Resolution No. 4814 - Laserfiche WebLink

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potential dams are often located in predominantly rural or agricultural areas. Dams assigned to the high hazard potential classification are those where failure or mis-operation will probably cause loss of human life. Failure of dams in the high classification will generally also result in economic, environmental or lifeline losses, but the classification is based solely on probable loss of life. Of the dams in the NID, nearly 60% are privately owned. In addition to the dams in the NtD, there are many thousands of dams too small to meet the selection criteria for the NID. Most of these small dams are also privately owned. The NID is available online through several links at FEMA and the United States Army Corps of Engineers. However, since September 11,2001, access is somewhat restricted. Basic NID information and links to the database are available at http://crunch.tec.army, mil/nid/webpages/nid.cfm 12.2 Dam Primer In the simplest terms, dams are impervious structures that block the flow of water in a river or stream and thereby impound water behind the dam. Dams have been built for thousands of years from a wide range of materials, including earth, stone, masonry, wood, and concrete. Large modern dams are almost always embankment dams (built primarily from soil, rock, or mixtures) or concrete darns. Large modern dams almost always have control mechanisms such as gated spillways or outlet pipes for releasing water in a controlled fashion. Typically, dams are operated to smooth natural variations in water flow. During high water flow periods, water is stored behind a dam, while in low water flow periods, water is released to increase flows. Controlled releases typically result in lower peak (flood) flows and higher minimum flows than in uncontrolled streams. The specific patterns of water storage and release vary from dam to dam, depending on the primary purpose(s) of the dam and on a wide variety of economic, regulatory and environmental considerations. 12,2.1 Dam Nomenclature and Types of Dams Modern dams, whether embankment dams or concrete dams, are typically constructed on a foundation, which may be concrete, natural rock or soils, or compacted soils. Dams are usually constructed along a constricted part of a river valley to minimize cost. Dams are also connected to the surrounding natural valley walls, which become the abutments of the dam structure itself. Embankment dams are commonly termed earthfill or rockfili dams, depending on the primary material used in their construction. Historically, a wide range of earth and rock materials have been used to construct embankment dams, with various construction techniques including hydraulic fill and compaction. Embankment dams are broad flat structures, typically at least twice as wide at the base as their height. In cross section, embankment dams are typically trapezoidal, with a wide flat base, sloping slides and a narrower flat top. Depending on the permeability of the materials used in an embankment dam, Public Review Draft: August 6, 2004 12-2