Rising Tide: The Great Mississippi Flood of 1927 and How It Changed America by John Barry might provide some interesting information
I suppose the worst case scenario would be a repeat of the flood of 1927, which reached right up to Lafayette, but didn't flood it. Almost all of St. Martin Parish was underwater. Of course Lafayette is much bigger now, so there may be some lower lying areas, or areas around the Vermillion that might be flooded.
I'm pretty sure there would have to be some massive levee failures for this to happen.
The green is what flooded in 1927.
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No.The worst case is getting to the base of the coteau ridge. The coteau ridge is very prominent in the area if you know where to look for it. Starting in the south you can see it at Spanish lake north of new Iberia. Traveling east from Broussard to St. Martinville you will go down a hill about a mile east of 90. Highway 90 at the vermilion river is a cut in the ridge, as is Surrey street. If you have ever been to Oakbourne country club you are traveling on the edge of the ridge as you enter the gate. The new UL Golf practice facility is built on the edge of the top of the ridge. Traveling on the old breaux bridge highway you go down a fairly large hill right before the turn off on the Cypress Island road. This is the historic location of the edge of the 1927 flood. My father and uncles volunteered to help rescue people in Breaux Bridge during that flood. The water wasn't too deep as they used national guard trucks to evacuate people. As you travel east on I 10 you will go down a very pronounced hill and traverse a cut in the ridge just east of the Moss street over pass. If you travel east on Pont Des Mouton, Butcher Switch and Gloria Switch from Moss street you will travel down a very pronounced hill. But the most classic example is the over look you get on I-49 near Doctors Hospital in Opelousas. Look to the east and you will see what was the edge of the Mississippi river over 10,000 years ago. There are cattle grazing on that property most of the time. Highway 190 just east of Opelousas going to Port Barre is another pronounced cut. I 49 north of 190 heading to Washington is very hilly and that is the Coteau Ridge.
Once you get north of Washington on I 49 you will get to and travel through the Pleistocene era Mississippi flood plain. There is a reason why I-49 is built up on about a 20 foot embankment. The fill for the embankment was obtained from the lake that was created at the rest area on the east side of I-49 about 4 miles north of La Highway 10 exit. This is called Dubuisson Lake and was a 640 acre lake purchased from Nick Dubuisson in the late 1970's for the borrow pit. The coteau ridge veers to the north and west north of Washington.
On a side note. The Contractor that did the embankment work on the Interstate was from either Minnesota or Wisconsin so they didn't know too much about floods. They were leaving one weekend in the spring for a few days off. An Old guy that lived on the access road to the pit told the guys "you better get you machines out of dat pit". They asked him why. He said "De flood is coming". They said "what flood" The old guy said "it's going to be here in about 2 to 3 days." They thought he was crazy. When they came back about four or five days later only the top parts of the booms of the drag lines were sticking above the water and every other piece of equipment was under water. It took them 6 MONTHS after the flood receded to pump the water out of the pit.
While that may be true to a degree. It was found after the Kat-Rita event of the fall of 2005 that the entire portion of South Louisiana from Mississippi to the Texas border and beyond was in a generalized subsidence area. From Lafayette west subsidence varied from 0.5' to 1.5 feet. Subsidence was most pronounced in areas where heavy ground water pumping was taking place by rice farmers. Lafayette area was 0.5 feet to 0.8'. To the east of Lafayette through the basin and over to New Orleans it was more pronounced as the geologic age of the land was very small and was never really stabilized prior to human habitation and development. Subsidence is more than 2.0' in many places. That was one of the reasons why FEMA and the USC&GS decided to void all published elevations for bench marks and require all elevation work with respect flood zone work and Flood elevation certificates to be perform using GPS observations. I know that for a fact because I had to purchase over $20,000.00 in equipment to perform that work. All flood certificates and flood studies to this date are done using GPS observation using the latest GEOID.
So even though the levees are blocking natural soil deposition, the main culprit is subsidence in the eastern portion of the state. That along with storm surge that basically washes away the barrier islands makes it a multi faceted problem that in many ways cannot be solved because it is very difficult to fight subsidence.
Last I heard subsidence was at the rate of about 0.5 to 1.5 centimeters per year. Our GPS equipment is sub centimeter accuracy.
This publication by the USGS might be helpful
https://pubs.er.usgs.gov/publication/70028078
As usual, great info Helmut. Thanks.
Your memory is correct. I think everybody along the Mississippi and Atchafalaya river needs to be aware of this situation and have plans in place now. They are predicting four to five days of rain in the upper Ohio River basin starting this weekend. All of that will run off almost immediately due to saturated soils in the area.
That is on top of the major flooding they are already dealing with. A lot of that flooding is due to them not letting more water flow downriver due to the high river stage we are currently at.
This will be an interesting next few months. It may be ok in the end, but it is definitely lining up to be a catostrophic spring for the Lower Mississippi River.
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