Elevation Data for Floodplain Mapping (2007) / Chapter Skim
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2 Flood Mapping
2 Flood Mapping
Pages 16-23
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From page 16... ...
The blue area is inundated during the 1 percent 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, and the flood map is displayed over a gray-shaded relief map of digital elevation data from the National Elevation Dataset. FEMA Map Modernization involves creating maps such as Figure 2.1 for the nation's higher-flood-risk areas, which will cover greater than 92 percent of the population and 65 percent of the land area of the nation.
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From page 17... ...
2.1 FLooD MoDELING 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.
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From page 18... ...
in Davis, California, is the principal source for flood simulation models used in FEMA flood map studies. The Hydrologic Modeling System is a computer program that transforms storm rainfall input to streamflow discharge output using watershed characteristics such as drainage area, slope, length of the longest flow path, and land cover and soil type, to modulate the conversion of storm rainfall to streamflow.
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From page 19... ...
SOURCE: San Antonio River Authority.
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From page 20... ...
In planning flood damage mitigation projects, the USACE uses the HEC-HMS FIGURE 2.4 Floodwater surface elevation computed using HEC-RAS model for one cross section on Rosillo Creek, San Antonio, Texas. SOURCE: San Antonio River Authority.
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From page 21... ...
The FEMA flood mapping process uses a simplified form of flood analysis employed in the USACE flood damage reduction studies. The flood hydrology and hydraulic modeling components are similar, but the third component in the USACE process is the conduct of an economic analysis of flood damage, while in the FEMA process it is the construction of a floodplain map.
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From page 22... ...
Indeed, in many FEMA flood mapping studies, the design flood discharge is estimated directly from USGS regression equations contained in the USGS National Flood Frequency program, and no rainfall-runoff model such as HEC-HMS is used. The large geographic areas of interest to the FEMA flood mapping program makes this a qualitatively different task than traditional hydraulic engineering studies for flood damage reduction.
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From page 23... ...
FEMA Map Modernization seeks to create a floodplain map such as that shown for Bexar County in most counties in the United States, as shown in Figure 2.5. This involves flood modeling for several million stream reaches, and logistical considerations require the use of automated hydrologic and hydraulic methods as described in Chapter 3.
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