Local and federal officials soon will complete a multiyear effort to update many of Northern Virginia's flood maps, in a process that will reshape the area's insurance landscape and cost hundreds of property owners thousands of dollars.

The Federal Emergency Management Agency has been working with the Army Corps of Engineers and officials from Alexandria, and Fairfax and Arlington counties, for the last six years to develop more accurate area flood maps.

Those maps determine which property owners must buy federally mandated flood insurance policies. Flood insurance costs the average property owner $570 annually, but can run into the thousands depending on where a resident's property lies within the flood zone.

"We were seeing higher and higher amounts of damage and insurance claims from outside of the floodplain," said FEMA spokesman David Bollinger, outlining his agency's reasons for redrawing the flood maps.

Bollinger said property owners who were not previously mapped into the floodplain soon could be included, and therefore would have to buy flood insurance.

About 7,000 properties in Fairfax, Arlington and Alexandria lie within the current flood maps, which date to the 1980s and 1990s.

Local and federal officials were unable to give the exact number of properties that would be included or excluded once the floodplains are redrawn, but thousands will likely be affected once the maps are finalized.

Fairfax County's floodplain update is scheduled to take effect in September, with Arlington and Alexandria's to follow in 2011.

Alexandria City Engineer Emily Baker said the city will have a six-month comment period before the new maps are finalized, during which time property owners can dispute the redrawn floodplains.

"We're going to be reaching out to all the property owners to make sure they understand the insurance implications," Baker said. "If you purchase your flood insurance prior to the new maps taking effect, you can get a more favorable premium."

Bollinger explained the "grandfather clause," which allows property owners to take advantage of a reduced insurance rate before the new maps are finalized. But the lower premium only lasts a couple of years, he said.

Bollinger said his agency and the Army Corps were updating flood maps nationwide.

"Congress did not want to end up spending a lot of additional taxpayer money" to pay for flood damage outside of the flood map, he said.

Bollinger said FEMA backs more than $1 trillion in flood insurance policies annually.