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Mapping the Zone: Improving Flood Map Accuracy
these landscapes. Such analysis is beyond the resources of this committee.
Recommendation. FEMA should commission a scientific review of the hydrology and hydraulics neededto produce guidelines for flood mapping in pondedlandscapes.
The main insights arising from case studies of elevation uncertainty at stream gages and flood mapping uncertainty are the following:
The sampling uncertainty of the base flood elevation at 31 USGS stream gages in North Carolina and Florida is 1 foot with a range of 0.3 foot to 2.4 feet, as inferred from frequency analysis of long records of annual maximum stage heights. This uncertainty does not show any systematic pattern of variation with drainage area or geographic location at these sites. Thus, there is a lower bound of approximately 1 foot on the uncertainty of the BFE as normally determined in floodplain mapping, since indirect methods of computing BFEs at ungaged sites will have uncertainty at least as great as uncertainties observed at stream gages.
On three stream reaches in North Carolina, the lateral slope at the boundary of the floodplain is such that a 1-foot change in flood elevation has a corresponding 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.
Observed flood discharges at stream gages are the most critical component for estimating the base flood discharge in the three study reaches because all hydrologic methods are calibrated using these data and each stream reach contained a stream gage. BFEs computed from the peak discharge estimated from the various hydrologic methods do not differ much, so the choice of hydrologic method does not introduce much uncertainty in the BFE beyond the lower bound uncertainty (1 foot) estimated by frequency analysis of USGS stage records. The most significant effect of hydrologic variations on BFEs is produced by introducing the average error of prediction into the regression flow estimates (from 42 to 47 percent), which changes the BFE by an average of 1 to 3 feet at the three study sites.
Structures in the channel induce backwater in all three study reaches, with backwater effects extending over the entire length of the reach in the coastal plain but less far in the rolling hills and mountains. The maximum backwater elevation increase found was 2.5 feet in the coastal plain reach, and the backwater effect extended an average of 1.1 miles upstream. In the mountains, the backwater effect extended an average of 0.3 mile upstream.
The greatest effect by far of any variant on the BFE is from the input data for land surface elevation: lidar or the National Elevation Dataset. At Long Creek, the BFE computed on the NED is 21 feet higher than on lidar because of a misalignment of the stream location on the NED. At the other two study sites, the average elevation of the BFEs for the two terrain data sources is about the same, but differs at particular locations by 3 to 10 feet. This result overturns the conventional view that map accuracy is affected at least as much by the accuracy of the hydraulic model and hydraulic parameters as by the accuracy of the topographic data.
The floodplain boundaries produced using lidar and the NED differ from one another, but at two of the three study sites the number of acres enclosed within the Special Flood Hazard Area is about the same for a detailed study using lidar data and an approximate study using the NED. At the third site (Long Creek), the difference in the number of acres within these areas is about 20 percent. This suggests that while floodplain boundary locations are more uncertain in approximate studies than in detailed studies, the total areas they encompass can be reasonably similar, provided the stream and topographic data are properly aligned.
These conclusions were based on limited studies in small areas of North Carolina and Florida, which were carried out to examine the uncertainty of riverine flood mapping quantitatively rather than qualitatively. They are indicative but not definitive of what more comprehensive analyses of a similar character done nationwide might reveal. The importance of the results lies not in the specific numbers but rather in the insights they provide about the relative effect of variations in hydrologic, hydraulic, and terrain methods on flood map accuracy.