a national effort covering entire counties in each study, and large regions of the nation as completed county studies accumulate, while the USACE process is local to a particular river or stream with flood problems. The large spatial extent of the FEMA flood mapping process means that it has to place a greater reliance on automated hydrologic and hydraulic methods. Indeed, in many FEMA flood mapping studies, the design flood discharge is estimated directly from USGS regression equations contained in the USGS National Flood Frequency program, and no rainfall-runoff model such as HEC-HMS is used. The large geographic areas of interest to the FEMA flood mapping program makes this a qualitatively different task than traditional hydraulic engineering studies for flood damage reduction.

Framework data are used at all three stages in the FEMA flood mapping process but are most crucial for the flood hydraulics and hazard zone mapping. Flood hydrology depends on a more generalized view of the landscape and its watershed characteristics and is less reliant on precise elevation or base map information. Precise elevation information is critical to both the input and the output of flood hydraulic analysis—the input because it is from such information that stream cross sections are developed, and the output because the boundary of the flood hazard zone has to be interpolated from one cross section to the next using a contour map or a digital elevation model. To ensure that the flood hazard map and the flood hydraulic model conform to one another, it is important to use consistent elevation information throughout each of the component processes.

This discussion indicates that uncertainty in mapped flood hazard zones and BFEs arises from a variety of sources—uncertainty in the magnitude of the flood discharge, in the modeling

FIGURE 2.5 The 3,140 counties of the United States.

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