EARTH DAM OR SPILLWAY?

Authors: Temple, Darrel; Hanson, Gregory

Submitted to: United States Society on Dams Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 5/15/2002
Publication Date: 6/1/2002
Citation: Temple, D.M., Hanson, G.J. Earth dam or spillway?. Proceedings of 2002 USSD Annual Meeting and Conference, San Diego, CA. U. S. Society on Dams. 9 p.

Interpretive Summary: Aging of the US water control and management infrastructure is increasing the likelihood that some dams will be overtopped during extreme floods. This is the result of changes in watershed hydrology as well as decreased flood storage resulting from filling of the reservoir with sediments. Because it is not economically feasible to modify all of the smaller watershed dams to prevent overtopping from occurring, it is important to be able to predict when a dam will fail as a result of overtopping and how such failure will affect the flood in the channel downstream. Research presently underway will extent results of previous research to allow better prediction of the breach potential of earth embankment dams during extreme floods that cause flow over the top of the dam. This improved performance prediction will assist engineers in determining which dams are most at risk and how to most effectively utilize the limited resources available.

Technical Abstract: Aging of the US water control and management infrastructure is increasing the likelihood that some dams will be overtopped during extreme floods. As structures approach their planned service life, sediment pools fill and continued sedimentation results in a reduction in the volume available for flood storage. Changes in land use within a watershed upstream can also contribute to the problem by increasing rates and volume of runoff. Development in the flood plain downstream may change the hazard classification of the structure thereby, increasing the size of the flood that needs to be controlled by the dam, and the importance of being able to predict the performance of the structure during the flood event. There is, therefore, an increased interest in being able to predict the response of an earth embankment that becomes a spillway as a result of a flood that exceeds the capacity of the existing reservoir and spillway system. Vegetated earth auxiliary spillways have long been used to pass major floods around dams. Recent advances in the understanding of the erosion processes experienced by these spillways has allowed significant refinement in their design and analysis. Although the fundamental processes governing the performance of earth spillways and embankments are the same, the requirements of tools to provide the needed analyses are somewhat different. Therefore, research has been initiated to allow the spillway erosion technology to be refined and expanded for application to overtopped earth embankment dams. This report reviews the fundamental processes involved, the differences between the spillway and embankment erosion problems, and the research that is presently underway to address knowledge gaps. Preliminary research results and observations, including those related to the performance of un-reinforced vegetation on embankment slopes, are discussed.