Laserfiche WebLink
• • <br /> Memo to File M-81-302 -2- May 26 , 1997 <br /> floodplain management measures . Encroachment on floodplains, such <br /> as by artificial fill material , reduces the flood-carrying <br /> capacity, increases the flood height of streams, and increases <br /> flood hazards in areas beyond the encroachment . For purposes of <br /> FEMA studies, the 100-year flood area is divided into a floodway <br /> and a floodway fringe, as shown on the attached figure . The <br /> floodway is the channel of a stream plus any adjacent floodplain <br /> areas that must be kept free of encroachment in order for the 100- <br /> year flood to be carried without substantial increases in flood <br /> heights . FEMA' s minimum standards allow an increase in flood <br /> height of 1 . 0 foot, provided that hazardous velocities are not <br /> produced. The floodway fringe is the portion of the floodplain <br /> that could be completely obstructed without increasing the water <br /> surface elevation of the 100-year flood by more than 1 . 0 foot at <br /> any point . <br /> Flood hazard maps are a type of floodplain inundation map that <br /> shows the extent of inundation as determined from a thorough <br /> technical study of flooding at a given location. Flood hazard maps <br /> are commonly used in floodplain information reports and require <br /> updating when changes have occurred in the channels, on the <br /> floodplains, and in upstream areas . These changes include <br /> structural modifications and channel or floodplain modifications in <br /> upstream areas . Development of new buildings on the floodplain, <br /> obstructions, or other land use changes can affect the stream <br /> discharges, water surface elevation, and flow velocities, thereby <br /> changing the elevation profile defining the floodplain. <br /> attachment <br /> C:\WP51\DEEPEPIT.2 <br />