County to use LIDAR to map floodplain February 1, 2009Posted by Bahadir Sahin in English, Haber (News).
Tags: 100-year floodplain paper maps, Bitterroot River, floodplain maps, National Flood Insurance Program
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 minimize property damage and protect the riverine ecosystem in the Bitterroot Valley.
The National Flood Insurance Program and the Federal Emergency Management Agency are working with states and local communities across the nation on a map modernization program 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destructive natural force in the United States, but most floodplain maps nationwide are more than 10 years old and are outdated because of changes in the landscape caused by land use, development, erosion and natural forces, according to FEMA.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 modernization project, Ravalli County is using aerial reconnaissance technology, known as Light Detection and Ranging, or 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.
“LIDAR is really fantastic,” said Laura Hendrix, the county’s floodplain administrator. “Getting more data is always exciting. It’ll help our residents to understand their potential for floodplain hazards.”
The LIDAR lasers are capable of recording elevation measurements with a vertical precision of 6 inches, according to the National Oceanic and Atmospheric Administration.
The LIDAR sensor sends a narrow, high-frequency laser beam pulse toward the earth, then records the time difference between the emission of the laser beam and the return of the reflected laser signal to the aircraft.
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.
The updated maps not only help to determine flood risk but can be used by road departments, emergency services, developers, conservation districts and others, Hendrix said.
Floodplain regulations regulate development and other land uses in the 100-year floodplain, the area that has a 1 percent chance of flooding in a given year.
The last recorded 100-year flood in Ravalli County occurred in 1899. The most severe floods in recent decades occurred in 1972 and 1974 when the county was hit with 50- and 60-year floods.
In Montana, eight counties and cities have completed the digital mapping conversion, another nine counties are underway and eight counties are in the early stages, including Ravalli County.
FEMA and Ravalli County’s planning department have given their mapping priorities to a Bozeman contractor, PBS & J.
The first areas where mapping is being updated are the Eight Mile Creek area near Florence and the Three Mile Creek area near Stevensville .
Those regions were selected because of the high level of development and potential for further development, Hendrix said.
Preliminary digital maps and files are expected out by September and the final ones are scheduled to go into effect in November 2010, but the FEMA program often has had delays, so the project likely won’t be completed by then, Hendrix said.
FEMA is paying for most of the project, whose cost hasn’t been determined.
For the LIDAR work, Hendrix sought a $100,000 state grant in 2006 after she heard that other communities in Montana were doing the same.
The county received the grant in 2007 for the first phase of the LIDAR mapping, which covered 185 square miles from Florence to Victor. The work was completed last year.
The county likely will receive another $100,000 grant from the state legislature this spring for the second LIDAR phase, which will cover 156 square miles from Victor to Skalkaho Highway. If the grant is approved, the work could take place this fall.
A third LIDAR phase is planned for 135 square miles from Skalkaho Road to the East and West Forks of the Bitterroot River.
Hendrix said modernizing floodplain maps will reduce errors of interpretation when planners, landowners and others try to determine whether specific sites are in the floodplain.
The current paper maps have “very coarse lines,” mislabeled streets and other problems, she said. “These hard copies aren’t all that accurate, so going digital will be a big help.”
The LIDAR data is available to the public for a nominal fee.
The data already has helped a number of landowners in the northern part of the county to determine whether their property is in the 100-year floodplain, Hendrix said.
“This is the greatest data we’ve ever had,” she said.
Local communities have six months to adopt updated floodplain management regulations after receiving revised flood hazard data from FEMA.
Communities that don’t adopt new regulations will be suspended from the federal flood insurance program, leaving homeowners without flood insurance and communities ineligible for federal disaster aid and other assistance.
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