Elevation Data for Floodplain Mapping (2007) / Chapter Skim
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3 FEMA'S Map Modernization Program
3 FEMA'S Map Modernization Program
Pages 24-56
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mapping activities and describe the basis for the use of various mapping technologies in the Flood Map Modernization program. The chapter reviews the basic elements of the FEMA Flood Insurance Rate Map (FIRM)
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and the column of text or Notes to Users (left side of the map) ; and the map data comprise the main map features with flood boundaries delineated.
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: • Zone labels: these labels show an insurance agent what rates to charge the property owner for flood insurance. Typical 1 percent recurrence interval zones will begin with either the letter A or the letter V
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: these are lines on the map that represent the 1 percent annual chance flood elevations. BFEs are used to replicate the flood profile that is contained within the FIS narrative and are used by stakeholders as a general guide for building elevations near or within the 1 percent floodplain.
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from which the floodway is calculated. In the upper panel, the green line represents the 0.2 percent annual recurrence interval flood, the orange line represents the 1 percent annual recurrence interval flood, the blue line is the stream centerline, and the dashed lines represent the floodway.
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New FEMA engineering studies produce more accurate flood maps and allow reevaluation of property locations relative to the floodplain. The study types include the approximate study, the limited detailed study, and the detailed study.
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Accurate flood maps are necessary to lessen the likelihood of property owner hardships. Semiautomated hydrologic, hydraulic, and mapping tools, coupled with digital elevation data, allow prediction of the floodplain limits, especially in lower-risk areas.
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In parts of the country where flood risks have not changed significantly since the date of the hazard analysis of an existing FIRM, FEMA provides an alternative flood study method involving the redelineation of effective detailed study data using updated digital elevation data. This simply involves the transfer of effective BFEs to the current land surface and results in a floodplain fit to existing terrain conditions (Figure 3.4)
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A limited detailed study does not include as much field survey data. Both types of studies include the 1 percent annual probability event and may also include floodways and profiles, although these are optional for a limited detailed study.
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3.3 FLooD INSURANCE Flood insurance is mandatory for the homeowner if the homeowner has a federally backed mortgage and is within the 1 percent annual chance floodplain (100-year flood) as determined by the FIRMs.
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Subsidence creates problems for flood mapping; its effects can vary from riverine to coastal environments, and the location and type of subsidence will determine the impact on the mapping of the floodplain boundary. Potential areas of subsidence in the continental United States.
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For instance, if the 1 percent annual coastal stillwater elevation extends 1 mile in from the shoreline and the land subsides 1 foot, the stillwater elevation may now extend 2 miles in from the shoreline. This effect also occurs when sea levels rise.
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For instance, if a structure has a finished basement, then the premiums are based on the difference between the 1 percent annual chance flood elevation and the elevation of the basement, not the first floor of the structure. In areas with elevated structures, if a property owner encloses the bottom area, this could become the lowest finished floor of the structure.
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A "map panel" is a representation of a geographic area that depicts flood risk information as well as basic features such as the road network and community boundaries. Prior to Flood Map Modernization, approximately 100,000 maps were required to cover most flood-prone lands in the nation.
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of the remaining 2.9 million miles that are or could be subject to some degree of flood risk to property due to development, there are approximately 247,000 miles of streams with flood hazards identified by detailed study methods and approximately 745,000 miles of streams identified by approximate study methods (FEMA, 2006d)
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3.6 FEMA'S PRoCESSES AND PRoCEDURES -- 2002 To 2006 FLooD MAP MoDERNIZATIoN In 2002, FEMA changed its mapping process based on guidance and funding supplied by Congress. Flood Map Modernization uses digital technology to provide better flood hazard maps and data and to compile a digital archive of the resulting materials.
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. The Multi-year Flood Hazard Identification Plan details FEMA's five-year plan for providing updated digital flood hazard data and maps for areas with flood risk.
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Census block group across the entire nation. This National Flood Risk analysis is intended to assist both FEMA and local authorities in prioritizing regional flood study mapping projects at the sub-county level.
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Figure 3.6 shows the results of this effort and provides a breakdown of flood risk in selected ranges, where the highest-risk areas are associated with the top 25 percent of the data and are identified in a purple color. As can be seen from this figure, the highest-risk areas of the nation are near the coastline, with particular risk in the Gulf and southern Atlantic coasts as well as the lower, central, and northwestern areas of the Pacific coast.
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FIGURE 3.7 National flood risk percentages by FEMA region. The Gulf coast states have the highest flood risk percentages in the country.
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overall, 75 percent of the mapped stream miles within the Map Modernization initiative will meet the 2005 Floodplain Boundary Standard, specified in Table 3.2, meaning that the floodplain boundaries on the maps are drawn consistently with the best available elevation data. TABLE 3.2 Risk Classes of Flood Insurance Rate Maps Risk Typically Delineation Reliability Class Characteristics Achieved by of the Flood Boundarya A High populations and densities within the Zones AE, ±0.5 foot/95% floodplain; high anticipated growth VE, AO, AH B Medium populations and densities within Zones A ±1.0 foot/95% the floodplain; modest anticipated growth and AE C Low populations and densities within the Zones A ±½ contour interval/ floodplain; small or no anticipated growth and AE 90%b D Undetermined risk; likely subject to flooding Zone D N/A E Minimal risk of flooding; area not studied (area not N/A mapped)
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Based on the risk class determined for each flooding source, varying methods of analysis can be employed in a flood study. Within the table, zones that begin with A refer to different areas of the annual 1 percent chance flood, and zones that begin with V refer to areas of the 1 percent annual chance flood where waves of greater than 3 feet exist.
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A-5) : FEMA has reduced the complex requirements to two standard choices for digital elevation data, expressed as equivalent contour intervals: • Two-foot equivalent contour interval for flat terrain (Accuracyz = 1.2 feet at the 95 percent confidence level)
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. These specifications also define the acceptable currency of the base map data used in flood map revisions: "The data must have been created or reviewed for update needs within the last 7 years" (FEMA, 2002b, p.
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Elevation field surveys are among the more costly components of a detailed study, and the development of accurate elevation maps also can be costly. However, ground elevation information has a multitude of other uses and often is already available.
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USGS quadrangle Acceptable for all risk Limited acceptability for Limited acceptability for maps (tagged classes (A-C) risk classes A and B; floodplain mapping for risk vector contour acceptable for risk class C classes A and B; acceptable information)
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Linear features, such as streams, drainage ditches, ridges, and roads, are often lost in a DEM if the grid spacing is larger than the dimensions of the feature. Furthermore in a DEM, it is unlikely that the sharp edge of the feature will be represented correctly in the elevation model.
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A TIN is a vector data structure generated from the mass points and breaklines in a DTM. TINs also preserve abrupt linear features and are excellent for calculations of slope, aspect, and surface area and for automated generation of topographic contours, which are all important functions to flood study engineering.
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In addition to the two-dimensional or surface view, both grids and TINs can be viewed in perspective in three dimensions, as shown for the TIN in Figure 3.9e. Floodplain mapping on a TIN surface involves complex calculations.
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Floodplain defined on a TIN e. Three-dimensional view of the landscape represented as a TIN FIGURE 3.9 Comparisons between gridded DEM surfaces and two- and three-dimensional TIN surfaces that are part of the base for floodplain maps: (a)
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Therefore, the triangulation of DEMs should be avoided for use in floodplain mapping since results are difficult to reproduce. In summary, the intersection of the computed water surface elevation with TINs creates the most accurate flood boundary and should be used whenever possible.
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FEMA's Map Modernization Program Triangulation Using a Digital Elevation Model (DEM) FIGURE 3.10 Inconsistencies experienced when triangulating DEMs.
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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.
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