Lessons Learned
A complete and thorough dam record is essential.
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All dams need an operable means of drawing down the reservoir.
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Concrete gravity dams should be evaluated to accommodate full uplift.
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Concrete-lined spillways are vulnerable to significant damage and potential reservoir breach if flows are not safely contained within the conveyance structure.
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Dam failure sites offer an important opportunity for education and memorialization.
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Dam incidents and failures can fundamentally be attributed to human factors.
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Dam owners, engineers and regulators need to address public safety at dams.
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Dams in cold regions should account for ice.
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Dams located in seismic areas should be evaluated for liquefaction, cracking, potential fault offsets, deformations, and settlement due to seismic loading.
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Dams should be thoroughly assessed for risk using a periodic risk review process including a site inspection, review of original design/construction/performance, and analysis of potential failure modes and consequences of failure. The completed review supports a case for taking risk-informed actions at individual dams and for prioritizing actions for an inventory of dams.
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Downstream flooding can be caused by spillway flows that exceed channel capacity or as a result of reservoir misoperation.
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Downstream of constructed spillway exit channels, spillway outflows can erode and even breach dam embankments and can adversely impact the operation of outlet works.
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Dozens of dams can fail or be in danger of failing during a single event (i.e. swarming failures). Dam owners and regulators need to prepare for these types of events.
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Drum gates are complex mechanical systems that must be carefully operated and maintained.
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Early Warning Systems can provide real-time information on the health of a dam, conditions during incidents, and advanced warning to evacuate ahead of dam failure flooding.
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Earth and rockfill embankment dams must be stable under the full range of anticipated loading conditions.
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Embankment dams and levees are vulnerable to damage from wildlife intrusions and animal impacts.
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Emergency Action Plans can save lives and must be updated, understood, and practiced regularly to be effective.
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Floods can occur due to unusual or changing hydrologic conditions.
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Forensic investigations are needed for major dam failures and incidents in order to determine the history of the contributing physical and human factors, and the culminating physical failure modes and mechanisms.
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Gates and other mechanical systems at dams need to be inspected and maintained.
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Hazardous hydraulic conditions, such as hydraulic rollers, can occur at dams of all sizes.
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High and significant hazard dams should be designed to pass an appropriate design flood. Dams constructed prior to the availability of extreme rainfall data should be assessed to make sure they have adequate spillway capacity.
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High and significant hazard embankment dams should have internal filter and seepage collection systems.
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Intervention can stop or minimize consequences of a dam failure. Warning signs should not be ignored.
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Many earth-cut spillways have been constructed in erodible material that can result in unsatisfactory performance and breaching of the spillway. The integrity of all earth-cut auxiliary spillways should be evaluated to ensure that the design storm can be safely passed.
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Natural dams can form quickly through processes such as landslides, avalanches, and river ice jams. In some cases, the formation of natural dams requires prompt intervention to protect people and property.
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Regular operation, maintenance, and inspection of dams is important to the early detection and prevention of dam failure.
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Seepage along penetrations through embankment dams should be controlled using a filter diaphragm instead of anti-seep collars.
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Site security is a critical aspect of dam safety that shouldn’t be overlooked or disregarded.
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Stability of the dam foundation and other geologic features must be considered during dam design.
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Static liquefaction should be considered as a potential failure mode for dams that have loose sands or silts in their embankments or foundations.
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The first filling of a reservoir should be planned, controlled, and monitored.
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The hazard classification of a dam can change over time (hazard creep).
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The study of past incidents and failures aids in the assessment of existing dams.
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Timely warning and rapid public response are critical to saving lives during a dam emergency.
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Uncontrolled vegetation on and around dams can hinder inspection and lead to serious structural damage, significant maintenance costs, and possible failure.
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