LAPLACE, La. — At the urging of residents who have long felt forgotten in the shadow of more densely populated New Orleans, the Army Corps of Engineers says it will look into whether the city’s fortified defenses pushed floodwaters into outlying areas.
However, the Corps has said it’s unlikely scientific analysis will confirm that theory suggested not only by locals, but by some of the state’s most powerful politicians. Instead, weather specialists say a unique set of circumstances about the storm — not the floodwalls surrounding the New Orleans metro area — had more to do with flooding neighborhoods that in recent years have never been under water because of storm surge.
Isaac was a large, slow-moving storm that wobbled across the state’s coast for about 2½ days, pumping water into back bays and lakes and leaving thousands of residents under water outside the massive levee system protecting metropolitan New Orleans. The storm was blamed for seven deaths and damaged thousands of homes on the Gulf Coast.
The Corps’ study was prompted by the suggestion that Isaac’s surge bounced off the levees and floodgates built since Hurricane Katrina in 2005 and walloped communities outside the city’s ramparts.
Blaming the Army Corps of Engineers is nothing new in southern Louisiana, a region that is both dependent on the Corps and distrustful of an agency that wields immense power in this world of wetlands, rivers, and lakes, all of which fall under the agency’s jurisdiction.
The Corps was roundly criticized after Hurricane Katrina, which pushed in enough water to break through the levees that had surrounded New Orleans. Much of the city was left underwater, and since then the government has spent millions rebuilding the system of floodwalls protecting the metro area.
Before that, the Corps was blamed for the unraveling of coastal marshes by erecting levees on the Mississippi River.
In towns like the bedroom community of LaPlace, people want answers. There, neighborhoods were under water even though they had never before flooded because of storm surge.
‘‘It has a lot of us questioning,’’ said Ed Powell, a 47-year-old airport emergency worker who has lived in LaPlace for 15 years and had never seen flooding on his street until Isaac hit.
On Friday, Republican Senator David Vitter asked the Corps to commission an independent study to determine whether the new floodwalls, gates, and higher levees around greater New Orleans caused water to stack up elsewhere.
The Corps is expected to complete its study within two months, said Senator Mary Landrieu, a Democrat, who joined Vitter in seeking the study. The Corps said its researchers will conduct the study and it will be peer-reviewed.
The Corps said it expects the study will find ‘‘minimal’’ changes in surge elevation because of its work around New Orleans. It based that assessment on previous modeling. The agency said it would not comment further until the scientific work is done.
Isaac came ashore as a Category 1 storm, but that classification is based on wind speed, not surge predictions. In the past, much stronger storms have produced much smaller surge levels. Isaac had a broad wind field — at times, more than 200 miles from its center — that made it capable of scooping up a lot of water, said James Franklin, the chief of hurricane operations at the National Hurricane Center in Miami.
Really strong hurricanes can sometimes produce small surge levels while weaker hurricanes can kick up massive surge. For example, in 2004, Hurricane Charley hit Florida as a compact Category 4 storm and produced a surge of only about 7 feet. By comparison, Isaac created storm surge of more than 6 feet at Lake Pontchartrain, according to US Geological Survey sensors. It reached about 12 feet near Braithwaite, a community flooded to its rooftops in Plaquemines Parish.
Furthermore, the storm’s size, slow speed, and the way it angled into the state ‘‘worked together to produce incredibly high surge,’’ said Jamie Rhome, a surge specialist with the National Hurricane Center.
Other scientists agreed it was unlikely the fortified defenses caused flooding in neighboring communities. Instead, numerous factors combined to create the flooding conditions. For instance, the storm was virtually stationary for a time and dumped rain far longer than many other tropical systems.