Flood maps may have out-of-date data

Maps used to calculate flood danger in the U.S. rely on data that are decades out of date, according to a report that calls for a new national program to remap land levels.

The National Research Council on Thursday proposed an "Elevation for the Nation" program to produce up-do-date data.

The most immediate need is for the Federal Emergency Management Agency's flood mapping program. Those maps are used by mortgage companies and FEMA's National Flood Insurance Program to determine whether property owners should be required to purchase flood insurance.

The report notes that FEMA has been working with state and local governments in a $200 million-a-year effort to replace paper floodplain maps with digital ones. Generally, it is up to state and local governments to provide the data upon which the maps are based, the report said.

The new report was requested by Congress so it could consider the recommendations in upcoming appropriations decisions. FEMA has requested a more comprehensive study of flood map accuracy, which is expected to take two years to complete.

While two-dimensional map data is available and accurate, most of the elevation information dates to 1970s data from the U.S. Geological Survey.

Since then, there have been significant changes, including subsidence in coastal areas, urban expansion and land development across the country. Such changes affect floodwater movement and depth.

The NRC committee recommended use of lidar to develop the new elevations. Lidar sends short light pulses from an aircraft and measures the time it takes them to bounce back, allowing it to calculate variations in ground level.

David R Maidment, chairman of the committee that prepared the report, said such a project would be costly but would be useful for many agencies in addition to the flood mapping.

Improved elevation data would be valuable to the Corps of Engineers, Agriculture Department, Bureau of Land Management, Forest Service and other agencies, he said.

It would help in planning for hurricane surges, for example, and even building logging roads, said Maidment, a civil engineering professor at the University of Texas at Austin.

A program to update map elevations in North Carolina has cost about $26 million, he said, but it is not clear what it would cost for a similar effort nationwide. It would take several years, he added.

The National Research Council is part of the National Academies, an independent organization chartered by Congress to advise the government on scientific matters.

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National Research Council: http://www.nationalacademies.org/nrc