WATER OVER THE DAM

Authors: Temple, Darrel; Hanson, Gregory

Submitted to: National Watershed Conference National Watershed Coalition
Publication Type: Proceedings
Publication Acceptance Date: 5/19/2001
Publication Date: 4/1/2002
Citation: Temple, D.M., Hanson, G.J. 2002. Water over the dam. CDROM. Proceedings, 7th National Watershed Coalition Conference, National Watershed Watershed Coalition, May 2001, Richmond, VA.

Interpretive Summary: Recent legislative activity in the form of PL 106-472 has placed a focus on the aging the nation's watershed flood control dams. Many of these earth embankment dams are reaching the end of their planned service life and will require rehabilitation if they are to continue to serve their intended purpose. Changes in population density and land use in the watersheds and floodplains have created the potential for some of these dams to be overtopped during extreme flood conditions. Correct prediction of the response of these structures to overtopping can assist in determining which structures should receive attention first and how to most effectively use the funds available. Research presently underway will lead to the development of improved tools for analysis of effects of embankment overtopping and the risk that such overtopping presents to people and property downstream. Results of recent research indicate that it may be possible to make minor modifications to some vegetated earth embankment dams to allow them to safely experience limited overtopping through utilization of the protective capability of grass on the downstream slope. Procedures for predicting the protective capability of the vegetal cover on the downstream dam face are introduced.

Technical Abstract: Aging of the Nation's watershed dams and the move to rehabilitate them raises new questions related to overtopping of earth embankment dams. Changes in watershed land use and filling of the sediment pools may combine to cause overtopping of structures by floods that would have been contained at the time of reservoir design. Changes in land use in the downstream floodplain may result in the need for the reservoir to contain larger floods than those for which it was originally designed. Analysis of these conditions suggests the need for re-evaluation of the traditional assumption that overtopping of an unreinforced earth embankment always results in failure. Research is presently underway with the goal of providing a means of evaluating the performance of overtopped earth embankment dams protected only by a vegetal cover. This research will provide the basis for prediction of the extent of overtopping that may be withstood without breach, the timing of the breach when failure does occur, and the hydrograph resulting from a failure. Preliminary results of the research confirm that these performance characteristics are strongly influenced by the nature and uniformity of the vegetal cover, the properties of the embankment materials, and the presence of berms and/or unprotected areas of the downstream toe. This report discusses these preliminary research results and their implications for dam rehabilitation, with focus on the concept of allowable overtopping.