Water driven by the large and powerful storm flooded over an 18-mile stretch of one levee in Plaquemines Parish south of New Orleans, flooding some homes in a thinly populated area.
The levee, one of many across the low-lying coastal zone, is not part of New Orleans' defences. Officials say the levee had not broken.
As the rain continued to fall and winds pushed across the Gulf Coast, it remained far too soon to determine the full extent of the damage.
Parish authorities believe some people may be trapped, but were not sure how many may have remained despite an earlier evacuation.
Rescuers were waiting for the strong winds to die down later in the day before moving out to search.
"We did have two parish police officers that were stuck in a car there. We just found out they were rescued and are safe," said emergency management spokeswoman Caitlin Campbell.
She said officials expected the water to recede to the Gulf as wind direction changes with the storm's movement.
The Canadian Press