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OCR for page 107
Mapping the Zone: Improving Flood Map Accuracy Appendix A Methods for Estimating Base Flood Elevations in Approximate Studies Method Comments Base flood profile extrapolation Data extrapolation is acceptable when a site is within 500 ft upstream of a stream reach for which a 100-year profile has 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 method Determine the ground elevation in the field where the shaded Special Flood Hazard Area (SFHA) is located on both sides 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 method This method is similar to the topographic study approach, but the SFHA crosses contour lines 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 mark plus a factor of safety Historical high-water marks often signify “worst case scenarios.” Communities may utilize them as BFEs and may also add a safety factor, commonly 1 to 3 ft above historical high-water marks Water control structures plus freeboard Communities may determine the elevation at the high end of a water control structure, such as the top of a berm at a 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
OCR for page 108
Mapping the Zone: Improving Flood Map Accuracy 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 Insurance Study Communities have discretion in using data from studies and maps that are in progress and have not yet been given final 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 Insurance Study This involves comparison of the location of the site on the FIRM to cross-section lines, and then utilizing that relationship to read a BFE on the appropriate profile sheet included in the FIS report Floodway data tables from a Flood Insurance Study The tables identify the BFE with and without the computed floodway at each cross section for a stream reach. Rather than 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.”