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•® 0 i 1 r I y Jim Cooper, Regional Hydrologist Minnesota Department of Natural esoures Everybody has read about disasters occurring when dams have failed and resulted in millions of tons of water cascading down- stream,destroying lives and property. Probably the most well known of this type of disaster is the Johnstown Flood which occurred in 1889 when the dam of the South Fork Reservoir burst, send- ing a wall of water downstream into Johnstown, Pennsylvonia, killing 2200 people. But, what is the potential for dams to cause disaster that ,. otherwise operate as Intended by design? The construction of flood control dams and associated reservoirs ore one of the many techniques employed to reduce the down- stream risk of loss of life and the occurrence of property damage due to flooding. The concept of a flood control dam is simple. Water is stored behind a dam during a flooding event and released at a lesser rate of flow downstream until the waters causing the flood have all been discharged. ff this is the case, how can a disaster result during the normal operation of a dam if failure and sudden release of aN the stored water does not occur? To understand how a disoster could occur, ~e must be familiar with the functional components of a dam and also the minimum standards associated with floodplain land use controls as imposed by Minnesota law and related rules, as well as the Federal Emergency Management Agency In the administration of the National Flood Insurance Program (NFIP). Figure 1 portrays the floodway-floodpiain management concept. Adam consists of three basic functional components: the barrier that holds back the water; the principal spillway which releases normal flow from the reservoir; and the emergency spillway which passes large volumes of water from the reservoir that results from runoff due to a large storm event occurring over the watershed upstream of the dam (see Figure 2). ...Pleaso tum to pa 2