Assessing Irene damage with eye to future

As Long Island residents take stock of damage from last month's storm, officials are measuring Irene's footprint across Nassau and Suffolk counties -- just how high storm tides swelled and just how far inland they swept -- with an eye to improving future flood predictions.

"It will be better information than what we had because it's newer," said Mary Colvin, the Federal Emergency Management Agency's regional chief of the floodplain management and flood insurance.

Documented by storm surge sensors, tidal gauges and surveys of damaged buildings, that data will sharpen the computer models that federal agencies use to forecast floods and storm surge.

Such forecasts matter on Long Island. Federal storm surge maps guide emergency managers deciding which communities should be evacuated as hurricanes loom. Flood maps outline areas most at risk for inundation -- a classification that requires property owners with federally backed mortgages to buy flood insurance.

The latest FEMA flood maps place more than 90,000 Nassau and Suffolk buildings in flood hazard zones, according to agency figures. The updated maps drew protests from some residents who said they were based on inaccurate elevation data.

Some newly added areas in Valley Stream and other areas in the Town of Hempstead could be remapped under a provision inserted in the Senate version of a bill to reauthorize the National Flood Insurance Program.

That legislation has not yet come to a full Senate vote, and the provision -- not contained in the House version passed earlier this year -- would need to be added to a compromise bill drafted by both houses to become law.

The maps are prepared by the federal Army Corps of Engineers and other agencies using local tidal data, topography and information from historic storms.

When a big storm such as Irene sweeps over Long Island, scientists and engineers gain valuable information on which areas get swamped, and what conditions combine to make a nor'easter or hurricane truly devastating.

Last week the U.S. Geological Survey collected 26 sensors that were bolted to bridges and jetties in the region before the storm to measure how high waters rose.

"Once they're bolted on, as storm surge comes in, they collect data every 30 seconds," said Ronald Busciolano, data chief for the survey's Coram office. "The data can be plugged into models and used to assess future storms -- how the surge moves through the embayments, how it moves through Long Island Sound."

On land, federal, state and local officials walked through flood-damaged coastal neighborhoods to record high water marks from Irene.

"That will give us an idea of how close Hurricane Irene came to the elevations that are presently shown on our most recent studies of Nassau and Suffolk County," said Paul Weberg, a senior engineer with FEMA Region 2.

While the data from Irene is one of many considerations that will shape future maps, officials said there isn't a direct relationship between flood maps and the expected impacts of individual storms.

Federal flood maps are based on the likelihood that an area will experience what insurers call the "100-year flood" -- a statistical term meaning there is a 1 in 100 chance a flood that size will happen during any given year.

"It's a flood of a certain size," Colvin said. "The storm surge in a larger hurricane can affect a lot more land and property than would be affected by the 100-year flood."