Officials search streets for residents who remained after evacuation order as storm pushes on towards New Orleans

Low-lying areas in south-east Louisiana faced serious flooding on Wednesday as Hurricane Isaac began to slowly move inland and towards New Orleans.

After making landfall on Tuesday night in extreme south-eastern Louisiana, Isaac remained stationary for several hours, unleashing 80mph winds on a sparsely populated neck of land that stretches into the Gulf of Mexico. Early on Wednesday it started moving towards New Orleans at around eight miles an hour – exactly seven years after the city was devastated by Hurricane Katrina.

Floodwater has already lapped over an 18-mile (29km) stretch of the levee along the Mississippi river in Plaquemines Parish, south-east of the city, threatening serious flooding.

There were reports of four-to-nine feet of water in the streets, houses flooded and perilous attempts to flee. Sheriffs' deputies made house-to-house visits looking for residents who had remained after an evacuation order.

"Say prayers: Plaquemines Parish has decided to attempt a roof rescue despite conditions that should prohibit it. #ToughChoices," tweeted local reporter Jen Hales.

Billy Nungesser, the president of the coastal Plaquemines Parish, said the 3-metre levees had not yet collapsed, but warned that roads in the area were already impassable, hindering the evacuation of the few remaining residents.

"As that water flows over the top, it eventually will eat out portions of that levee, which then it washes away. Either that or the inside of the levee will fill up. One or the other will happen. Either way that area's going to be totally inundated with water," he told CNN.

Gene Oddo, trapped by floodwaters in his attic with his family in Braithwaite, part of Plaquemines Parish, said the water appeared to have stopped rising. "I'm not scared. I ain't scared of anything. It looks like it's stabilising," he told local TV channel 4WWL. Oddo said at least three neighbouring families were also trapped. Weather experts said the storm was slowly moving westwards from Plaquemines.

Ryan Bernie, a spokesman for the city of New Orleans, said the storm had caused only minor street flooding before dawn and felled trees but had left roughly 125,000 people without power.

The National Hurricane Center had warned in its 5pm ET update that Isaac was getting better organised as it neared south-east Louisiana. The hurricane had picked up force, with maximum sustained winds measured at 80mph.

President Barack Obama signed emergency declarations for Louisiana and parts of Mississippi and warned against overconfidence. "We're dealing with a big storm and there could be significant flooding and other damage across a large area. Now is not the time to tempt fate. Now is not the time to dismiss warnings," he said.

Speaking in Mississippi, Craig Fugate, head of the Federal Emergency Management Agency, warned the storm was moving slowly and that heavy rains could cause inland flooding. "This is not just a storm for the coastal counties," Fugate said.

The governor of Louisiana, Bobby Jindal, said the storm's slow speed of 10mph meant some areas of New Orleans would experience six to 10 hours of hurricane-force winds and that storm surges could top 10ft. He said 41 parishes had declared emergencies, and seven were at least partially evacuated.

Seven years almost to the day after Katrina hit, New Orleans was hoping that $14.5bn-worth of civil works – a 133-mile chain of levees, flood walls, gates and pumps – would avert another catastrophe.

Mitch Landrieu, the mayor of New Orleans, did not order an evacuation of the city, but those living outside the levee system were strongly encouraged to leave. "We don't expect a Katrina-like event, but remember there are things about a category one storm that can kill you," Landrieu said.

In the lower ninth ward, a sprawl of largely ramshackle one- and two-storey homes, which were hit hard by Katrina, appeared abandoned at first glance. St Claude Avenue, the main drag, was empty.

But as the sky turned an ominous metallic grey, you could find those who had stayed behind. "You need money to leave," said Scott McMorris, 47, a mechanic. "A lot of people can't afford to run. You either pay your bills or you run. Can't do both."

Further up the street, bunkered in his home, was Gregory Richardson, 56, a retired oil rig worker with rheumatism. He had stocked up on water, ice, batteries and tinned food. He would have left, he said, but didn't have the cash. "My social welfare comes on the third of the month. If it came this week I wouldn't be here talking to you now."

He narrowly survived Katrina in 2005, which killed 1,800 people after the levees broke in the storm surges that followed the hurricane. "It was madness. I was rescued after four days by the army." He prayed Isaac would be more merciful. As he spoke the wind opened and slammed shut his front door.

Mandatory evacuations were ordered for St Charles Parish and parts of Jefferson Parish and Plaquemines Parish: low-lying regions outside the city's defences.

About 7,000 Plaquemines residents out of the population of 23,000 were ordered to evacuate, and many of those who could not or would not leave town were holed up in a well-preserved old hall with gold-painted chandeliers and ceiling fans. Folding beds were arranged in neat rows and seats faced a flat-screen television and a kitchen; naval officers and police looked on

"This building holds 80, next building 200, the backup location which we've overflowed to we have a capacity of 200. We're already pretty full. We're at 340," Gina Meyer, the superintendent of emergency medical services, said on Tuesday.

One resident, 21-year-old Joseph Buras, was sanguine about the risks. "The worst it's probably going to get is 90mph winds, so people aren't in so much of a panic as they normally would. Then again the fear of Katrina is still in a lot of people's hearts. So when they heard a hurricane was coming they just hauled it," he said. "We've survived the worst, this isn't the worst."

Back in the lower ninth, others stayed because they trusted the repaired and fortified levees would withstand whatever Isaac threw at them. The bolder ones intended to enjoy nature's show.

Darrel Walters, 48, and Kerwin Brown, 50, watched an exodus of neighbours with nonchalance, perching on chairs in their yard, bare-chested, smoking and sipping Budweiser. "We got some medicine right here," said Walters, tapping his beer.