report are the floodplain boundaries and base flood elevations. Floodplains are low-lying, relatively flat areas adjoining inland and coastal waters. The most common floodplains mapped are those created by the 1 percent annual chance flood (also known as the 100-year flood) and the 0.2 percent annual chance flood (also known as the 500-year flood). The base flood elevation is the computed elevation to which floodwater is expected to rise or that it is expected to exceed during a 1 percent annual chance flood, and it forms the basis for setting flood insurance premiums and structure elevation regulations.
The extent of potential flood inundation must be predicted from statistical analyses and models. For riverine flooding, statistical estimates of flood discharges at U.S. Geological Survey (USGS) stream gages and digital representations of the land surface topography provide data for hydrologic and hydraulic models. The output is used in geographic information systems to delineate the predicted floodplain area. The process is similar for coastal flood mapping, except the existing repository of observational data (hurricane winds, topography, and bathymetry) is smaller and extreme events are more difficult to capture. As a result, coastal flood maps rely more heavily on modeling of wave and erosion processes and storm surge (water that is pushed toward the shore by the force of winds swirling around a storm) to predict coastal flood elevations. All of the inputs have uncertainties that affect the accuracy of the resulting flood map.
Finding 1. Topographic data are the most important factor in determining water surface elevations, base flood elevation, and the extent of flooding and, thus, the accuracy of flood maps in riverine areas.