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Miami-Dade expands hurricane evacuation zones

When hurricane season opens in less than a month, the number of Miami-Dade County residents living in evacuation zones will triple to nearly 1.8 million.

The county’s new evacuation maps were developed with more accurate surveys and more sophisticated computer models that revealed vast new swaths potentially at risk from storm surge — the hurricane-driven flooding that caused much of the record $75 billion in damage inflicted last year on the northeastern U.S. by Superstorm Sandy. The result for Miami-Dade: Dramatically expanded “storm surge planning zones” covering nearly three-quarters of the county’s population.

Curt Sommerhoff, director of Miami-Dade’s Office of Emergency Management, stressed that no single hurricane, not even the hypothetical “big one,” would force a mass exodus of everyone in every zone.

“No one storm can impact 1.8 million people,” Sommerhoff said Friday as he unveiled the new maps at the county’s emergency operations center in Doral. “But what people should understand is that they need to plan for the possibility that there is at least one storm in all of those thousands of storm scenarios out there that could put that kind of water over land where they live.”

That includes inland locations once considered safe havens for residents seeking shelter from the most vulnerable locations in the county: barrier islands and heavily populated coastal communities.

Almost the entire county south of Tamiami Trail — from Coral Gables to Country Walk and even the Redland west of Krome Avenue — now falls in one of five surge zones, labeled A to E. North of the Trail, chunks of Hialeah, Miami Springs, North Miami and North Miami Beach and other cities are now also in surge zones.

The new analysis also altered the highest risk coastal areas, A and B, which typically endure the strongest winds and surge that could possibly range to 20 feet in the most powerful major storms. Heavily populated Miami Beach was bumped down from the highest-risk A zone, the first areas that emergency managers order evacuated. Much of the low-lying coast of Biscayne Bay was bumped up into Zone A.

Sommerhoff stressed that the huge new inland zones, labeled D and E, aren’t likely to see flooding or face evacuations in most hurricanes, but the threat would rise with major storms. Under at least some of nearly 30,000 scenarios, using storms of every strength, size and path, those new zones could be swamped by up to 18 inches of surge — the measuring stick used to draw the lines on the map.

The risk isn’t from the ocean sweeping across suburban streets inland but instead from it pushing up canals and rivers like the Miami River, he said, spilling over and potentially swamping streets. Some areas are more at risk from hurricanes crossing the state from the Gulf, others by veering in from southern Biscayne Bay.

Lt. James Brinkley, a storm-surge specialist at the National Hurricane Center, which is now in an evacuation zone on the main campus of Florida International University, said forecasters have improved at forecasting surge, which — as Sandy showed last year — typically has the deadliest and most damaging impacts.

“I can’t tell you that at the corner of first and eighth streets, you’re going to see this much storm surge, but I can tell you a specific region can expect to see a range of surge,” he said.

Surge impacts aren’t necessarily tied to the power of a storm. Category 5 Andrew, for instance, was a small, fast-moving storm with surge that peaked at 16.5 feet near the Deering Estate but affected a relatively narrow section of the South Miami-Dade coast. Sandy dropped near Cat 1 strength when it neared the Northeast but the massive, slow-moving storm drove the sea deep inland.

“It depends on a whole bunch of variables,’’ Brinkley said. “The size of the storm, the forward speed, the angle of approach to land, the timing with high tide and low tide, all of these factors come into play and can have big implications.’’

Though the evacuation zones have expanded, Sommerhoff believes that improved NHC predictions will help reduce unnecessary evacuations by helping the county pinpoint only the pockets most likely threatened. Depending on the storm, only small sections of inland zones might be evacuated, he said, with the county issuing the street boundaries of the areas considered at risk.

In the wake of the brutal 2004 and 2005 hurricane seasons, Florida lawmakers tapped $29 million in federal funding to pay for a statewide evacuation study that included aerial surveys using laser technology that provides the most precise measures of elevation. Those maps, and NHC surge models, were used to develop the new zones for Miami-Dade, which had not updated its map in about 10 years.

Miami-Dade worked with other agencies, including the South Florida Water Management District, which runs the regional flood-control system, to develop a final map, Sommerhoff said. He said he wasn’t sure if other counties would do similar updates but said he believed Broward had examined its maps and did not plan changes. Broward emergency managers and the Florida Division of Emergency Management did not return calls.

The surge zones don’t affect maps that the Federal Emergency Management Agency use to set national flood insurance policy rates.

Though Miami-Dade began working on the update early last year, Hurricane Sandy proved a powerful reminder that water, rather than wind, is the biggest threat.

Sommerhoff recalls watching one woman in New Jersey on TV wondering how so much water was in her home when she lived two miles from the beach.

“Here,’’ he said, “it could be 10 miles from the coast.’’