So Aunt B has
thrown down the gauntlet to all her engineer friends and asked:
Could New Orleans have a safer life, granted, as a smaller city, if the main
branch of the Mississippi didn’t run through it?
First off...... I haven't heard that. And I doubt it's as clear as your average news article claims that it is.
I'm not that familiar with NO or the Atchafalaya River, but I'll take a stab at it based on what I've seen on the news, read in the trade journals, and my general knowledge. And we'll leave aside the enormous political and social impacts of blowing out that many towns and drying up the lifeblood of NO.
To really answer the question, you have to know a little about the reasons New Orleans floods. The most basic fact....... NO is below sea level. So the only thing keeping the water out is the levees. If you look at the FEMA floodmaps for the area, nearly the entire city is in the 100 year flood plain. It has quite a few flooding sources. The Mississippi runs through the center of the city, and Lake Pontchatrain is immediately to the north. There are lowland swamps all around. So there's really no where else for the water to go once it gets to NO.
The main cause of flooding during Katrina, and any hurricane of size, was storm surge. Simply put, that's a giant wind induced wave. The wind blows across flat surface of the ocean and pushes the water ahead of it. Sometimes it works with the tidal effect. So the wind was working on all that open water around NO and pushing the water right over the top of the levees. It probably blew some waves from the Gulf straight up the
MRGO and the river, and even off the lake. There's some noise in environmental circles that the swampland between NO and the Gulf has been eroding away for many years. That means the ocean creeps closer to the city, and removes some of the buffer area that would absorb the storm surge before it gets to the city.
Removing the river itself from the situation would take away one flood source, but you still have plenty of other water around the city. It would lessen the chance of flooding caused by a hard rain or a wet spring in the Midwest (which drains down through the Mississippi), but wouldn't appreciably change the flooding potential during a hurricane. And hurricanes are the most dangerous form of flooding in a coastal city.
The second question Aunt B
asks:But I’m just curious as to why we don’t let the river do what rivers do, especially since artificially keeping it from doing that seems to be exacerbating a problem.
True enough. Trying to keep the river in place does probably exacerbate the problem at NO, but we've been doing it too long now to quit.
It's important to note, there's always going to be a Mississippi River flowing through NO. Even if the river diverts into the Atchafalaya there's still going to be water in it at NO. The only Atchafalaya River I found was waaaay up above the state line into Mississippi, and lots more water enters the river downstream of that, so it will still be flowing in the riverbed. Plus, there's almost no chance that the entire Mississippi would divert. A large portion of it might, but there will still be plenty going down the old river channel. Letting it flow down the Atchafalaya would relieve the river levees at NO (but not Lake Pontchatrain), but it will still be there.
I think the primary reason you can't just let it blast down the Atchafalaya is all the new damage it would do. The Mississippi River drains the entire interior of the state from the Appalachians up into the Pacific Northwest. It's huge. I'm not familiar with the Atchafalaya, but I know it doesn't approach the size of the Mississippi. So if the Mississippi were allowed to flow unchecked down the Atchafalaya it would have to blast a much bigger channel. Think along the lines of a hose (one of those that takes two guys to hold it) being directed into a kid's sand box.
The environmental impacts would be astronomical. It would wash a huge amount of dirt out into the bay killing all kinds of plant and animal life around the estuary, and scour the banks clean for the entire length. You'd have monster trees washing out and floating down the river. Probably clogging the channel and causing the river to divert even more. The lower water levels in the Mississippi would drain wetlands throughout lower Louisianna. There's no doubt that sort of catastrophic adjustment has happened in the past. Mother Nature is pretty good at adjusting, but it takes a long time.
Rivers are tricksy. Once they get out of control, they want to stay out of control and you can't be sure what it will want to do. Reelfoot Lake was caused by an earthquake changing the path of the Mississippi River flooding hundreds of acres, and it's much smaller at Reelfoot than it is down at NO. It's like letting Mrs. Wigglebottom off her leash. Sure she'll probably go eat cat poo and come back to you, but she may decide to run off to the park on her own instead.
We haven't even mentioned the economic losses. A lot of cargo ships into NO and then up the Mississippi. The Corps of Engineers spends a lot of money dredging the river, but it's not as much money as they'd have to spend starting over in some new riverbed. And it's not as much as it would cost to truck all that cargo into the interior parts of the country.
Those are my off the cuff thoughts on it. The answer to B's question is, controlling it is better than letting it run loose. Better the devil you know.
