SACRAMENTO, Calif. (AP) Federal officials proposed rules Jan. 15 to limit building in a flood-prone neighborhood after concluding that levees protecting the area from the Sacramento River could be overwhelmed in a major storm.

Construction in the Natomas area, where roughly 70,000 people live, can be allowed only if building occurs 20 ft. (6 m) above the ground because the levees are too weak or too low, the U.S. Army Corps of Engineers and the Federal Emergency Management Agency said.

The building restrictions are scheduled to take effect in December, when FEMA plans to release updated flood maps for the area. Homeowners with federally backed mortgages and businesses with federal loans or grants also must buy flood insurance.

The assessment marks a shift in how the federal government is calculating flood risks to better gauge threats to homeowners around the country since Hurricane Katrina devastated New Orleans in 2005.

“People need to be aware of what the risk is,” said Kevin Knuuti, the Corps’ chief of engineering in Sacramento. “One of the problems of Katrina was people said they didn’t know it was that bad.”

The government’s advice Jan. 15 to Natomas homeowners: Buy flood insurance and store all valuables on a home’s second story if it has one, said Kathy Schaefer, a regional engineer with the Federal Emergency Management Agency.

Sacramento, which ranks at the top of the nation’s most flood-prone cities, sits at the confluence of the Sacramento and American rivers, and below Folsom Lake, which the Corps has said does not have enough storage to contain a major flood.

Natomas, a natural basin that is home to Sacramento International Airport and Arco Arena, where the NBA’s Sacramento Kings play, would be under 15 to 20 ft. of water in a catastrophic flood of the Sacramento River.

Developers began building homes, strip malls and office parks in Natomas in 1998, after the Corps ruled that the levees were strong enough to provide the area with 100-year flood protection. That rating means there is a 1 percent chance of being flooded every 100 years.

More than half the city’s new businesses settled there over the past decade.

“Local government officials have known there are problems, and they still keep telling people it’s OK to build,” said James Pachl, an attorney for environmental groups trying to limit development in Natomas. “Everyone knows it’s a flood hazard.”

A review of the levees in 2006 prompted the Corps to drop its 100-year certification, a change that led FEMA to propose building restrictions. Sacramento officials requested a thorough inspection, hoping it would lead to a loosening of building restrictions while the levees are being repaired.

The Corps’ latest study was even worse news for local officials.

Sacramento Mayor Heather Fargo said she was frustrated that the federal government had responded by recommending restrictions she said essentially impose a building moratorium for the next two years.

Local officials also complained that the Corps had used more stringent criteria in examining the levees.

“This is theoretical math,” Sacramento County supervisor Roger Dickinson said. “There’s a lot of debate over how these calculations ought to be done.”

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