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A study of sampling uncertainties in extreme stage heights at USGS stream gages in North Carolina and Florida found that for 30 of 31 gages, the average uncertainty is approximately 1 foot with a range of 0.3 feet to 2.4 feet. Uncertainties do not appear to vary with the size of the drainage basin or its topographic slope. It may thus be inferred that the lower bound on the uncertainty of the base flood elevation is approximately 1 foot. For the river reaches studied in North Carolina, a 1-foot change in flood elevation corresponds to a horizontal uncertainty in the floodplain boundary of 8 feet in the mountains, 10 feet in the rolling hills, and 40 feet in the coastal plain. This uncertainty has a significant impact on the delineation of inundated areas on flood maps.
The constriction of flood flow by bridges and culverts raises the base flood elevation in the three study areas. Such backwater effects are largest just upstream of the constriction and diminish progressively upstream. They are most pronounced in the coastal plain, extending an average of 1.1 miles and raising base flood elevations by up to 2.5 feet (average 0.9 foot). They are least pronounced in mountainous areas, raising the base flood elevation an average of 0.2 foot, which is not significant, given the sampling uncertainty noted above.
The largest effect by far on the accuracy of the base flood elevation is the accuracy of the topographic data. The USGS National Elevation Dataset (NED), developed from airborne and land surveys, is commonly used in flood map production, even though the elevation uncertainties of the NED are about 10 times greater than those defined by FEMA as acceptable for floodplain mapping. Data collected using high-resolution remote sensing methods such as lidar (light detection and ranging) can have absolute errors on the order of centimeters, consistent with FEMA requirements, but they are not available nationwide. A comparison of lidar data and the NED around three North Carolina streams revealed random and sometimes systematic differences in ground elevation of about 12 feet, which significantly affects predictions of the extent of flooding (e.g., Figure S.2). These large differences exceed FEMA’s stated error tolerances for terrain data by an