Technical Manual: Overtopping Protection for Dams, Best Practices for Design, Construction, Problem Identification and Evaluation, Inspection, Maintenance, Renovation, and Repair [open pdf - 12MB]
"Inadequate spillway capacity is a common problem with many dams. Thousands of dams throughout North America have been determined to have inadequate spillway capacity and would be overtopped during the inflow design flood (IDF), which is often equated to the probable maximum flood (PMF) or to some frequency flood associated with a particular return period. The PMF is defined as the flood that may be expected from the most severe combination of critical meteorologic and hydrologic conditions that are reasonably possible in the drainage basin under study (Federal Emergency Management Agency [FEMA], 2004). Reservoir inflow from storm events which exceeds the available storage and/or spillway discharge capacity can result in the dam being overtopped. Dam failure from overtopping can lead to a potential for loss of life and significant downstream damages. [...] This document assumes that a hydrologic deficiency exists at a dam and that traditional approaches to safely accommodate a larger design flood have first been investigated. Designers and dam safety personnel should fully evaluate all options available when dam overtopping is a possibility. While choosing an alternative that avoids flow over the top of the dam has clear engineering benefits, providing project-specific protection during dam overtopping can be a viable method in some instances to safely convey large flows downstream from the dam. Overtopping protection should generally be reserved for situations with some combination of very low annual probability of occurrence (e.g., 1 in 100), physical or environmental constraints on constructing other methods of flood conveyance, and prohibitive cost of other alternatives, or where downstream consequences of dam failure are demonstrated to be low."
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