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8/31/2016 1:25:16 PM
WESTERN DAM ENGINEERING NEWSLETTER, VOLUME 4, ISSUE 2 AUGUST 2016
RISKS OF AGING DAMS, HYDROLOGIC INADEQUACIES, INTERNAL EROSION
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Western Dam Engineering <br /> Technical Note <br /> <br /> August 2016 <br /> <br />6 <br />Failure due to lack of filters can take days or years <br />and problems can develop in older dams despite <br />years of good performance. Modern practice and <br />guidelines dictate the use of zoned embankments that <br />include, at a minimum, engineered filters downstream <br />of earth cores and around conduit penetrations and <br />toe drains. <br /> <br />Figure 5. The Result of an Unfiltered Penetration through an Earth <br />Dam in Wyoming <br />Hazard Creep <br />The U.S. Census Bureau maintains a POPULATION CLOCK <br />that ticks off a current average net gain of one person <br />every 12 seconds in the U.S. This rate of population <br />gain affects the role of dams in our nation. Land <br />development reduces water infiltration and thus <br />increases runoff and associated flooding. This in turn <br />increases the need for flood control structures. The <br />increase in population also increases the demand for <br />renewable power and recreational sites, both of which <br />are provided by dams. <br />This population growth will likely move development <br />further into the unpopulated areas below aging dams, <br />increasing the population at risk and reclassifying many <br />low or significant hazard dams as high hazard. This <br />change in hazard classification, or hazard creep, brings <br />owners of dams that were originally constructed as low <br />hazard, costly new challenges to modify the existing <br />structures to meet stricter criteria. There are limited <br />means for owners to be notified and participate in the <br />decision to develop downstream of their dams. Once <br />the development and hazard classification upgrade <br />occurs, the owner is responsible to upgrade the dam <br />for larger flood and seismic loads. <br />See the related article in this issue for a discussion of <br />hazard creep effects on hydrologic design criteria and <br />methods of managing the hydrologic deficiencies of <br />aging dams. <br />Managing Effects of Aging Dams <br />Dams are expensive to build and to fix. According to <br />the Association of State Dam Safety Officials (ASDSO), <br />it would cost over $50 billion to rehabilitate all of the <br />aging dams in the country . So how do owners, <br />engineers, and regulators manage and prioritize the <br />effects of aging on dams? <br />Monitoring <br />Monitoring programs can significantly reduce the risk <br />of a dam failure by identifying deterioration in the <br />early stages, giving the owner the opportunity to repair <br />or remediate the problem and avoid severe <br />consequences. Direct evaluation of the effects of aging <br />is possible by monitoring changes in structural <br />properties and physical features. Indirect evaluation <br />results from monitoring the response of the dam to <br />various loading conditions. The following are just a few <br />of the key review and monitoring activities to help <br />evaluate potential effects of aging. <br />Dam Safety Visual Inspections: Regular inspection of <br />the dam is the most effective means of risk <br />management, as it may identify potential problems <br />before they become a dam safety issue. Inspections <br />should include a review of the embankment (potential <br />changes in grade, surface erosion, seepage, vegetation, <br />animal activity), and all of the appurtenant structures. <br />Inspections should focus on changing conditions that <br />may be an indication of deterioration or other internal <br />mechanisms that are not directly visible. The <br />importance of effective dam inspections warrants its <br />own article. See our previous article, DAM SAFETY <br />INSPECTIONS...A CLOSER LOOK. <br />Design Reviews: Dam safety reviews should be <br />performed by an engineer experienced in dam design <br />and include a review of existing data including <br />analyses, drawings, specifications and construction <br />photos. Although for older dams, much of this <br />information is limited, there are usually a handful of <br />documents including previous inspection reports that <br />can shed light on potential problems and improve the <br />inspection process.
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