FEMA has agreed to examine additional data to prove its controversial Nassau flood map is accurate and to make public information that it used to put together the 2009 diagram, officials said Thursday.

The Federal Emergency Management Agency now will look at Town of Hempstead tidal gauge data and other storm readings it did not use before to show the map was accurate in adding 25,000 county properties to the flood zone.

Many owners of those properties were forced to buy flood insurance with premiums of up to $3,000 a year by their mortgage banks. If the additional data show changes in the map are necessary, it could remove some properties from the map - and the requirement to buy insurance.

FEMA said this week it believed the flood map is accurate. When FEMA prepared the map for the county that was adopted two years ago, the agency determined the probable depth of storm surge water by using an unpublished Army Corps of Engineers study for the Suffolk shore from Fire Island to Montauk.

Critics claimed the study could not provide reliable information for Nassau. And while most independent experts agreed with FEMA's contention that the study could be extrapolated into Nassau, many said FEMA did not look at all of the available information to prove that results were valid.

FEMA initially refused to share data from the Corps study with local officials who wanted to challenge the map, saying the study was unpublished and not fully vetted by the outside scientific community. But the Village of East Rockaway - where the number of houses in the flood zone jumped from 570 to about 1,200 so the village made two unsuccessful appeals of the map - and other critics said they could only make a scientific challenge if they had the data used by FEMA.

FEMA now says it will make all data available to make the process more transparent.

"Our mission from Congress is that we use the best science and technology available" and cooperate with local officials and the public, said Tim Crowley, FEMA's new regional director for flood mitigation.

"I think it's a fantastic way to begin the process" of confirming the validity of the map, said recently retired East Rockaway planning director Dennis McCabe.

Two components were used to construct the map. The first is ground elevation data supplied by Nassau County based on aerial surveys. FEMA last year agreed to check the accuracy of the aerial surveys by conducting 100 spot check surveys on the ground. The results released this week showed the elevations were accurate, FEMA said.

The second was the Corps study. McCabe and other critics questioned using a Suffolk model for Nassau. "It's for an entirely different area" where the barrier beaches are much closer to Long Island proper than they are in Nassau, and the bays in Nassau are filled with islands and marshland that absorb winds and stormwater, McCabe pointed out.

But FEMA said the Corps model took those differences into account.

Jay Tanski, coastal processes researcher for the New York Sea Grant Extension Program, said the Corps Suffolk model should work for Nassau. But "it becomes less certain as you move to the west" so using Hempstead's tidal data and other available information is critical to showing the map is accurate.

"It's good that FEMA has come to its senses on providing the data for communities to challenge these maps," said Sen. Charles Schumer (D-N.Y.). "Credible concerns have been raised."

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