discharge estimate, stream cross sections, and BFE. The floodplain boundaries of individual reaches are merged to delineate the Special Flood Hazard Area on a map panel. The digital information describing a single flood study is stored in hundreds or even thousands of files, which must be compiled for each county mapped in the nation. A key purpose of FEMA’s MIP is to store these files so that they will be available for later retrieval. Two types of files are involved: the files that comprise the flood map (DFIRMs) and files of raw field data analyzed in engineering studies to define the BFE (Data Capture Standard database; FEMA, 2003, Appendix L).

Walker and Maidment (2006) examined the design of a geodatabase model to store flood map information. They showed that the most critical parts of the data capture standards are the stream centerlines and cross sections used in the flood hydraulics model. If accurate geographic information system (GIS) files of these are maintained along with the flood hydraulics model, the model could be georeferenced and used in subsequent applications. This involves preserving data defining the connection between two coordinate systems: the Cartesian (x, y, z) coordinate system used to record the meandering of the channel through the landscape and the (s, n, z) coordinate system used in the river hydraulics model, in which s represents stationing distance along the river and n represents the distance across a particular cross section in the river. In effect, the hydraulic model “straightens” the channel by ignoring the bends and considering only how far along and transverse to the stream centerline the water flows. Unless both sets of coordinates are stored in the archived map and model information, it will be difficult or impossible at a later date to place a hydraulic model cross section at the correct map location along the stream.

One limitation of FEMA studies is that they are done county by county and there is no requirement that the underlying streamlines match across county boundaries. This difficulty can be overcome if FEMA streamline data are matched with those of the U.S. Geological Survey (USGS) National Hydrography Dataset (NHD).7 The NHD is a seamless, digital representation of streams and water bodies at map scales of 1:24,000 and 1:100,000 in the continental United States.8 Walker and Maidment (2006) showed that for Fayette County, Texas, the 1:24,000 NHD streamlines cover all the streams mapped in the Map Modernization Program, and that each FEMA-mapped stream segment could be located in a corresponding position on the NHD. Thus, the flood study data collected by FEMA could be linked to and become a part of the nation’s larger repository of hydrologic information, enabling it to be used for much more than flood mapping.


Recommendation. FEMA should reference all stream and coastal studies within its Mapping Information Platform to the USGS National Hydrography Dataset.

7

Presentation to the committee by Sally McConkey, Association of State Floodplain Managers, on November 8, 2007.

8

See <http://nhd.usgs.gov/>.



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