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Elevation Data for Floodplain Mapping
FIGURE 2.1 Floodplain map of Bexar County, Texas. The blue area is area inundated during the 1 percent annual flood event; the white area shows the urbanized area of the City of San Antonio and its surrounding communities. The yellow and black dashed line is the boundary of Bexar County. The flood map is displayed over a gray-shaded relief map of digital elevation data from the National Elevation Dataset. SOURCE: San Antonio River Authority. Reprinted with permission.
The floodplain map for a county is created by constructing flood simulation models for each stream reach under investigation. FEMA flood maps are created for streams with a drainage area that typically exceeds 1 square mile and has risk that warrants a map based on input from the local community officials and FEMA regional staff. Where flood simulation models already exist, they may be used as is or updated with new framework data to support a flood mapping study. Figure 2.3 shows flood modeling for Salado and Rosillo Creeks, two of the watersheds within San Antonio. The left-hand side of Figure 2.3 shows a Hydrologic Engineering Center—Hydrologic Modeling System (HEC-HMS) flood hydrology model for the whole of the Salado Creek watershed, of which Rosillo Creek is a