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These FIRMs are used in identifying whether a land or building is in flood zone and, if so, which of the different flood zones are in effect. In 2004, FEMA began a project to update and digitize the flood plain maps at a yearly cost of $200 million. The new maps usually take around 18 months to go from a preliminary release to the final product.
Theoretically a 100-year flood has a 1 percent chance (1/100 = 0.01 or 1 percent) of occurring in any given year and a 500-year flood has as a 0.2 percent chance (1/500 = 0.002 or 0.2 percent) of occurring in any given year. [12] However, these expected flood elevations actually occur more or less often than expected. [13]
Another commonly encountered term is the Special Flood Hazard Area, which is any area subject to inundation by a 100-year flood. [25] A problem is that any alteration of the watershed upstream of the point in question can potentially affect the ability of the watershed to handle water, and thus potentially affects the levels of the periodic floods.
The county is currently using 500-year flood maps to indicate areas where residents should be ready to evacuate. "We are using the 500-year flood plain maps as kind-of a guide.
Based on the expected 100-year flood flow rate, the flood water level can be mapped as an area of inundation. The resulting floodplain map is referred to as the 100-year floodplain. Estimates of the 100-year flood flowrate and other streamflow statistics for any stream in the United States are available. [3]
The map to the right shows a flood map for the City of York, including the floodplain for a 1 in 100-year flood (dark blue), the predicted floodplain for a 1 in 1000 year flood (light blue) and low-lying areas in need of flood defence (purple). The most sustainable way of reducing risk is to prevent further development in flood-prone areas and ...
FEMA Flood Insurance Study predicts the 100-year flood by the Red Falls stream gauge to be 18,130 cubic feet (513 m 3) per second, and the 500-year flood to be 27,040 cubic feet (766 m 3) per second, therefore flows during Irene surpassed the projected 500-year level. Bank erosion and flood-related damages to buildings occurred in Maplecrest.
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