Agenda - 05-06-2010 - 4m - Laserfiche WebLink

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necessary to provide for the safety of the public and those that may have to respond to an incident at those locations. River Basins Dam Failure There are approximately 80,000 dams listed in the National Inventory of Dams. This number includes impoundment structures greater than or equal to 25 feet in height or impounding 50 acre-feet (an acre-foot equals water 1 foot deep across one acre of land) or more of water, or structures above 6 feet in height whose failure would potentially cause damage downstream. Nine thousand dams nationwide have been designated as high hazard dams. For purposes of this report, dams meeting these criteria, shall be termed regulated dams. The high hazard designation does not indicate the inherent stability or instability af a dam but instead measures the potential threat posed to downstream populations in the event of a dam failure. Background Information on Dams Dams provide a life-sustaining resource to people in all regions of the United States. Unlike most infrastructure, dam owners are solely responsible for the safety and the liability of the dam and for financing upkeep, upgrade and repair. While most infrastructure facilities (roads,bridges, sewer systems,etc.) are owned by public entities, the majority of dams in the United States are privately owned. Across the nation, about 58% of dams are privately owned, 16% are owned by local governments,4% by states, and the rest by the federal government and public utilities. Manmade dams are classified according to the type of construction material used, the methods used in construction, the slope or cross-section of the dam,the way the dam resists the forces of water ressure the means used for controllin see a e and occasi n p , , o all , g Pg Y according to the purpose of the dam. The materials used for construction of dams include earth,rock, tailings from mining or milling,concrete,masonry, steel,timber, miscellaneous materials (such as plastic or rubber) and any combination of these materials. Embankment dams,the most common type of dam, are usually constructed of natural soil or rock or waste materials obtained from mining or milling operations. An embankment dam is termed an "earthfill"or"rockf'ill"dam depending on whether it is comprised of compacted earth or mostly compacted rock. The ability of an embankment dam to resist water pressure is primarily a result of the mass, weight, type and strength of the materials from which the dam is made. Overtopping of an embankment dam is very undesirable since embankment materials may be eroded away. Water normally passes through the main spillway or outlet works; it should pass over an auxiliary spillway only during periods of high reservoir levels and high water inflow. All embankment and most concrete dams have some seepage; however,it is important to control the seepage to prevent internal erosion and instability. Proper dam construction, maintenance, and monitoring of seepage provide this control. 18