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Mapping the Zone: Improving Flood Map Accuracy (2009)

Chapter: Appendix A: Methods for Estimating Base Flood Elevations in Approximate Studies

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Suggested Citation:"Appendix A: Methods for Estimating Base Flood Elevations in Approximate Studies." National Research Council. 2009. Mapping the Zone: Improving Flood Map Accuracy. Washington, DC: The National Academies Press. doi: 10.17226/12573.
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Page 107
Suggested Citation:"Appendix A: Methods for Estimating Base Flood Elevations in Approximate Studies." National Research Council. 2009. Mapping the Zone: Improving Flood Map Accuracy. Washington, DC: The National Academies Press. doi: 10.17226/12573.
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Page 108

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Appendix A Methods for Estimating Base Flood Elevations in Approximate Studies Method Comments Base flood profile Data extrapolation is acceptable when a site is within 500 ft upstream of a stream reach for which a 100-year profile has extrapolation been computed by detailed study methods, and the floodplain and channel bottom slope characteristics are similar to the downstream reaches. However, the area must be free of backwater effects from downstream structures Point of the boundary Determine the ground elevation in the field where the shaded Special Flood Hazard Area (SFHA) is located on both sides method of the structure for which the base flood elevation (BFE) is needed. Assuming water seeks its own level, interpolate between these two elevations to the location of the building Redelineation Many approximate studies entailed no fieldwork and their floodplains were calculated by “stripping” cross sections from topographic maps to determine the volume of water in a watershed then overlaying the approximate floodplains onto base maps of variable accuracy. Because these floodplains are based on topographic configurations, overlaying a Flood Insurance Rate Map (FIRM) onto a topographic map at the same scale can produce an estimated BFE if the floodplain boundary generally conforms to the contour lines along the flooding source in question Contour interpolation This method is similar to the topographic study approach, but the SFHA crosses contour lines method In riverine areas, the difference between the water surface elevations on opposite banks of a flooding source must be within one-half of the map contour interval to meet national map accuracy standards.a In these cases, the approximate BFE will be equal to the elevation of the lower of the two bank elevations plus one-half the contour interval. This method should be performed at each structure location In lacustrine areas, the difference between the highest and lowest determined water surface elevations around the flooding source must be within one-half of the map contour interval to be acceptable (FEMA, 1995) Historical high-water Historical high-water marks often signify “worst case scenarios.” Communities may utilize them as BFEs and may also add a mark plus a factor of safety factor, commonly 1 to 3 ft above historical high-water marks safety Water control structures Communities may determine the elevation at the high end of a water control structure, such as the top of a berm at a plus freeboard detention basin. The high end approximates the worst possible scenario of overbank flooding. Communities may add freeboard to this elevation, typically 1 to 3 ft above the highest point of the water control structure Stream gage data Stream gages measure fluctuations in water height. Data recorded during flood events can yield a BFE in the location of the stream gage. Because of varying conditions along a watercourse, gage information from various locations should be utilized to determine the variation of BFE along that watercourse. Gages that were not operational during known flood events should not be relied on to establish BFEs continued 107

108 APPENDIX A Method Comments Flood study Agencies other than FEMA may have elevation information that may not appear on the FIRM or the Flood Insurance Study (FIS) report. These include • Federal sources of floodplain studies, technical information, and design manuals (e.g., Army Corps of Engineers, Tennessee Valley Authority, U.S. Geological Survey, National Resources Conservation Service, Federal Highway Administration) • State agencies (e.g., environmental agencies, departments of transportation, state geological surveys, state floodplain management agencies) • Local or regional agencies (e.g., river basin commissions, flood control districts, local and county planning commissions, public works departments, utility companies and agencies, dam commissions) Preliminary Flood Communities have discretion in using data from studies and maps that are in progress and have not yet been given final Insurance Study approval or adopted and published. The information from draft or preliminary studies may be the “best-available” data in areas with only approximate A zones Profiles from a Flood This involves comparison of the location of the site on the FIRM to cross-section lines, and then utilizing that relationship Insurance Study to read a BFE on the appropriate profile sheet included in the FIS report Floodway data tables from The tables identify the BFE with and without the computed floodway at each cross section for a stream reach. Rather than a Flood Insurance Study reading the profiles, the floodway data table provides the BFE at the cross section, eliminating interpolation or profile reading errors FIRM While the FIRMs may indicate BFEs, they are graphical depictions of the observations and computations reported in the FIS report and are not as accurate or precise as information within the report. Aside from graphical approximations or errors in transferring information from the report to the map, BFEs on FIRMs are shown to whole feet, while information within the FIS report is shown to one-tenth of a foot, a big difference aNational map accuracy standards are available at <http://rockyweb.cr.usgs.gov/nmpstds/nmas.html>. “Vertical accuracy, as applied to contour maps on all publication scales, shall be such that not more than 10 percent of the elevations tested shall be in error more than one-half the contour interval. In checking elevations taken from the map, the apparent vertical error may be decreased by assuming a horizontal displacement within the permissible horizontal error for a map of that scale.”

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Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps portray the height and extent to which flooding is expected to occur, and they form the basis for setting flood insurance premiums and regulating development in the floodplain. As such, they are an important tool for individuals, businesses, communities, and government agencies to understand and deal with flood hazard and flood risk. Improving map accuracy is therefore not an academic question--better maps help everyone.

Making and maintaining an accurate flood map is neither simple nor inexpensive. Even after an investment of more than $1 billion to take flood maps into the digital world, only 21 percent of the population has maps that meet or exceed national flood hazard data quality thresholds. Even when floodplains are mapped with high accuracy, land development and natural changes to the landscape or hydrologic systems create the need for continuous map maintenance and updates.

Mapping the Zone examines the factors that affect flood map accuracy, assesses the benefits and costs of more accurate flood maps, and recommends ways to improve flood mapping, communication, and management of flood-related data.

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