Isaac Drenches Gulf Coast and High Water Cuts Off Many.

NEW ORLEANS — Seven years to the day that Hurricane Katrina and levee failures unleashed a deluge of devastation on the Gulf Coast, Hurricane Isaac brought its own distinctive mode of destruction on Wednesday, drenching the coast not with a quick blow but with an unremitting smothering.

It pummeled the Mississippi Coast with relentless roundhouse jabs, while pinning southern Louisiana under a saturating rainfall. On its crawl up from the coast, Isaac dumped more than a foot of rain in some places and shoved before it a violent storm surge that would soon bring back the terrible old images of 2005: people marooned on rooftops, rescue workers breaking into attics with axes and the rescued clutching what little they had left.

The worst-hit part of the coast was Plaquemines Parish, La., the finger of land that follows the Mississippi River from Orleans Parish out into the Gulf of Mexico, and the place where both Isaac and Katrina first made landfall.

Fears that a locally built gulf-side levee would be overtopped by Isaac’s massive surge were well founded. Many of those on Plaquemines Parish’s east bank who ignored Monday’s order to leave were forced into their attics when the gulf poured in, filling up the bowl between the levees with up to 14 feet of water.

Dozens of people had to be pulled to safety by rescue workers and neighbors. As of Wednesday evening, water was beginning to creep up the west bank of the parish as well, prompting officials to go door to door to evacuate what is effectively the bottom two-thirds of the parish.

“We’ve never seen anything like this, not even Katrina,” said a visibly rattled Billy Nungesser, the parish president, in a briefing to reporters.

The same theme was repeated everywhere, by Kim Duplantier, a school principal whose home in Plaquemines had survived multiple hurricanes but was filled to ruin with water on Wednesday; by the mayor of Grand Isle, La., a coastal community that was flooded and cut off from the mainland; and by A. J. Holloway, the mayor of Biloxi, Miss., who now wishes he had ordered people to leave.

The skepticism with which Gulf Coast residents, including Mr. Holloway, viewed Isaac — which was downgraded from a Category 1 hurricane to a tropical storm by midafternoon on Wednesday — proved treacherous.

“I really didn’t anticipate this,” said Mr. Holloway, as he wheeled his sport utility vehicle to the edge of Highway 90, a cozy coastal road usually filled with carloads of beachgoers and casinogoers but now a steadily swelling river. “There’s a lot more water than I would have thought.”

In New Orleans, the decision by most residents to stay did not turn out to be disastrous. Trees were down across the city, and streets flooded, and three-quarters of the city was without power, as it will be for several days for more than 600,000 across the state, until the wind dies down enough for utility workers to come in. But despite a few nervous moments, the city’s all but finished $14.5 billion flood protection system seems to have worked.

Outside the city, severe flooding was widespread as Isaac sat defiantly on the coast. The National Hurricane Center expected the storm to drop up to 25 inches of rain in some areas. Officials said Wednesday night that they were working to evacuate up to 3,000 people from floodwaters in St. John the Baptist Parish, about 30 miles west of New Orleans. Tornado warnings were also in effect in several Mississippi counties.

Gov. Bobby Jindal of Louisiana said Wednesday that more than 4,000 people were in shelters across the state, and that 5,000 members of the National Guard had been deployed to help in response efforts. What is perhaps most remarkable about the storm is that there are still no reported fatalities in the United States, especially considering the degree to which it caught gulf residents by surprise.

“Initially, the storm only being a tropical storm instead of a hurricane, many people, especially the people who live down there, didn’t have a whole lot of concern,” said Deano Bonano, an aide to a parish councilman, referring to the town of Lafitte outside the levee. By Wednesday afternoon, the bayou that splits the town was rising so rapidly that scores, if not hundreds, of people were facing potentially days of being cut off from the world.

“I think everyone was surprised by this,” said Denny Mecham, the executive director of the new Ohr-O’Keefe Museum of Art in Biloxi, which was inches from taking in water. “They try to prepare you, but for people who are used to a Cat. 3 or Cat. 5, this doesn’t seem like much,” she said. “Everyone was saying, ‘We’ll be open by Thursday morning.’ Well, this is not how this one is turning out.”

The same calculus was relied upon in Plaquemines (pronounced PLAK-uh-men) Parish, whose residents are almost by definition hardy and self-reliant. Shrimpers, oystermen, ranchers and workers in the oil patch live together on this stretch of coastline divided by the Mississippi River nearly from head to foot, and they have been through it all: multiple hurricanes, the worst of the BP oil spill and a preference for occupations that are not generally associated with comfort and security. The parish was largely walled out of the federal levee system, much to the anger of the residents. They know what that means.

“We knew it was a matter of time,” said Ms. Duplantier, 44, who moved with her husband to the now-submerged community of Braithwaite so they would have space to keep their horses, pigs, dogs, goats and cats. “We just figured we’d ride it out and see how long it would last. But we did not think in our wildest dreams that a Cat. 1 would do this.”

She evacuated, and her husband and her parents stayed behind to look after the animals. But they spent much of Wednesday watching the water rise, and were reached by boat after eight hours of being stranded. They had figured, she said, that “this was not the big one.”

Until it was. Solutions to getting the water out of the east bank of Plaquemines, which could take days to drain, are not straightforward. The Army Corps of Engineers is rounding up portable pumps from Baton Rouge and elsewhere that can pump floodwaters into the Mississippi, but such pumps are slow.

Mr. Nungesser, with the support of Mr. Jindal, said that the plan was to punch holes in the gulf levee to speed up the draining, as they did after Hurricane Gustav in 2008, and that a team could begin doing that as early as Thursday afternoon.

And still Isaac trudged on, drenching the towns of the north bank of Lake Pontchartrain on Wednesday night and heading at an agonizing 6 miles per hour in the direction of Baton Rouge. Officials warned that the risks were far from over, as flooding was a threat not only along the coast but in mid-Louisiana, upstate Mississippi and the drought-starved regions north. On Wednesday afternoon, Isaac was flooding towns farther inland with its unceasing rain, and was far from finished with southern Louisiana and the Gulf Coast.

“There is another half of the storm to go for most people who have already begun to experience it,” W. Craig Fugate, the administrator of the Federal Emergency Management Agency, said on a conference call with reporters. “For some folks in the path, the event and the weather haven’t even begun. We are still way early before this is all over.”