NEW ORLEANS (Reuters) - Hurricane Isaac dumped heavy rains on the U.S. Gulf Coast and caused widespread flooding on Wednesday, but elaborate defenses built to protect New Orleans after the 2005 Hurricane Katrina disaster withstood the onslaught, officials said.
The massive cyclone, which weakened to a tropical storm on Wednesday afternoon as it moved slowly into Louisiana, pushed water over a levy on the outskirts of New Orleans and threatened to flood oil refineries and towns in the state and neighboring Mississippi.
But fears of a repeat of the catastrophe of Katrina that swamped large parts of New Orleans exactly seven years ago and killed 1,800 people, did not materialize and there were no immediate reports of injuries or deaths.
Police and National Guard units, many armed with assault rifles, patrolled the virtually empty downtown area of New Orleans, a port city which normally hums with tourists drawn to its jazz bars, Creole cuisine and French colonial architecture. They were deployed to prevent a repeat of 2005's widespread looting.
Authorities reported four arrests for looting on Wednesday. The mayor said he was clamping a dusk-to-dawn curfew on New Orleans nonetheless.
Storm surges climbed up to 12 feet and top sustained winds were up to 70 miles per hour (110 km per hour).
Highway 90, a key interstate roadway on Mississippi's Gulf Coast, was washed out by the storm, which triggered widespread power outages and was expected to bring rainfall accumulations totaling as much as 25 inches to some areas.
New Orleans Mayor Mitchell Landrieu told local radio the city's flood defenses, strengthened since 2005 with a $14.5 billion system of walls, floodgates, levees and pumps had done their job.
"The federal levee system ... is fine," he told local radio. "There are no risks. It is holding exactly as we expected it to and is performing exactly as it should."
Still, as torrential rains continued to hammer the city -- one location reported 17 inches -- Landrieu later cautioned that it was too soon to declare victory.
The storm taxed the city's sewage system, prompting Landrieu to urge residents to "keep the flushing to a minimum."
Tree limbs and street signs littered the streets, drainage canals filled with storm water and power was out in much of the city.
Isaac was wobbling northwestward near six mph, a slow pace that increases the threat of rain-induced flooding.
Outside the city, in low-lying Plaquemines Parish, which stretches southeast from New Orleans, emergency officials said floodwaters had flowed over an 8-foot (2.4-metre) high levee between the Braithwaite and White Ditch districts.
Parish President Billy Nungesser said about 2,000 residents had been ordered to evacuate but only about half were confirmed to have left before Isaac made landfall late on Tuesday.
About 118 people were rescued in Plaquemines, including 25 trapped on their roofs or attics as water rose 14 feet, authorities said.
"This storm has delivered more of a punch than people thought," Nungesser told CNN. "We're not out of the woods yet."
Plaquemines Parish is cut in two lengthwise by the Mississippi River as it flows to the Gulf of Mexico. Much of it lies outside the greater New Orleans levee system, and construction projects to bolster protection are not complete.
Private citizens in their boats led the rescue effort, Nungesser said, referring to boatmen from the Mississippi Delta and bayous popularly known as the "Cajun Navy."
Jesse Shaffer, a 25-year-old Braithwaite resident, told reporters he and his father, 53, personally rescued more than 20 people during several outings in their fishing boat.
Local television showed rescuers in a small boat chopping through the attic window of a house to pull a man and his four dogs to safety in the Braithwaite subdivision, a neighborhood of brick houses that were submerged up to the first floor.
In Belle Chase, in Plaquemines Parish, canals overflowed and threatened to swamp houses by midday on Wednesday, though several residents said they planned to ride the storm out even as forecasts threatened another 10 to 12 hours of rain.
Patty Mattison, 58, was watching floodwater rise in her yard, threatening to swamp her laundry room. "I'm looking at a little water in the back," Mattison said.
Crews were also rescuing residents after a levee breach in Madisonville in St. Tammany Parish, on the north bank of Lake Pontchartrain on the opposite side from New Orleans, a also outside the federally operated flood protection ring.
Rescue crews used boats and jet-skis to rescue up to 100 people in houses and condominiums there, Jindal said.
"This is a slow-moving storm and it is going to cause a tremendous amount of damage," Jindal warned. He said Louisiana could face 18-26 hours of tropical storm-force winds before Isaac leaves the state on Friday
"This is not over, and there is no such thing as just a tropical storm," said Craig Fugate, director of the Federal Emergency Management Agency and U.S. President Barack Obama's top advisor on disaster response. "You have significant weather impacts still to occur to the state."
Official estimated that Lake Pontchartrain had risen nine feet (three meters) since the start of the storm with peak expected at 6 p.m., according to WWL radio.
While not nearly as strong as Katrina - a Category 3 hurricane on the five-step Saffir-Simpson scale when it slammed into New Orleans on August 29, 2005 - authorities have warned repeatedly against underestimating Isaac.
Before moving to the Gulf Coast, Isaac killed at least 23 people and caused significant flooding and damage in Haiti and the Dominican Republic before skirting the southern tip of Florida on Sunday.
"The slow motion and large size of this system are making the impacts more severe and more wide ranging than some folks might have perceived would be the case from a Category 1 hurricane," said Rick Knabb, National Hurricane Center director.
"It's human nature to think that if I think back to my experience with some past hurricane, if this one's a lower category, then I'm going to be fine," Knabb said.
More than 700,000 Louisiana customers of Entergy Corp and other local utilities were without power as of midday. Entergy warned that it would be unable to begin restoring power until winds drop below 30 mph.
At 5 p.m. EDT (1900 GMT), Isaac was centered about 60 miles west of New Orleans, the NHC said.
Oil production in the U.S. Gulf of Mexico nearly ground to a halt as Isaac closed in on Louisiana on Tuesday and ports and coastal refineries curtailed operations.
But on Wednesday, big oil companies including Royal Dutch Shell and Exxon Mobil Corp began making plans to assess or restart operations.
Intense hurricanes such as Katrina -- which took out 4.5 million barrels per day of refining capacity at one point -- have flooded refineries, keeping them closed for extended periods and reducing fuel supplies.
Louisiana usually processes more than 3 million barrels per day of crude into products like gasoline.
Perceptions that the area's oil facilities would not sustain major damage left international benchmark Brent crude little changed in Wednesday afternoon trading at about $112.70 a barrel.
(Additional reporting by Ben Gruber and Kathy Finn in New Orleans, Emily Le Coz in Tupelo, Missisippi, Kristen Hays, Erwin Seba and Chris Baltimore in Houston and Jane Sutton and David Adams in Miami; Writing by Tom Brown and Anna Driver; Editing by David Brunnstrom)