Those statistics, along with an updated Federal Emergency Management Agency map showing areas flooded by the powerful October storm, provide the best overview yet of how Sandy hammered the region. The data -- supplied Friday in response to a Newsday request -- came from FEMA's staff, as well as measurements and observations by other agencies and governments.
The previous best estimate was the Long Island Power Authority's calculation that 100,000 structures were damaged or destroyed.
Ten percent of the 948,540 households in the two counties experienced some flooding or storm damage, FEMA said. Also, 38,189 structures had damage totaling more than 50 percent of their value, and 182 structures -- 117 in Nassau and 65 in Suffolk -- were destroyed.
Among the flood-damaged structures were 44 power substations or other electrical facilities; 28 fire stations; 26 schools; three police stations; and one medical facility, Long Beach Memorial Medical Center, located adjacent to Reynolds Channel and heavily damaged.
The new inundation map, in many cases, shows the flood line farther inland than on FEMA's 2009 Nassau County flood zone map or 2006 Suffolk County map. That was expected because the flood zone on the 2009 prediction map is based on a storm expected once in 100 years, such as a Category 1 hurricane.
In southwestern Nassau, southern Queens and lower Manhattan, Sandy came ashore as a storm expected once in about 300 years, coastal experts said. In western Suffolk, the storm also was more severe than a 100-year storm.
Coastal specialist Jay Tanski of the New York State Sea Grant Extension Program said the map shows that "the farther west you went, especially on the South Shore, the worse it was in terms of the storm tide. It represented anywhere from over a 100-year event to a more than 300-year event."
Timothy Crowley, FEMA's regional director of mitigation, said the latest flooding impact map was derived from tide gauges in the bays and observations along the coast. In places where there are no observations, the mapmakers used nearby known points to estimate the extent of the flooding.
"It's something we normally do" after any major storm, Crowley said of the impact map. "It's fairly accurate now, but as we get more information, we're always updating."
The agency also will compare the Sandy map with its 2009 flood zone map to see whether that map needs to be tweaked.
If the South Shore were to experience a 100-year flood, "hopefully the maps would match up perfectly," Crowley said. "But you never get the perfect 100-year flood."
While FEMA officials declined to comment on the accuracy of the maps, comparisons of the 2009 map with the Sandy inundation map show they line up closely in many areas.
In formulating a flood map, FEMA looks at historical data, Crowley said, "but the maps are really statistical modeling of a variety of storms of a variety of levels," including future possible storms more severe than Sandy. So, he said, "one singular event normally does not change the flood maps significantly."
Westhampton Beach coastal geologist Aram Terchunian said the map "looks to be dead-on in the locations that I know."
Nassau County Executive Edward Mangano said "Sandy's devastation to residents and employers was beyond imaginable, and that is why Congress must pass the full recovery measures needed to help our neighbors rebuild."
Suffolk County Executive Steve Bellone said FEMA's numbers seem on par with damage reported to the county.
"However, assessments are still continuing and the number of damaged properties may increase," he said, adding that the county "will continue to advocate for hardening of the shoreline and dredging of the waterways for the protection of all Suffolk County residents."
With Sarah Crichton, Joe Ryan and Keith Herbert
Sandy impact analysis by FEMA
FEMA compiled these federal statistics in response to a Newsday request.
Total number of flooded, damaged or destroyed structures: Nassau: 74,736; Suffolk: 20,798
Structures that experienced only stormwater inundation: Nassau: 17,405; Suffolk: 5,942
Structures with damage totaling less than 50 percent of their value: Nassau: 27,178; Suffolk: 6,638
Structures with damage totaling more than 50 percent of their value: Nassau: 30,036; Suffolk: 8,153
Total debris (cubic yards): Nassau: 3,312,311.22; Suffolk: 1,166,067.35
Estimated structural debris (cubic yards): Nassau: 3,139,330.31; Suffolk: 828,521.34
Estimated tree debris (cubic yards): Nassau: 172,980.91; Suffolk: 337,546.01