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<br />areas that are within the limits of the SOO-year floodplain (shaded <br />on' the work map) and the Zone X areas outside the limits of the SOO- <br />year floodplain (unshaded on the work map). <br /> <br />Zone A <br /> <br />Zone A is the flood insurance rate zone that corresponds to the 100- <br />year floodplains that are determined in the Flood Insurance Study <br />by approximate methods, Because detailed hydraulic analyses are not <br />performed for such areas, no BFEs or depths are shown within this <br />zone. <br /> <br />Zone AO <br /> <br />Zone AO is the flood insurance rate zone that corresponds to the <br />areas of laO-year shallow flooding (usually sheet flow on sloping <br />terrain) where average depths are between I and 3 feet, Average <br />whole-foot depths derived from the detailed hydraulic analyses are <br />shown within this zone, <br /> <br />Zone AH <br /> <br />Zone AH is the flood insurance rate zone that corresponds to the <br />areas of laO-year shallow flooding (usually areas of ponding) where <br />average depths are between I and 3 feet, Whole-foot BFEs derived <br />from the detailed hydraulic analyses are shown within this zone, <br /> <br />A2-4 SHALLOW FLOODING CLASSIFICATION AND DESCRIPTION <br /> <br />Shallow flooding can occur as the result of several phenomena, However, the <br />following classification of two broad types of shallow flooding, into which <br />almost all individual cases can be assigned, has been determined as an <br />appropriate level of detail for purposes of the NFIP, <br /> <br />A. Ponding <br /> <br />Ponding is the result of runoff or flows collecting in a depression that may have <br />no outlet, subterranean outlets, rim outlets, or manmade outlets such as culverts <br />or pumping stations. Impoundments behind manmade obstructions (levees, road <br />fills, railroad grades, canal banks, and other similar structures) are included <br />in this type of shallow flooding as long as they are not backwater from a defined <br />channel, or do not exceed 3,0 feet in depth, <br /> <br />B. Sheet Runoff <br /> <br />Sheet runoff is the broad, relatively unconfined downslope movement of water <br />across sloping terrain that results from many sources, including intense rainfall <br />and/or snowmelt, overflow from a channel that crosses a drainage divide, and <br />overflow from a perched channel onto deltas or plains of lower elevation, <br />Generally, it enters a channel or drainage system that intersects its flow, but <br />occasionally it dissipates before reaching a channel, Sheet runoff is typical <br />in areas of low topographic relief and poorly established drainage systems, <br /> <br />A2-2 <br />