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Western Dam Engineering <br /> Technical Note <br /> <br /> August 2016 <br /> <br />9 <br />Retrofitting Old Dams to Address <br />New Hydrologic Inadequacies <br />Introduction <br />At the time when dams are designed and constructed <br />they must meet certain hydrologic requirements to <br />ensure their proper operation and safety. However, <br />these requirements can change over the life of a dam, <br />which can impact a dam’s compliance with regulatory <br />agencies. The most prominent of these hydrologic <br />requirements is the inflow design flood (IDF), which is <br />the minimum flood event that must be safely <br />discharged through the dam. While IDF requirements <br />vary between regulatory agencies, they are generally <br />based on potential downstream consequences <br />resulting from a dam failure or flood outflow event. <br />So why do these hydrologic requirements change over <br />time? There can be several reasons, but some of the <br />primary reasons include advancements in knowledge <br />and understanding leading to updates associated with: <br />1) Hydrologic analyses (precipitation and runoff) <br />2) Regulatory agency requirements <br />3) Hazard classifications <br />These are currently considered the most influential <br />factors driving change in hydrologic requirements for <br />dams and are further discussed in subsequent sections <br />below. However, climate change shouldn’t be ignored <br />and is another factor becoming more influential in <br />hydrologic requirements for dams. With the potential <br />to increase the frequency of extreme storm events, <br />climate change will likely continue to become more <br />pertinent and prominent in the future and initiate <br />updates to hydrologic data and regulatory agency <br />requirements. <br />Hydrologic Updates <br />Dam engineering is continuously evolving based on <br />improved understanding, additional data, and lessons <br />learned. This is particularly true when considering <br />hydrology and development of the IDF. <br />Larger and more refined precipitation data sets <br />combined with increased understanding, have <br />prompted numerous hydrologic updates to be <br />undertaken, both domestically and internationally. <br />These updates can both increase and decrease <br />precipitation depths and runoff potential as compared <br />to those used during previous designs and evaluations. <br />In cases where precipitation depths increase, a dam <br />with sufficient capacity to safely discharge the <br />previously developed IDF may no longer have sufficient <br />capacity to safely discharge the updated IDF and would <br />not comply with regulatory requirements. <br />For scenarios in which precipitation depths decrease, <br />existing outflow capacity is likely to be sufficient and <br />potentially in excess of what is required from a dam <br />safety perspective. On this basis, there could be <br />opportunities to safely store additional water in <br />selected existing facilities with only modest structural <br />improvements. There are of course caveats about <br />water rights, property rights, etc., but it can be argued <br />that in general, the environmental permitting for an <br />incremental increase in storage at an existing facility <br />would require less effort than developing a new dam <br />and reservoir from the ground up. <br />Regulatory Agency Updates <br />As industry understanding of the physical processes of <br />precipitation and the engineering processes of runoff <br />calculation evolve, regulatory agencies are tasked with <br />periodically updating their rules and regulations to <br />keep pace. Although dam owners would prefer to <br />remain exempt from regulatory changes (i.e., <br />“grandfathered”), regulatory agencies must maintain a <br />common level of safety. Therefore, in the interest of <br />public safety, grandfathering of older dams cannot be <br />justified. <br />Regulatory updates pertaining to hydrologic adequacy <br />typically impact evaluations of existing facilities with <br />static downstream consequences and facilities where <br />the downstream consequences have increased and <br />their hazard classification must be changed due to the <br />condition known as ‘hazard creep.’ <br />Hazard Classification Updates <br />Hazard classification updates can be initiated based on <br />hydrologic and regulatory agency updates as well as <br />the hazard creep resulting from development <br />downstream of dams. Although most local zoning laws <br />prevent downstream development from occurring <br />within Federal Emergency Management Agency <br />(FEMA) flood delineations, no such zoning restrictions