Saturday, May 7, 2011

"Old Man River . . ."

The mighty Mississippi is up to its high jinks again. Daddy used to say, "There is no problem in the lower Mississippi valley unless the Missouri rises early and the Ohio rises late." And that seems to be the problem.

The Mississippi south of St Louis is flooding. Already the US Army Corps of Engineers have blown the levee to protect Cairo, IL. Cairo is important because it is the confluence of both the Mississippi and the Ohio rivers. If they had not relieved the pressure from the upper Mississippi, Cairo would have flooded and quite possibly been washed away. I have cross over the Mississippi from the Illinios into Missouri from Cairo on my way home to New Roads from Chanute Air Force Base, IL. When we went to Chanute we crossed over the Ohio from Paducah, KY into Cairo. So I have seen it from both directions so to speak.

The lower Mississippi is constrained at Memphis, Vicksburg, Natchez and Baton Rouge south to the Gulf of Mexico. Between those cities are places along the river that can get up to 10 miles wide and thus act as an accumulator slowing down the flow of the river. But the river really rushes though those locations. The accumulators or balloons help slow down the rate of flow, dampen things so to speak but they do not retain water for long and it all drains back into the river.

When the river rises like it is now, it backs up water in all its tributaries. One has no idea how many those there are - creeks, small rivers even just drainage areas. So while Memphis itself sits on a bluff and is relatively safe, the low lying areas drained by the local watershed is soaked and full to the top. That means all the low lying areas are flooding. That is why those areas are called the flood plain, they are subject to flooding. And that is a well known factor.

The US Government provides flood insurance to those people. No, it is not free. And it has a $15,000 deductible. So it is crisis insurance only. And it is not cheap to get.

When the levee breaks it is called a crevass. Pointe Coupee Parish is most vulnerable to crevasses. It has the Mississippi on the North and East sides of the parish and the Atchafalaya forms the West boundary. In 1927 the Atchafalaya crevassed at McCrea in Pointe Coupee, down river from the present day Simmsport bridge. It also crevassed at Melvillie across the river (West side) and little further south of McCrea. That is why towns of Simmsport, Melville and Krotzsprings on the West side of the Atchafalaya are surrounded by ring levees. You would not notice it too much today as both Krotzsprings and Simmsport have out grown the ring levees and are sprawled wll out beyond the levees.

Just above the Old River Structure, where a US Corps of Engineers constructive effort to keep the Mississippi out of the Atchafalaya river, is a stretch of levee that can be dynamited if necessary. The fear is that if that is done, the raging waters may cut a new channel to the Atchafalaya and leave Baton Rouge South high and dry. So that is truly a last resort location to cut the levee at that point.

The next location is the Morganza Spillway. The town of Morganza is not new to crevasses and once there was a town of New Texas out on the river above Morganza. It has long washed away. The spillway structure is just North of the town on Highway 1 which crosses the structure. Basically, there is a fore bay that stretches from the flood gate structure to the river, perhaps three or four miles in depth. This is one of the accumulators I refer to. The river at that point can reach perhaps 10 or 12 miles wide. Pretty significant size body of flowing water.

The spillway is a series of guide levees that extend down into the great Atchafalaya Swamp and eventually to the Gulf of Mexico. The spillway runs abreast of the Atchafalaya river as it more or less leaves Pointe Coupee Parish to the South. A great deal of water can be absorbed by the swamp and if need be, the lower Atchafalaya levee can be breached to allow the water to flow back into the river making an easier path to gulf.

The guide levees cut off upper Pointe Coupee from any natural drainage. That enclave drains toward the South end of the parish but there the water is cut off. Pumps have been installed to lift the drainage into the Atchafalaya. But if there is a crevass in that area, all there will be flooded severely, up to the roof tops as the levees often reach a design level of 50 feet. Give the terrains ranges from 20 to may 35 feet above sea level, well you can judge for yourself. We are talking about a land mass of say 150 square miles.

Trouble is that Terrebone Parish and cities like Morgan City sit in the path of all this water. Morgan City has a ring levee but like Simmsport it has way outgrown the ring levee. So it is a prime target for flooding if the Morganza Spillway is opened up completely.

New Orleans is protected by the Bonnet Carrie Spillway that goes from the river at Norco to Pontchartrain Lake. The Lake fishermen complain bitterly about its use but there has been not significant damage to the lake ecology due to its usage. And it is has been used often. I can recall crossing it with water up to but not over the elevated roadway (bridge). I was but a small child and my father worked for the US Corps of Engineers in New Orleans.

He always said the Bonnet Carrie was opened when the flood gauge at Carrolton Street read 18 feet. New Orleans levees could not tolerate much more than 20 feet at best. So the Bonnet Carrie has been used many times since its construction pre war days some time after the great 1927 flood.

The new John J. Audubon Bridge opened yesterday as the St Francisville ferry was shut down due to high water. The ferry had no place to dock on the New Roads side of the river. This is a sort of annual event anyway, not necessiarily caused by the current anticipated flooding. The bridge is open prematurely with little fan fair. I am sure there will be festivities later in the year.

Let us hope that the new bridge does not become an evacuation route for Pointe Coupee. Ironically, New Roads in all the history of crevasses in Pointe Coupee has never gotten more that 3 or 4 feet of water, and that was only once. However, you can imagine what happens to False River. The camp would be under water!

And it would take a while for it to drain off. It would be a grand disaster of epic proportions. Now you know why the Morrison plantation house is up high on brick pillars.

2 comments:

Tobin said...

Thanks for the great blog entry! I am thinking of driving up LA-1 tomorrow to see the Morganza floodway and Old River Control before they open the sluices.

Cypress-myrtle said...

Lets keep our fingers crossed for no breaks in any levee around here. Made the journey over the JJA Bridge Saturday, it was a nice drive but I was shocked at how much smaller the bridge was in person compared to the web cam on the bridge building site. Those are some very narrow lanes with no left shoulder and the right shoulder will not take a vehicle completely out of the travel lane. For the time being they should have made the bridge two lanes since both approaches are only two lanes.