ALLOWABLE OVERTOPPING OF EARTHEN DAMS

Authors: Temple, Darrel; IRWIN, WILLIAM - USDA-NRCS

Submitted to: State Dam Safety Officials Association Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 8/15/2006
Publication Date: 9/10/2006
Citation: Temple, D.M., Irwin, W. 2006. Allowable overtopping of earthen dams. In: Dam Safety 2006. Proceedings of the Association of State Dam Safety Officials Annual Conference, September 10-14, 2006, Boston, Massachusetts. 2006 CDROM.

Interpretive Summary: Aging of the nation’s flood control infrastructure has resulted in a need for reevaluation, and, in some instances rehabilitation, of existing earthen dams. Because of changes in the watershed, sediment deposition in the reservoir flood pool, and changing regulatory requirements, a common deficiency of these older dams is inadequate spillway capacity. Correcting this problem by increasing the spillway capacity or flood storage capacity of these dams can be expensive, and available resources are limited. Research has resulted in improved understanding of the protective capabilities of natural materials such as vegetation and rock when used for erosion protection on the downstream face of dams subjected to overtopping during extreme floods. Mathematical relations quantifying the performance characteristics of these materials have been developed and incorporated into computer software for use in analysis of dams that may be overtopped. These tools may be used for evaluation of the benefits of utilizing vegetation or rock riprap to economically increase the safety of these dams. This report discusses the development and use of these relations and software.

Technical Abstract: Aging of the nation’s flood control infrastructure has resulted in a need for reevaluation, and, in some instances rehabilitation, of existing earthen dams. Inadequate spillway capacity is often one of the deficiencies identified for these structures. Inadequate spillway capacity may be the result of changes in the watershed, sedimentation within the flood storage pool, changes in hazard classification, refined hydrologic information, or a combination of these. In many instances, correction of the problem will require an increase in flood storage or in spillway discharge capacity. However, experience has shown that vegetated earthen dams can withstand limited overtopping with little or no damage. Research utilizing actual vegetation on test embankments and experience with vegetated earth spillways has allowed quantification of the hydraulic attack required to generate failure on a vegetated embankment face. The stability of riprap used to protect an embankment from erosion during overtopping has also been investigated. The results of this research and experience have been used to develop simplified computational procedures for use in predicting an acceptable amount of overtopping flow during a major flood event. Allowing overtopping of dams to the point of incipient failure of the slope protection is consistent with the Natural Resources Conservation Service approach of requiring passage of the design flood without breach of the embankment dam or earth/vegetated spillway. This report discusses the primary considerations in predicting performance of an overtopped earth dam protected by a vegetal or riprap cover and discusses potential application of the analysis.