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<br />" <br /> <br />Guidelines and Specificationsfor Flood Hazard Mapping Partners [April 2003] <br /> <br />may be found on FEMA' s Flood <br />http://www.fema.gov/fhmlen modl.shtm. <br /> <br />Hazard <br /> <br />Mapping <br /> <br />Web <br /> <br />site <br /> <br />at <br /> <br />Wide floodplains with significant storage areas often exist along large rivers in relatively flat <br />watersheds. The Mapping Partner performing the hydrologic analysis may use the unsteady- <br />flow models (one-dimensional models with quasi-two-dimensional capabilities and two- <br />dimensional models) that appear on the FEMA accepted models list to simulate flood attenuation <br />due to this type of storage. <br /> <br />Controlled Flood Storage <br /> <br />Most large reservoirs on large river systems are operated with outflow controls. In these <br />reservoirs, gates are used for regulating flow through outlet structures. The gates an: operated <br />according to established rules that determine the relationship between inflow, outflow, storage, <br />and water demand. <br /> <br />The Mapping Partner performing the hydrologic analysis normally shall not consider storage <br />capability below the Normal Pool Elevation of reservoirs operated primarily for purposes other <br />than flood control because the availability of such storage is uncertain. The exception is when <br />all of the following conditions have been met: <br /> <br />. Operation of the project in accordance with its documented water control plan could <br />affect the 1-percent-annual-chance flood elevations in a community by 1 foot or more. <br /> <br />. The storage capability to be considered is totally dedicated to flood control. Where <br />different amounts of storage can be totally dedicated during different parts of the year, <br />the Mapping Partner shall obtain flood discharges from the joint probability combination <br />of frequency curves established for each part of the year that the different storage levels <br />are dedicated. Joint use storage based on forecasted inflow is not acceptable for NFIP <br />purposes. <br /> <br />· A project water control plan providing explicit details of operation during flooding <br />conditions is in effect and has been reviewed and approved by FEMA or another Federal <br />agency responsible for Federal flood-control activities. The Mapping Partner shall <br />contact the RPO to discuss the review and approval process. <br /> <br />· A written commitment to dedication of the flood-storage capacity and to the approved <br />reservoir operation plan is assured through a mandatory condition of Federal or State <br />licensing or through a direct agreement between the project operator and FEMA for non- <br />Federal projects. <br /> <br />C.1.1.2 Gaged Streams <br /> <br />[February 2002] <br /> <br />Flood discharges may be determined directly from gage data in areas where river gages are <br />located, or may be estimated based on data from gages in nearby areas having similar <br />characteristics. <br /> <br />C-3 <br /> <br />Section C.l <br />