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County to use LIDAR to map floodplain

USA: Like all streams, the Bitterroot River and its tributaries are constantly shifting, their beds and banks altered by runoff, land use and other factors. But like most communities across the nation, Ravalli County hasn’t kept up to date with its floodplain maps, which were created more than a decade ago.

Now, the county is going digital and airborne to update its 100-year floodplain paper maps in an effort to minimise property damage and protect the riverine ecosystem in the Bitterroot Valley.The National Flood Insurance Program and the Federal Emergency Management Agency (FEMA) are working with states and local communities across the nation on a map modernisation programme to develop more accurate floodplain mapping data.

Flooding is the most destructive natural force in the United States, but most floodplain maps nationwide are more than 10 years old and are outdated because of changes in the landscape caused by land use, development, erosion and natural forces, according to FEMA. As part of the map modernisation project, Ravalli County is using LIDAR, to create highly detailed topographic maps of private land in the Bitterroot Valley. Nationwide, Ravalli County is one of the few small communities to use LIDAR, a remote sensing system that uses aircraft-mounted lasers.The new digital maps will use LIDAR data, ground surveys and hydrology models to determine the 100-year floodplain.

The county currently uses antiquated U.S. Geological Survey topographical maps that have 20 foot to 40 foot contour levels, but LIDAR can detect surface variations down to two foot contours – or enough to detect things as small as a typical backyard burn pile, Hendrix said.

FEMA’s goal is to update paper maps and data, known as Flood Insurance Rate Maps, and convert that information into geographic information systems (GIS) maps and files. The project will upgrade the flood map inventory into a national database that is publicly available in GIS format.