USGS Atchafalaya & Mississippi River Deltas Study
Posted on 09/08/2005 11:38:38 AM PDT by syriacus
In the 1950’s, engineers observed that the Mississippi would soon cease to inhabit its current channel as the mainstream, and instead migrate to the Atchafalaya River Basin.
The path by which the Mississippi would migrate was a small stretch of water, named the “Old River”, that connected the Mississippi to the Red River. Old River was formed when Captain Henry Shreve dug a shortcut across the the neck of Turnbull’s Bend in 1831. The Mississippi abandoned its old course and took the shortcut provided by Old River. As a result, the Atchafalaya River received more and more discharge from the Mississippi. Discharge was also increased into the Atchafalaya in 1840, when a 30 mile long log jam was removed from its headwaters by the state of Louisiana. This increased discharge caused most of the problems the Army Corps of Engineers would have to face.
[snip]
No one can tell whether the capricious river will flood its banks for a final time and permanently send its main flow to the Atchafalaya.
Will the mighty Mississippi winding past New Orleans be reduced to a bayou?
How much wetland habitat will be lost to subsidization and how far will the Louisiana coasts recede?
These questions remain unanswered. Much work remains to be done to counteract the damage caused by our attempts to control nature; it is up to us to see that matters don't become worse.
(Excerpt) Read more at tulane.edu ...
The Mississippi Levee System and the Old River Control Structure
"One who knows the Mississippi will promptly aver...that ten thousand River Commissions, with the mines of the world at their back, cannot tame the lawless stream, cannot curb it or confine it, cannot say to it Go here or Go there, and make it obey; cannot save a shore that it has sentenced."
Mark Twain
Life on the Mississippi
The Levee System
The muddy Mississippi has wound its way through this country's history since the first European settlers set foot on the shores of America. Since the dawn of mankind, humans have built their civilizations next to the water, and the early Americans were no exception. To the settlers of Mid-America, the Mississippi River was one of their most valuable resources. It provided them with a means of transportation for developing commerce and industry, as well as water for crops and irrigation. While settlers enjoyed their ready access to the river, they did not enjoy its ready access to them. Floods frequently swept away their attempts at permanent settlements. The consensus grew that the Mississippi would need to be artificially controlled in order for society to benefit from its proximity.
The history of man's attempts to control the Mississippi is full of both success and failure. Levees already existed when the first French trappers ventured into the wilds of Louisiana. These levees were formed naturally by the Mississippi's fluvial processes and tended to be no more than a meter or two in height. Building up these natural levees was the first solution to the flooding problem. In 1717, the first manmade levee system was started by Bienville, the founder of the city of New Oreans. The construction of the first levees, which reached only three feet in height, was completed in 1727. After that, it was left to private interests to extend the levees. By 1743, French landowners were required to build and maintain the levees along their riverfront property or forfeit their lands to the French crown. However, it soon became obvious that these small levees, although augmented through the efforts of the settlers, were not enough protection against Mississippi flood waters. During large floods, the river would frequently break through at weakened points in the levees, referred to as crevasses. Many crevasses, such as the Macarty Crevasse of 1816, took many lives and caused extensive property damage.
The unorganized levee system was finally turned over to the Army Corps of Engineers. The levees’ were designed to protect populated areas from potentially disastrous flooding and keep the Mississippi safely within its banks. However, not everyone agreed that levees were the best way to decrease flooding. In 1852, the federal government appropriated $50,000 in order to conduct studies on how to further eliminate the flooding problem. The first study was done by an engineer named Charles Ellet Jr., whose study produced some startling conclusions. His report to Congress attributed the increase of flooding in the Mississippi River Basin to four major developments, including:
"The extension of the levees along the borders of the Mississippi, and of its tributaries and outlets, by means of which the water that was formerly allowed to spread over many thousand square miles of low lands is becoming more and more confined to the immediate channel of the river, and is therefore, compelled to rise higher and flow faster, until, under the increased power of the current, it may have time to excavate a wider and deeper trench to give vent to the increased volume which it conveys."
Ellet also mentioned the effects of increased cultivation, manmade cutoffs/shortcuts, and the lengthening of the delta all of which will increase the probability and magnitude of floods. He concluded that the flooding problem would worsen with time as the Mississippi Basin becomes more settled. According to Ellet, “It is shown that each of these causes is likely to be progressive, and that the future floods throughout the length and breadth of the delta, and along the great streams tributary to the Mississippi, are destined to rise higher and higher, as society spreads over the upper States, as population adjacent to the river increases, and the inundated low lands appreciate in value”.
Unfortunately, Ellet’s opinion was ignored in favor of two Army Corps Engineers, Captain Andrew Humphreys and Lieutenant Henry Abbot, whose views became the consensus for the next 140 years. In their study, Report Upon the Physics and Hydraulics of the Mississippi River, Humphreys and Abbot emphasized that levees were the best method of flood damage control. Since 1882, the USACE in conjunction with the Mississippi River Commission extended the levee system so that it included mainly the area from Cairo, Illinois to the mouth of the Mississippi delta in Louisiana. However, the relief brought by the levees would prove to be short-lived. Soon, an even greater challenge would take precedence in the minds of both government official and citizen alike.
The Old River Control Structure
As time progressed, it became increasingly apparent that the Mississippi was diverting more and more of its flow down the Atchafalaya River. In the 1950’s, engineers observed that the Mississippi would soon cease to inhabit its current channel as the mainstream, and instead migrate to the Atchafalaya River Basin. The path by which the Mississippi would migrate was a small stretch of water, named the “Old River”, that connected the Mississippi to the Red River. Old River was formed when Captain Henry Shreve dug a shortcut across the the neck of Turnbull’s Bend in 1831. The Mississippi abandoned its old course and took the shortcut provided by Old River. As a result, the Atchafalaya River received more and more discharge from the Mississippi. Discharge was also increased into the Atchafalaya in 1840, when a 30 mile long log jam was removed from its headwaters by the state of Louisiana. This increased discharge caused most of the problems the Army Corps of Engineers would have to face.
In their study of the Atchafalaya River, the USACE was able to deduce several possible effects of the diversion. The discharge of water into the current Mississippi channel would decrease until it resembled a bayou. All the levees along the previous Mississippi channel would no longer be needed to prevent flooding. In addition, towns such as Morgan City, located within the current Atchafalaya flood plain would be swept away by the newly expanded river. An expensive levee system would have to be built along the Atchafalaya in order to preserve current standards of flood control. The old Mississippi channel would no longer be able to be used for navigation by industry without expensive and extensive dredging. Industry would lack the water it needed to perform many of its processes such as cooling and the dumping of wastes. Agriculture would suffer from the lack irrigation water, and cities such as New Orleans would suffer economically from the lack of trade and drinking water. The only thing the diversion of the Atchafalaya promised to bring to society was disaster, and legislators decided to prevent this disaster at all costs.
The Army Corps of Engineers was given the job of maintaining the current distribution of water between the Lower Mississippi and the Atchafalaya River channels (70%-30%). They did so by building the Old River Flood Control Structure which consisted of massive floodgates that could be opened and closed as needed at the entrance to the Old River. This structure was completed in 1963. In 1973, a large flood tested the ORCS to its limits. Huge scour developed underneath the large steel pilings which anchored the structure to the river bottom. The structure was almost swept away, and emergency concrete was poured into the holes as a kind of large Band-Aid. After the '73 flood, the corps saw the need for a backup structure, and built the Old River Control Auxiliary Structure (ORCAS) to alleviate some of the pressure on the main control structure during large scale flooding.
Despite several close calls, the ORCS still manages to keep the Mississippi River in check. How long this will last, however, is a matter of opinion. The Army corps claims to have the situation in control; the Mississippi will not divert to the Atchafalaya as long as they are there to prevent it. However, what if the control structures necessary to prevent the Mississippi's diversion to the Atchafalaya River were completely undermined and swept away during a flood such as the one in 1973? The ORCS has almost failed in the face of the Mississippi's might before, and it could still do so. Can the Army corps withstand nature's might indefinitely, or will physics and the Mississippi River win out in the end?
Researcher Raphael Kazmann at LSU suggested that the Mississippi would be the victor in the struggle of man against nature. In his 1980 study on the possible effects of the Atchafalaya diversion he states, “Probably the most important single conclusion reached by this study is that in the long run the Atchafalaya will become the principal distributary of the Mississippi River and that the current main-stream will become an estuary of the Gulf of Mexico…the final outcome is only a matter of time” (Kazmann 1).
In addition to the flooding problem, engineers now face problems caused by the lack of flooding. The channelization produced by the levees and control structures deprives natural wetlands of the sediments normally deposited during flooding. Wetlands rely on sediments from distributaries and flooding to counteract subsidence, the compaction of sediments under their own weight. Water flows faster in subsidized areas, and distributaries can rapidly expand into wide channels, causing wetlands to disappear under the influx of water. The coastal marshes of Louisiana provide a natural barrier against the erosion causes by the fierce storms which often come from the Gulf. Because of the loss of these wetlands, the Louisiana coast has receded several thousands of feet over the past few decades, and commercial fishermen have also been deprived of a ready source of income.
Most of the problems resulting from the levee system, including wetland degradation, stem from channelization. While the levee system could not be scrapped without a large financial loss, the USACE realized that diversion structures could help alleviate some of the problems caused by channelization. Diversion structures diminish some of the force of flood waters and the likelihood of crevasses (breaks in the levee) by providing flood waters with established escape routes. The first diversion structure, the Bonnet Carre Spillway, was built in response to the great flood of 1927. It was designed to discharge excess flood waters into Lake Pontchartrain and thence into the Gulf of Mexico.
The USACE has recently started to build other diversion structures whose main purpose would be to divert sediment-rich water into wetland areas in order to stop subsidence. The Caernarvon diversion structure, completed in 1991, was the first of these modern structures. It has significantly restored wetland acreage and wildlife in the area. The success of the Caernarvon diversion structure has encouraged the government to develop more of diversion structures. Construction of the Davis Pond Diversion Structure began in 1997. Further in the future is a possible third freshwater structure located on the Bonnet Carre spillway itself. The Bonnet Carre Freshwater Diversion Structure would divert river water into Lake Pontchartrain, and finally the Western Mississippi Sound. With the help of diversion structures, the wetlands of the Mississippi River Basin may be able to offset the effects of subsidence and coastal erosion.
As the year 2000 approaches, the future seems uncertain for the lower Mississippi. Many questions regarding its fate reside in the hearts of both citizen and legislator alike. When will the next record-breaking flood take place, and what will be its effects? No one can tell whether the capricious river will flood its banks for a final time and permanently send its main flow to the Atchafalaya. Will the mighty Mississippi winding past New Orleans be reduced to a bayou? How much wetland habitat will be lost to subsidization and how far will the Louisiana coasts recede? These questions remain unanswered. Much work remains to be done to counteract the damage caused by our attempts to control nature; it is up to us to see that matters don't become worse.
I knew I had read this long ago. Left to its own devices, the Mississippi would have moved west many many years ago and abandoned NO. It's not nice to fool Mother Nature.
We also were taught that it was the deepest river in America and produced the best tasting catfish.
I didn't learn this until last week....when I came across this folowing article
. The levee system that has been built along the river, coupled with the canal system to keep the interior of the city dry, prevent the land from being replenished by the annual spring floods. As a result, the land will continue to sink until eventually there will be nothing to stop the waters of the Gulf to rush back upon the fragile land.In addition, the fresh water that is pumped into the brackish wetlands surrounding the city is creating an ecological disaster. ...
As a result, in order to save itself from the waters surrounding it, the city of New Orleans is slowly destroying its own environment.
The final death knoll of the city may very well come from the river itself.
Scientists and environmentalists know that the Mississippi is trying to change its course that will bypass the city in favor of the shorter route to the Gulf through the Atchafalaya basin. So far, the Corps of Engineers has prevented the river from doing this. But, one day, it will happen, perhaps following a direct hit from a hurrican As late summer and early fall approach every year (hurricane season), New Orleans stands with the threat that it will lose its own lifeline. No canal system and no levee system will prevent the disaster that will follow.
How far from the main branch of the Mississippi does the the Atachafalya flow? I should answer my own question and look at a map, I guess.
NEW ORLEANS – The U.S. Army Corps of Engineers has increased the diversion of Mississippi River water into the Atchafalaya Basin through the Old River Control structures to help Louisiana’s crawfish industry.The decision was made by Brig. Gen. Edwin J. Arnold, division engineer of the Corps’ Mississippi Valley Division in Vicksburg, Miss., at the request of Louisiana Gov. Mike Foster.
“Although the diversion aids the crawfish industry, only more rain and more water coming downriver can provide all the water needed to maximize the crawfish harvest,” Arnold said.
“This is the most water we can divert at this time and still maintain a stable relationship between the Mississippi and Atchafalaya rivers,” Arnold said
Three million people are depending upon the present channel of the Mississippi River.The problem is that the Mississippi is on the verge of switching to a new channel along what is now the Atchafalaya River.
A Billion dollars and a re-built Old River Control Structure says it won't happen anytime soon. It did almost happen about 30 years ago, but quick work saved the old Old River Control Structure from a washout.
There is also the Morganza spillway and the Bonnet Carre spillway to regulate the flow on the lower river.
Sounds good. Must be quite a system!!
Nature has a way of putting hubris on display.
They need to build the city on higher ground. There is such ground between the Mississippi and the Atchafalaya.
Start by moving the new city there - the surviving manufacturing and port facilities will still be close to the new metropolitan area.
Then, build a canal and an inland dredged port parallel to the Atchafalaya, and control structure to keep enough water coming down the existing channel for the canal, along with locks between the canal and the existing channel.
That way, when the river does jump, we'll be ready. It'll take 15 years, but IMO it's an insurance policy against the hazards that hang over the current locations and would ensure continued navigation up the Mississippi.
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