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Western Dam Engineering Technical Note August 2016 10 are in place for dam failure inundation delineations. Dam failure inundation delineations are typically much more extensive than those estimated and delineated by FEMA. This, in combination with the downstream flood protection benefits that dams often provide, can result in downstream areas becoming more desirable for development than they would be without the presence of the dam. Figure 1. Downstream Development near the Dam Toe—a Hazard Creep Scenario [1] A dam’s hazard classification is generally based on the potential downstream consequences of a dam breach in terms of life loss as well as infrastructure and environmental damages [2], [3]; therefore, downstream development can significantly impact a dam’s hazard classification and associated IDF. Dam Owner Impacts from IDF Changes To this point, the emphasis has been on potential changes to the IDF, but why are these potential changes so critical for a dam owner? The hazard categories and associated IDFs required by regulatory agencies vary, however, as an example let’s assume that a particular regulatory agency requires the following IDF events for each hazard classification (these requirements are typical for western states): • Low hazard –A 1 in 100 annual exceedance probability (AEP) event (a.k.a, the 1 in 100-year event) • Significant hazard – The 50 percent probable maximum flood event (PMF) • High hazard – The 100 percent PMF event Let’s consider a scenario in which a small dam was originally constructed in a relatively rural area for the purpose of irrigation water supply by a local consortium of farmers. The rural area had little to no downstream development, therefore, the downstream consequences from a potential dam failure were low and the dam was classified as a low hazard structure. Over time, the downstream floodplain was gradually developed and the potential downstream consequences are now severe. As a result, the dam is reclassified as a high hazard structure—a hazard creep scenario. Assuming the regulatory agency requirements did not change since construction, the hazard creep directly resulted in a change to the required IDF from a 1 in 100 AEP event to the 100 percent PMF event. To put this IDF change into perspective, let’s further assume the required spillway discharge is 1,000 ft3/sec for a 1 in 100 AEP event and 10,000 ft3/sec for the PMF event. By comparing the spillway crest length required for each of these discharges, we can illustrate the requirements associated with increasing the spillway capacity to comply with the revised IDF. Assuming a spillway discharge coefficient of 3.0 and spillway head of 5 feet, the spillway crest length required to discharge 1,000 ft3/sec is about 30 feet. This crest length increases to nearly 300 feet to discharge 10,000 ft3/sec —a significant increase, particularly for what was once considered a small, low hazard structure. This type of scenario would likely be cost prohibitive to a dam owner and could lead to storage restrictions or even complete breach and abandonment of the dam. Though, prior to undertaking any major dam modifications, detailed engineering studies would need to be conducted to optimize potential modifications or potentially justify no action. These and other opportunities to address inadequacies are discussed in the following section. Addressing Inadequacies As previously discussed, an IDF change can be prompted due to several different factors. Upon understanding the justification for an IDF change, dam owners commonly engage in engineering studies to proactively attempt to reduce or eliminate IDF