EARTH DAM OVERTOPPING AND BREACH OUTFLOW

Authors: Temple, Darrel; Hanson, Gregory

Submitted to: Proceedings of the World Water and Environmental Resources Congress
Publication Type: Proceedings
Publication Acceptance Date: 4/15/2005
Publication Date: 5/20/2005
Citation: Temple, D.M., Hanson, G.J. 2005. Earth dam overtopping and breach outflow. In: Walton, R., editor. Proceedings of the World Water and Environmental Resources Congress, May 15-19, 2005, Anchorage, Alaska. American Society of Civil Engineers. 8 p. Available: http://www.ascelibrary.org/ascecp.

Interpretive Summary: Over the past half-century, the USDA has assisted in the design and construction of approximately 11,000 flood control and multipurpose dams. As these dams age and the watersheds become more developed, the risk of overtopping increases as do the consequences of failure. There, therefore, is an increasing need for improved tools to analyze the performance of earth dams that may overtop during extreme floods. Research has been undertaken in both the US and Europe to address this need through the development of improved mathematical models. An essential component of a mathematical model describing earth embankment dam overtopping and breach is the computation of the quantity of water flowing over the dam and through the breach area as the erosion causing the breach progresses from the initial overtopping through dam failure. This report discusses potential approaches to computation of this outflow for the geometric configurations associated with the different stages of the breach. This information may be used by engineers to evaluate performance of specific dams and by researchers involved in the development of comprehensive mathematical models of the breach process.

Technical Abstract: Over the past half-century, the USDA has assisted in the design and construction of approximately 11,000 flood control and multipurpose dams. As these dams approach the end of their planned service life, sediment pools fill and sediment deposition in the flood pool reduces flood storage. Simultaneously, development in the watershed may increase runoff while development downstream of the dam increases the consequences of failure. Therefore, there is an increased need for improved procedures for predicting performance of overtopped earth embankment dams. Field experience and laboratory experiments indicate that the erosion process for overtopped earth embankment dams can be divided into sequential stages or phases for computational purposes. These stages include: 1) an initial stage prior to the failure of the vegetal cover or other protection on the downstream slope, 2) a breach development stage during which the resulting headcut or overfall moves from the point of formation through the dam crest, 3) a breach stage during which the headcut advances into the reservoir, and 4) a drawdown stage during which the breach continues to widen as the stored water in the reservoir is released. The flow geometry and the relations describing the discharge and erosion processes vary with stage. This report discusses the dominant characteristics of the identified stages of the progressive breach process and the impact of the resulting geometry on discharge prediction. Results of an idealized study of breach outflow during headcut entry into the reservoir are briefly discussed.