FIGURE 4.1 Map of stream gages analyzed in this report.

FIGURE 4.1 Map of stream gages analyzed in this report.

to a horizontal run of 329 feet. In the mountains, flood discharges for a given drainage area are large, but the floodwaters are confined within narrow valley floodplains. In the coastal plain, lower terrain slope leads to less flood discharge for a given drainage area, but once the banks overflow, floodwaters spread over a broader floodplain. The relationship between the terrain slope and the river slope is discussed below (see “Channel Slope”).

Peak stage data were also studied from 10 gages in southwest Florida (Table 4.1), which has a pitted landscape with many sinkholes where water ponds in depressions and flows from one pond to another until it reaches a stream or river. These stage height data were analyzed to determine whether BFE uncertainties were different in pitted landscapes compared to landscapes with dendritic drainage patterns. Altogether, 31 stream gage records were examined from North Carolina and Florida. The gages have an average length of record of 54 years and an average drainage area of 458 square miles. Although the spatial distribution of USGS stream gages is biased toward larger streams and rivers, the drainage area of the gages examined varied by three orders of magnitude—from approximately 5 square miles to approximately 5,000 square miles—which is a reasonable representation of the range of drainage areas for stream reaches used in floodplain mapping.

At each stream gage site, the historical record of both flood discharges and flood stage was analyzed using the U.S. Army Corps of Engineers (USACE) Statistical Software Package HEC-SSP.3 Although some stream gage records include estimates of “historical” floods before the period of gaged record, these were not included in the present study. In some gage records, there are notes that the flood flows were affected by factors such as urbanization or releases from upstream reservoirs. The committee did not separate out these records in the belief that riverine environments must be mapped, regardless of whether such events occurred. In a few of the coastal gages, the times of occurrence of the maximum flood stage and maximum flood discharge differ slightly, and in those cases, the largest value was used. For each gage, the log-Pearson III distribution was applied to both discharges and stage heights, as



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