In the recent past there has been an emphasis on dam safety, and certainly dams throughout the country are much safer because of this effort. However, spillways often do not get as much attention as dams. A developing failure that is overlooked could result in a spillway failure during the next flood. There could be serious consequences associated with a spillway failure, and there is a growing need to raise awareness of dam safety issues associated with spillways and the assessment of potential hydrologic failure modes. The ability to evaluate existing conditions and design deficiencies that can result in damage or failure of a spillway is a significant concern for dam owners and operators. Potential hydrologic failure modes include discharges that exceed spillway chute and stilling basin capacity, cavitation damage, foundation erosion, and hydraulic jacking during normal or flood operations. These failure modes can develop into a condition of head-cutting leading to loss of reservoir. Methods for assessing the risk of failure resulting in loss of reservoir have been refined over the past several years. Engineers and dam safety inspection staff should know how to identify and evaluate spillway failure modes during a review of these structures. A thorough evaluation is based on the evaluation of design and construction details as compared to current standards, and observed field conditions. Often signs of serious problems are overlooked by examiners because the observed damage appears to be minor in nature. Minor surface repairs are sometimes recommended when major modifications are actually needed. One of the most significant challenges is identifying subtle signs that can indicate a major problem is developing. Past operational history may or may not be any indication of future performance. Design deficiencies may take a while to develop into a failure. An understanding of historic developments in spillway design that can help spillway evaluators make assessments of likely design and construction details that were typical during the period of construction, even when as-built details are not available. Spillways on erodible foundations are of particular concern, because failure can lead to a breach of the reservoir. This paper will provide information that can be used to help identify potential spillway problems.
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