DWR_2952033 - Laserfiche WebLink

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