In recent years, several serious incidents occurred at embankment dams which highlight the importance of evaluating and understanding spillway exit flow conditions at dams. In all cases, the peak spillway flows during these incidents were significantly less than the design flood flows; and although the spillways performed satisfactorily, the outflows from the spillways led to significant erosion and near breaching of the dam embankments or adverse impacts to the outlet works. In most cases, had the spillway design flows occurred, they would have likely resulted in complete breaching and catastrophic failure of the dams. This paper presents the lessons learned from several case studies and discusses several failure mechanisms related to exit flows from spillways. Inspection techniques and modeling tools that are available to engineers to evaluate downstream spillway flow conditions and their potential to adversely impact embankment dams will also be discussed. I. Introduction In recent years, several serious incidents have occurred at embankment dams which highlight the importance of evaluating and understanding spillway exit flow conditions at dams. In all cases, the peak spillway flows during these incidents were significantly less than the design flood flows but still resulted in near breaching of the dam embankment, breaching of the spillway, or significant impairment of dam performance. A potential failure mode is "a physically plausible process for dam failure resulting from an existing inadequacy or defect related to a natural foundation condition, the dam or appurtenant structures design, the construction, the materials incorporated, the operations and maintenance, or aging process, which can lead to an uncontrolled release of the reservoir." (FEMA, 2003) Potential failure modes created or exacerbated by spillway exit flows can be categorized in one of the three following failure mechanisms: 1. Undermining or erosion of the spillway or exit channel 2. Erosion of the embankment 3. Impaired outlet works or spillway performance due to elevated tailwater conditions An infinite number of potential failure modes could be identified for any one of these failure mechanisms for a given dam. The following sections provide a description of these three failure mechanisms as well as examples of how a potential failure might progress.
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