Last year I drew a comic about the oil spill in which Michael Bay spun an over-the-top worst-case disaster scenario. One of the panels was actually slightly more plausible than the others. It was based on a real disaster which almost happened in 1973, and in two weeks it may come closer to happening than ever before.
I learned about this from John McPhee’s The Control of Nature (adapted from this article), a book that my mom gave me as a kid (Happy Mother’s Day!). I’m not any sort of an expert on the subject, but here’s what I’ve learned so far:
Every thousand years or so, the lower Mississippi changes course. It piles up enough silt at its delta that it ‘spills over’ to a new shortest path to the ocean. At times, the outlet has been anywhere from Texas to the Florida Panhandle.
Since the early 20th century, the Mississippi has been trying to change course again—sending its main flow down the Atchafalaya river, which offers a much shorter, steeper path to the ocean. The Army Corps of Engineers was ordered by Congress to keep that from happening. The center of their effort is the Old River Control Structure, which limits the flow down the Atchafalaya to 30%.
Every now and then there’s a massive flood which stresses the system. The fear is that if the Mississippi ever broke through the ORCS and the main flow was captured by the Atchafalaya, it would be very hard or virtually impossible to return it to its old route. This would devastate the people and industries around in Baton Rouge and New Orleans who depend on the river (as if they haven’t had enough problems lately). This almost happened in 1973, when a massive flood undermined the structure; this was the subject of John McPhee’s book.
They’ve since strengthened the structure, but the coming flood is quite a bit larger than the one in 1973. In order to save New Orleans and Baton Rouge, they have to send some of the floodwaters down the Atchafalaya.
Here is the working plan for routing the water from a nightmare flood:
The Mississippi River Commission document outlining the plan is here.
This plan, put together after the devastating 1927 floods, is based around the estimate of the largest possible flood the Mississippi could ever experience. In theory, the system is capable of handling such a flood, although much of it has never been put to the test.
The current flood moving down the Mississippi is going to stress this system to near its limit. Here’s a version of that map with the current flow rates, with the approximate expected coming flood shown at the top:
This is based on the diagram at the ACOE Mississippi River page, which is updated daily with new flow rates.
The floods above the system are expected to crest 6′ higher than in the 1927 flood, the highest in recorded history, and 7′ higher than the 1973 flood that almost destroyed the ORCS. Here’s the gauge just above the structure as of noon on May 8th:
The current Natchez gauge can be seen here.
The Morganza spillway has only been opened once (to take the stress off the failing ORCS in 1973), and then only partly. It’s fairly clear at this point that the Morganza spillway and the Bonnet Carré spillway will both be fully opened to route the flow away from New Orleans (which is expected to crest just a few feet below the tops of the levees there).
I have no idea how likely the Old River Control and Morganza structures are to fail, or whether a rerouting of the Misssissippi through a new channel would be irreversible. You can read some speculation on this here.
Wunderground blogger Barefootontherocks maintains a page full of resources on the current Mississippi flood, and there’s a lot of information in the comments. The excellent Jeff Masters will probably have a post on the subject in the next few days. You can see more gauges and a ton of information at the NWS page on the lower Mississippi.
Michael Bay can be reached here.