historical background of earth spillway design practices and experience in the United States, including lessons learned from spillway failures and the application of recent developments in earth spillway design and analysis. Common mistakes relating to the analysis, design, construction, and maintenance of earth spillways are discussed along with specific recommendations to designers, regulatory agencies and dam owners to maximize the performance and reliability of these spillways. Recommendations for field data collection and application of the SITES program are also presented based on the authors' experience evaluating existing and designing new earth spillways. 16 pp., 1 table, 10 figures, 14 references.