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Elevation Data for Floodplain Mapping
FEMA program maintains more than 100,000 map panels. The nation has approximately 4.2 million miles of streams and coastlines. Of this total inventory, FEMA has mapped approximately 1 million miles of floodplain. As development occurs, FEMA constantly has to update the existing floodplain maps and generate new maps for previously unstudied streams. The rate of re-study depends on how quickly development occurs within a watershed. Subsidence, the gradual settlement or sudden sinking of the earth’s surface caused by human development and natural causes, can create problems for the users of DFIRMs product. In areas of riverine flooding if the subsidence is widespread and uniform, the relative difference between the floodplain and the development is critical, and in areas of coastal flooding, the absolute elevation is critical. A large user of the DFIRMs is the insurance industry. The federal government requires any federally backed mortgage whose structure is within the 1 percent annual recurrence interval floodplain to purchase flood insurance. Accurate flood maps are necessary to lessen the likelihood of property owner hardships, and accurate elevation data are fundamental to generating an accurate flood map. The current Map Modernization program is performing a national update of the floodplains. This work is funded from 2004 to 2008, and the products will be available by 2010. This update will cover approximately 65 percent of the nation.
FEMA has ranked every area of the nation based on a risk analysis that incorporates 10 parameters. This risk ranking is used to prioritize where and when updates should occur within the nation. FEMA in 2005 adopted additional map quality standards specifying that the floodplain boundary must match the best available topographic data. The goal is that of the maps that become modernized, 75 percent of the streams and coastlines will meet this requirement.
For most cases, FEMA does not acquire digital elevation data during a floodplain study but usually relies on the best available information. Typically for high-population counties, the local government does have good elevation data, but in low-population areas, USGS 7.5-minute quadrangle maps are the best available elevation data.
The actual drawing of the floodplain boundary can occur by hand or with digital techniques using DEMs or TINs. The floodplains should be drawn based on the TINs since this will generally create the most accurate floodplain map.