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Western Dam Engineering Technical Note August 2016 11 changes, particularly those driven by hazard classification changes. Proactive Management of Hazard Creep A dam owner has little control over development downstream of their dam, but has potentially great financial liability. By actively participating in zoning and developmental planning and discussions, a dam owner can attempt to mitigate the impacts of downstream development on a dam’s hazard classification and associated IDF requirement. State dam safety regulators are often willing to participate in community discussions regarding planning and zoning downstream of a dam. They are able to explain to local community officials the dam modifications that would be necessary to adequately protect the incoming population associated with downstream development. Many state dam safety regulators possess data to help explain the costs associated with this hazard creep. In Colorado and elsewhere, engineers must submit cost estimates as part of the design review process for all new dams and for modifications and repairs to existing dams. As such, the relative costs associated with required modifications for spillways that are the direct result of hazard creep can be estimated and used for future reference. An additional proactive approach could include discussions and negotiations for some or all of a dam owner’s costs (to address dam inadequacies relative to downstream development) to be included as part of developer costs such that the dam owner is not saddled with those expenses. Engineering Studies The purpose of undertaking engineering studies is to: 1) Justify a reduced IDF, regardless of the hazard classification, based on an incremental damage assessment (IDA) – Recognizing that consequences (i.e., life loss, damages, etc.) will be present within the downstream floodplain during extreme flood events, regardless of the performance of the dam, the basic premise of an IDA is to compare downstream consequences resulting from the IDF event with and without a dam failure. If the incremental consequences between the two scenarios are insignificant, the impact from a dam failure is considered to be negligible and a reduced IDF is justified. Lower percentages of the initial IDF event (which is usually the PMF for high hazard dams) are then evaluated until unacceptable incremental consequences are present. The IDF is then selected based on the percentage of the initial IDF at which unacceptable incremental consequences result. It should be noted that an IDA may not be appropriate or beneficial for all dam sites [2], [3]. 2) Justify reductions in probable maximum precipitation (PMP) depths and the associated IDF through site-specific PMP evaluations – The PMP is theoretically the most severe precipitation event possible and is often assumed to result in the PMF event. The PMPs for various locations across the United States were estimated during the 1960s, 1970s and 1980s within a series of hydrometeorological reports [4] by the National Oceanic and Atmospheric Administration (NOAA). These studies were performed for large scale regions based on the data and understanding available at the time of each study. Using modern techniques and updated data sets, site-specific PMP studies have been used to justify reductions in design PMP depths of up to about 40 percent. As a result of these reductions in maximum precipitation estimates, the IDF can be reduced. It should be noted that site-specific PMP studies may not always result in a reduced IDF. Although rare, revised PMP depths have increased in some locations as a result of site-specific studies. 3) Develop dam modification alternatives to safely discharge the IDF event – If engineering studies to justify a reduced IDF are not appropriate or effective, dam modification alternative evaluations are required. The development of modification alternatives include numerous considerations and are discussed in the following section.