Flood-estimate maps showed water reaching depths of up to 20 feet and pooling out into every part of the floodway within eight days.
A week and a half later, those maps appear to have been a bit pessimistic. The water has taken an unexpected trajectory as it moves out into the floodway, and some areas will probably be spared significant flooding. The flooding has also moved more slowly than anticipated, forcing local officials to recalibrate by the day.
Mandatory evacuation orders have been lifted and then reinstated in some areas, and lifted altogether in others. At least one shelter has closed for lack of evacuees.
The promising outlook does not mean that all is clear. The water is rising, if slowly, and in some places is spreading over roads and moving toward homes. Backwater flooding, which occurs when tributaries, bayous and small bodies of water overflow, is a concern throughout the basin, and there is still an enormous amount of water that must funnel out into the Gulf of Mexico, threatening Morgan City and the other communities at the basin’s spout.
A barge that was sunk in a bayou just outside of Morgan City in an effort to protect populated areas from an influx of backwater appears to be working for the most part, said Bill Pecoraro, a member of the Morgan City port commission.
But the crest of the flood is not expected to reach Morgan City until Monday, and water will stay high for some time. It remains to be seen how well the barge, a temporary fix, will hold up over several weeks, Mr. Pecoraro said.
He added, however, that it appeared the flooding would not be as significant as some had feared. “It looks like it’s going to be anticlimactic,” he said.
It is too early to tell with any certainty why the flood projections in south Louisiana have diverged with reality, officials said.
“There’s probably not going to be one definitive answer to that,” said Jeff Graschel, a hydrologist with the National Weather Service.
But the outlook was improving even before the spillway, about an hour north of Baton Rouge, was put into use. While officials with the United States Army Corps of Engineers at one point anticipated having to send water out through the spillway at 300,000 cubic feet per second, or half its capacity, that projection was cut in half by the time the first gate on the spillway was opened.
The water is currently gushing out at 114,000 cubic feet per second, less than a quarter of capacity, and corps officials do not expect to use any more gates than the 17 that are now open. They might start closing gates in as soon as a week, in the same gradual manner as they were opened.
The floodwaters were originally expected to fill up the Atchafalaya River Basin within several days, curling around levees and threatening towns like Krotz Springs and Melville that lie in the western part of the basin.
But, Charles Shadie, the head of water control for the Mississippi Valley Division of the corps, said “the flows tend to be hugging the eastern side of the floodway.”
St. Landry Parish, which sits due west of Morganza, rescinded a mandatory evacuation order for certain areas on Monday, though a voluntary order was still in effect. With all these changing orders, residents are not entirely sure what to do.
“We’re frustrated, like many of the citizens here, with not being able to get a definite timeline,” said Lisa Vidrine, the director of St. Landry Parish’s Department of Emergency Preparedness.
Mr. Graschel explained that the flooding models were based on the last time — the only time — the Morganza spillway was opened, in 1973. There are a number of reasons the comparison is not perfect, he said.
In 1973, the floodway had already been pounded by heavy rainfall when the spillway was opened. As a result, the Atchafalaya River was running high, soils were saturated and pools and other potential water storage areas were full. The spillway itself was opened much more quickly in 1973, as opposed to the gradual opening that took place this month.
“It’s kind of comparing apples to oranges,” Mr. Graschel said.
Residents in the floodway have been trying to adjust.
“It’s sort of yo-yo living,” said Charlene Guidry, 57, who has left her riverfront house in Butte La Rose. A mandatory evacuation order was declared for Butte La Rose last week, then was lifted, then went into effect again on Tuesday at noon.
“It gets a little frustrating,” Ms. Guidry said. But, she added, better to leave and have nothing happen than to stay and experience the worst.
“When you look at people in Joplin, Mo., and Tuscaloosa, Ala., we’ve got nothing to complain about,” she said.