Just checked. Looks like opening the Morganza could flood an area 20 miles wide by 100 miles long, along the Atchafalaya River. Mostly rural I think. But not without consequences.
There are a few small towns and cities in the area. The difference is that they are working class and will do what needs to be done unlike the NOLA city dwellers.
It’s kind of a double edged sword in that the flooding will be bad for personal property but it will also be beneficial in replenishing the wetlands and fertile farming regions. I’m just wondering when the Mighty Mississippi decides it’s had enough of man trying to tell it where to go and changes course again.
http://www.2theadvocate.com/news/Corps-Up-to-25-feet-of-water-could-spill.html
Click on the map on the left side of this article. It looks like a parish wide swath, 7 parishes long will be flooded to at least 5 feet. IMO the River is changing course.