Hurricane Isaac hovered over the Gulf Coast on Wednesday, punishing southeast Louisiana with 75 mile per hour wind gusts, driving, horizontal rain and the threat of calamitous flooding. Forecasters said the rainfall may not let up for days.

The hurricane, which made its second landfall early Wednesday, was moving very slowly, at about 6 miles per hour near Houma, La., according to the National Weather Service, bringing with it the heightened risk of tornadoes and flash flooding hundreds of miles inland from Louisiana, and across Mississippi and Alabama to Florida.

Hurricane Isaac’s maximum sustained wind speeds have slowed from 80 miles per hour during the morning and are expected to weaken further as the system moves inland.

New Orleans so far appears to have avoided major damage, but the city is littered with fallen trees and downed power lines, and few traffic lights are working. Some streets were under several feet of water as rain continued to fall. More than 600,000 residents of Louisiana were without power, nearly a third of them in New Orleans.

The longer the storm lingers, the more pressure it is putting on the levees and other flood-protection systems along the coast.

In Plaquemines Parish, about 95 miles from New Orleans and where the hurricane first made landfall, water “overtopped a levee, causing extensive flooding and stranding dozens of people, officials said.

The levee is not one of the large, federally maintained earthworks lining the Mississippi River, but a locally maintained levee some 8 feet high, and lower than the 12-foot surge that hit it, according to officials from the Army Corps of Engineers. The water is threatening people living along the east bank of the parish, near the mouth of the Mississippi River, who did not comply with the mandatory evacuation.

Hurricane Isaac is the first test of a $14.5 billion, 133-mile ring of levees, flood walls, gates and pumps installed after Hurricane Katrina by the Army Corps of Engineers, the agency that built the defenses that failed New Orleans catastrophically in 2005.

While the current storm is nowhere near as powerful as Hurricane Katrina which struck seven years ago Wednesday its pounding, driving rains and surging waves are lashing towns from east of Morgan City, La., to the Mississippi-Alabama border.

2012 The New York Times Company