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<br />selected should be sufficiently large to cause significant discharge through <br />the spillway and associated significant downstream flooding without a failure. <br />The flood selected should be shown in the plan, such as, probable maximum <br />flood (PMF) , 1/2 PMF, or 18" rainfall in 24 hours. <br /> <br />The differences in flooded areas may be large enough to warrant <br />identifying different evacuation areas for different flood or failure <br />conditions. If appropriate, these areas should be identified in the EAP. <br /> <br />3. Dam Break Hydrograph and Routing Procedures. Several factors usually <br />have to be evaluated whenever dam failures are postulated. The type of dam <br />and the mechanism which could cause failure require careful consideration if a <br />realistic breach is to be assumed. Size and shape of the breach, time of <br />breach formation, hydraulic head, and storage in the reservoir contribute to <br />the dam failure hydrograph. Most of the methods for estimating dam break <br />hydrographs require the choice of size, shape, and time of dam breach. <br />However, simplified techniques do not require an estimate of the size and <br />shape of the breach or the time of breach formation. There are also several <br />available procedures for routing dam failure hydrographs to determine <br />information on areas inundated by the flood as it travels downstream. To <br />avoid unnecessary study and costs, the routing should cease at a point where <br />real-time flood warning information can be provided on a pre-planned basis. <br /> <br />Many methods for developing the dam failure hydrograph and routing dam <br />break flows downstream are available and equally acceptable. Regardless of <br />the method used, all emergency action plans for communities and other affected <br />areas in a watershed should be developed to the extent practicable using the <br />same computational procedures to ensure coordination, compatibility, and <br />acceptance. Many Federal agencies have developed procedures that are <br />available upon request. They may be obtained from the National Weather <br />Service, Bureau of Reclamation, Soil Conservation Service, Corps of Engineers, <br />Tennessee Valley Authority, Geological Survey and Federal Emergency Management <br />Agency. Procedures developed by the National Weather Service (NWS) are the <br />most widely used. Simplified NWS procedures are readily available from the <br />nearest FEMA office. However, considerations for using the detailed procedure <br />should be coordinated with NWS. <br /> <br />4. Flooding Information and Inundation Maps. <br /> <br />(a) Mapping. Whenever communities or significant numbers of <br />dwellings are located in the flood plain downstream of a dam, an inundation <br />map is usually needed to develop an adequate evacuation plan. These maps <br />should show an outline of the area covered by the dam break flood in enough <br />detail to identify dwellings and other significant features that are likely to <br />be directly affected. This is generally accomplished by superimposing the <br />flood outline on a existing map. Additional data included on the maps are <br />estimated flood travel time and depth at selected locations. The maps should <br />be of sufficient scale and detail to identify clearly the areas which would be <br />flooded if large flows occur from dam failure, misoperation, or extreme <br />storms. Clarity and simplicity are important. Therefore, the map scale <br />should be such that all important features can be identified. A typical <br /> <br />4 <br />