Report touts digital floodplain maps

New digital, high-resolution land elevation maps, created using LiDAR technology, are needed to support the Federal Emergency Management Agency’s (FEMA) effort to modernize the nation’s floodplain maps, says a new National Research Council report requested by Congress. The floodplain 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.

FEMA has been partnering with state and local governments in a $200 million-per-year modernization effort to replace paper floodplain maps with digital versions. FEMA sets accuracy requirements for the maps, but it is generally up to state and local governments to provide the data upon which the maps are based. Congress requested the report because of concerns that underlying base map information currently available for much of the nation is not adequate to support the new digital maps.

The committee that wrote the report focused on two layers of floodplain maps: base map imagery and base map elevation. It concluded that there is sufficient 2-D imagery available from digital orthophotos to meet FEMA’s standards for mapping landmarks such as streams, roads, and buildings that show the context necessary for mapping flood hazard areas. The committee also endorsed a program known as Imagery for the Nation, a joint federal-state effort to keep orthophoto databases current.

However, there is inadequate elevation information available to map the shape of the land surface in three dimensions, which is critical in determining the likely direction, velocity, and depth of flood flows, the committee says. In fact, most of the publicly available elevation data is more than 35 years old, with 1970 being the average date of origin in the U.S. Geological Survey’s National Elevation Dataset. Land development and urban expansion since then have significantly altered the surface. New road embankments and flood drainage structures also affect expected floodwater depths, as does land subsidence, which is particularly significant in coastal areas.

In addition, FEMA requires that elevation data in the new digital maps be about 10 times more accurate than most existing data in the National Elevation Dataset, which are used by many states and local communities partnering with FEMA to produce new flood maps. By last summer, digital floodplain maps had been prepared for about 1 million miles of the nation’s 4.2 million miles of rivers and streams, but only 247,000 miles had been mapped using high-resolution elevation data.

The committee calls for a new elevation mapping program, which it named Elevation for the Nation to parallel the existing Imagery for the Nation concept. The committee found a striking level of agreement among representatives of several federal agencies that LiDAR is the current technology of choice for measuring surface elevation.

Elevation for the Nation’s first focus should remain on areas where flood risks justify collecting new data, the committee says. Existing local and regional data may be used if they are sufficiently accurate and complete, the report adds. The committee emphasizes that a seamless nationwide elevation dataset would have many applications beyond FEMA’s flood insurance maps, although it acknowledges that the cost of creating such a dataset will be significant. Data collected in Elevation for the Nation should be disseminated to the public as part of an updated National Elevation Dataset.

The report-sponsored by the National Academies, which is made up of the National Academy of Sciences, National Academy of Engineering, Institute of Medicine, and National Research Council-was completed in a short time so that Congress could consider its findings and recommendations during the upcoming appropriations process. Meanwhile, FEMA requested a separate, more comprehensive Research Council study of flood map accuracy, which is expected to take two years to complete.

Copies of Base Map Inputs for Floodplain Mapping is available from the National Academies Press online at www.nap.edu/catalog/11829.html.