Resolution No. 4814 - Laserfiche WebLink

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failure. However, if the dam is full, relatively minor amounts of settling may cause overtopping to occur, with resulting scour and erosion that may progress to failure. For any dam, improper design or construction or inadequate preparation of foundations and abutments can also cause failures. Improper operation of a dam, such as failure to open gates or valves during high flow periods can also trigger dam failure. For any dam, unusual hydrodynamic (water) forces can also initiate failure. Landslides into the reservoir, which may occur on their own or be triggered by earthquakes, may lead to surge waves which overtop dams or hydrodynamic forces which cause dams to fai~ under the unexpected toad. Earthquakes can also cause seiches (waves) in reservoirs that may overtop or overload dam structures. In rare cases, high winds may also cause waves that overtop or overload dam structures. Concrete dams are also subject to failure due to seepage of water through foundations or abutments. Dams of any construction type are also subject to deliberate damage via sabotage or terrorism. For waterways with a series of dams, downstream dams are also subject to failure induced by the failure of an upstream dam. If an upstream dam fails, then downstream dams also fail due to overtopping or due to hydrodynamic forces. An excellent review of the common mechanisms for dam failures is given in the FEMA publication: Dam Safety: An Owner's Guidance Manuat.3 For further details, the reader is referred to this publication and the references therein. A National Research Council study4 of dam failures in the United States and Western Europe from 1900 to 1969 compiled historical data on the observed probability of failure as a function of type of dam. Dam failures are quite common in the United States. For example, FEMA data from Tropical Storm Alberto (1994) show 230 dam failures in the State of Georgia from this single event.5 Fortunately, most dam failures are of small dams where the failure poses little or no risk to life safety and only minor, localized property damage. Most failures are of dams that are too small to be included in the NID database or dams in the NID Low Hazard Potential Category. However, in the United States between 1960 and t997 there were 23 dam failures that caused at least one death, with total fatalities from these 23 failures estimated at 318 people.5 Since 1874, there have been six dam failures in the United States which killed over 100 people.2 The worst dam failure, in terms of casualties, was the 1889 Johnstown Pennsylvania dam failure which killed over 2,200 people. Three of the high fatality dam failures occurred in the 1970s: Black Hills, South Dakota, Big Thompson River, Colorado, and Buffalo Creek, West Virginia. These three failures alone resulted in an estimated 514 deaths.2 (Note: the published death statistics in this paragraph from these two FEMA sources are inconsistent, but these differences are not significant for the present purposes). 12.3 Oregon Dam Data The National Inventory of Dams (NID)lists 812 dams in Oregon. Of these NID dams, 34 are in Lane County. The statistical breakdown of these dams by NID Potential Hazard Categories is shown below in Table 12.2. Pubhc Review Draft: August 6, 2004 12-4