Resolution No. 5303 - Laserfiche WebLink

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Eugene -Springfield Area Natural Hazards Mitigation Plan 6. Appendices downstream side is broadly sloping. Much of the dams' capacity to impound water arises from the weight of the structure. Gravity dams are often anchored into bedrock foundations and abutments to increase stability. Concrete arch dams rely primarily on the strength of concrete to impound water. Much thinner in cross section than concrete gravity dams and they are always convex on the upstream side and concave on the downstream side because concrete is much stronger in compression than in tension. The arch design uses the pressure of impounded water to compress the concrete, making the dam stronger. Concrete arch dams are also keyed into bedrock foundations and abutments to provide stability. A less common variation of a concrete arch dam is a buttress dam. Buttress dams are arched or straight dams with additional strength provided by buttresses perpendicular to the long axis of the dam. H.2 Causes of Dam Failure Dam failures can occur at any time in a dam's life; however, failures are most common when water storage for the dam is at or near design capacity. At high water levels, the water force on the dam is higher and several of the most common failure modes are more likely to occur. Correspondingly the probability of dam failure is much lower when water levels are substantially below the design capacity for the reservoir. For embankment dams, the most common failure mode is erosion during prolonged periods of rainfall and flooding. When dams are full and inflow rates exceed the capacity of the controlled release mechanisms, overtopping may occur. Overtopping can scour and erode either the dam itself and/or the abutments which may lead to partial or complete failure of the dam. Internal erosion, piping or seepage through the dam, foundation, or abutments can lead to failure. Willamette River Basin dams are designed for spillway opening only during infrequent severe events. Increased use can cause wear on spillway parts and lead to greater maintenance needs and an increased risk of failure. Twenty percent of dam failures have been caused by piping (internal erosion caused by seepage). Seepage often occurs around pips and spillways; through animal burrows; around roots or woody vegetation; and through cracks in dams.5 Earthquake activity may cause embankment dams to settle or spread laterally. Such settlement does not generally lead to immediate failure. However, if the dam is full, relatively minor amounts of settling may cause overtopping to scour and erode the dam leading to potential failure. 5 United States. Association of State Dam Safety Officials. Dam Failures and Incidents. httos:Hdamsafety.org/dam-failures Accessed August 2019. 6-130 January 2020