Methods in Geotechnical Engineering- Group 1

20 Oct

Authors: Bill Wortner, Charlotte Kenny, Emily Woodson

Executive Summary:

In early spring and summer 2008, a combination of record rainfall and flash flooding led to severe flooding and overtopping of the levees bordering the Mississippi River. Weak, eroded levees failed and resulted in severe flooding that left thirteen dead; damage was estimated to be in the tens of billions of dollars. This has not been the only occurrence of flooding along the Mississippi since the construction of the first levees in the 1800.  Flooding has been recorded as early as 1879; the most recent floods occurred in the spring of 2011. Even with the constant expansion of existing levees, the threat of failure looms large over residents of the Mississippi river basin.  Research of the levees in 2008 helped conclude that certain types of vegetation will prevent failure; however,  over topping still occurs, and will continue to occur. Preventative measures to prevent mass flooding have began. These measure have helped some areas, but hurt others such as agriculture. A long term plan must be devised in order to repair the Mississippi River Basins flood plain with minimal damage to residential communities.

History of the Mississippi River Levee System

The Mississippi River is the lifeblood to many cities in the United States; it provides transportation and water for cities and industries.  However, it is also notorious for overflowing its banks and wreaking havoc in the communities in the surrounding areas.

 The first documented flood occurrence dates back to 1543 when the river flooded for about 80 days.  Since then, the floods have persisted and those living in the Mississippi River Valley have had to cope with the unpredictability of its floods.

Figure 1: Mississippi River Basin (1)

The Mississippi River has one of the largest drainage basins in the world; due to its vast size, (draining 41 of the 48 continental states) controlling its flow has been a real challenge. Figure 1 shows the Mississippi drainage basin and all its tributaries. In 1717, the French realized the devastating potential of the Mississippi and constructed the first levees along its banks in New Orleans.  These primitive levees consisted of earth embankments that were intended to keep river water from overflowing the banks.  By the early 1800s, the prim

Figure 2: The construction of the levees were major public works projects durin ghte 1800s. (2)

itive levees extended about 100 miles upstream of New Orleans. Creation of the levees was a major public works project, witch employed many civilians, and used a great deal of animal power, which can be seen in figure 2. From that point forward, states and levee boards began taking control of the levee building effort and were able to fund the projects by collecting money from taxpayers.  Despite these efforts, several major floods occurred during the 19th century causing mass devastation to those living in the River Valley.  Up until this point, the Army Corps of Engineers had been responsible for maintaining navigation along the river, but after Congress established the Mississippi River Commission (MRC) in 1879, the Corps acquired the task of improving navigation and developing better flood prevention measures.  Due to the limitations imposed by Congress, the Army Corps assisted levee boards by identifying better levee construction techniques, but its main function was repairing breaks in the levees.

Although federal government involvement improved the levee system, the system was still lackluster and inhabitants suffered from more devastating floods.  After the flood of 1882, levees were left overtopped or crevassed and major considerations were made on flood prevention measures.  Some proposed ideas included floodplain reforestation, reservoirs, and flow diversions; the MRC rejected each proposal due to its “levee only” position. In 1917, Congress passed the Flood Control Act of 1917 which gave the MRC the ability to construct an elaborate system of levees to protect the Mississippi River Valley.  The MRC maintained its “levee only” position and up until 1927, the new levee system had successfully protected citizens during multiple high water events.  However, the levees were unable to hold back waters in the flood of 1927.

For more information on the early levee system please visit:

http://www.mvn.usace.army.mil/bcarre/pastfloods.asp

http://www.mvn.usace.army.mil/pao/bro/misstrib.htm

Figure 3: Evolution of the levees used in the Mississippi River Valley. (3)

Figure 3 shows the history of levee expansion through the years. the expansions began in 1828. Due to the failures in the 1927 flood, the 1928 Federal Flood Control Act mandated much more stringent levee requirements and introduced the use of floodways.  The new levees were required to have a higher freeboard, used a trapezoidal shape, and required a design life of 30 years instead of 20 years.  The floodways that were introduced diverted the floodwaters if water rose above the height of a lower levee.  The water would pass through the lower levee and be directed to a secondary channel that would reconnect with the main river after it passed the area of large habitation.  In addition plug flow levees were constructed; if these levees were overtopped they would quickly deteriorate to slow the flow of the river to a desired flow rate.  Although these levees are designed to fail on their own, they can also be blasted with dynamite to initiate failure.

Figure 4: Image of a town near the Mississippi River during the 1993 flood. (4)

The levees held up fairly well up until the flood of 1973 which caused an estimated $184,000,000 worth of damage.  After the devastation of this flood, it became apparent that the levees needed to be raised again.  In urban environments that lacked land area for construction of levees, floodwalls were constructed along the river to block floodwaters from entering the city.  The levees were once again damaged in 1993, the extent of flooding can be seen in in figure 4, which prompted additional rebuilding and modification.  Unfortunately, these measures were not adequate in preventing the deadly floods in 2008.

A detailed description of the levee evolution can be found at: http://web.mst.edu/~rogersda/levees/Evolution%20of%20the%20Levee%20System%20Along%20the%20Mississippi.pdf

Mississippi Levee Failure of 2008

 

During the spring and early summer of 2008, record rain amounts fell in the Midwest on parts of southern Indiana, Ohio, Wisconsin, Iowa, and Minnesota. After months of heavy precipitation, the ground saturated, leaving it unable to absorb any more water. In June 2008, thunderstorms producing flash flooding dropped an additional 0.3m of rain in over 58 locations. This led to severe flooding throughout the Midwest as water overtopped the levees bordering the Mississippi River. The erosion associated with the overtopping resulted in the levee failures. figure 5 shows a typical seen along the Mississippi during these times.

Flooding continued as long as two weeks with central Iowa and Illinois being hit the hardest. The flood left thirteen dead and damage was estimated to be in the tens of billions of dollars.

                       

Figure 5: Failed levee during the June 2008 flood. (5)

Prior to the failures, millions of sandbags were placed to offer support to the weaker levees. But, according to the Army Corp of Engineers, there was no definite way to determine which levees would possibly fail. (http://www.foxnews.com/story/0,2933,367803,00.html ) In one town, Winfield, Missouri, muskrat burrows contributed to the levee breach. As the floodwaters traveled through the communities, they carried with them the additional threat of garbage, chemicals, fertilizers, manure, and fuel. Simply wading through the waters proved hazardous and local and federal officials warned the citizens.  With the flooding, thousands were forced from their homes and many had nothing to come back to.

Figure 6: Grass cover on a levee after the June 2008 flood. Notice little to no erosion due to dense root system. (6)

After the weeks of flooding, a post-flood field reconnaissance team was sent in to collect data to study how and why the levees eroded and failed. Seven different sites throughout the Midwest were setup. Some of the sites sustained overtopping for days without failure. However, the team found evidence that the river stayed at a stage level equal to or higher than the 100-year flood for approximately 8 days. For additional information and study results, see <http://casehistories.geoengineer.org/volume/volume2/issue2/IJGCH_2_2_3.pdf>. The team also discovered the importance of vegetation around the levees. On one site, shown in figure 6, Reedy Canary grass one meter tall was forced down by the overtopping waters and essentially created a barrier to prevent erosion. However, beneficial vegetation does not include trees due to the root system that spans a larger area and can travel through the levees and create paths through which water can initiate seepage and erosion.

http://aquadoc.typepad.com/waterwired/2008/06/after-1993-floods-call-for-change-ignored.html

http://casehistories.geoengineer.org/volume/volume2/issue2/IJGCH_2_2_3.pdf

Floods After 2008

Figure 7: Map indicating flooding severity in the US after May 2011.(7)

Floods continue to be a problem in along the Mississippi River.  The most recent floods occurred in May of 2011, the severity of these floods shown in figure 7. The floods of 2011 are the third largest and most damaging in the history of the Mississippi River.  Illinois, Missouri, Kentucky, Tennessee, Arkansas, Mississippi, and Louisiana have all experienced flooding in 2011. Kentucky, Tennessee, and Mississippi have counties that have been declared as federal disaster areas. The map to the left indicates USGS gauges that have recorded flooding this year; all boxes that are not green indicate some severity of flooding. As seen, most flooding locations are located along the Mississippi River and its tributaries.

The floods have a number of contributing factors. Four major storm systems deposited record setting rainfalls in April; both of these systems were over the Mississippi water shed.  The rain fall from these 2 storms made April, 2011 the 10th wettest month ever recorded.  Large snowfalls in the upper Midwest contributed to the amount of water being drained in 2011. These contributing, natural, factors brought on the 500-year flood that occurred this year.

Follow this link to see a series of Abc News videos  on the flooding from this year and the damage they have caused: http://abcnews.go.com/Technology/mississippi-river-crests-memphis-towns-prepare-high-water/story?id=13563217 – the videos report on how the levees affect the speed and level of the waters, as well as what has been causing the flooding, and what the community is doing to slow the effects.

Figure 8: Distribution of private and federal owned levees.(8)

Human factors have also contributed to this flood. And the dangers are not recognized by many residents of the basin.  The creation of levees protects people from the dangers of river flooding, but levees are not guaranteed to withstand all floods. Levees are designed to withstand a certain year flood, and any flood larger than that will overtop the levee. This has been the case in the 2008 and 2011 levee failures.  Many levees are privately managed, or not managed at all; these levees pose an even greater risk of failure due to their lack of upkeep and monitoring. red regions in figure 8 show the comparison of levees and public owned land, most of which do not coincide with each other.  However, the real dangers of the Mississippi come from years of settling, population growth and urbanization. Although m uch of this damage is not reversible, state and government officials can repair some of the damage and prevent it from continuing.

Sandra Postel, the director of the independent Global Water Policy Project, explains in her article for National Geographic (http://newswatch.nationalgeographic.com/2011/05/03/mississippi-floods-can-be-restrained-with-natural-defenses/) why the floods can be restrained with natural defenses.  Over 35 million acres of wetlands have been destroyed do to population and settlement growth.  This wet land is the rivers natural flood plain. The flood plain is the first line of defense for flood prevention. The problem is that many flood plains have been removed and are now inhabited.  The levees have been created to keep the water contained instead, but once overtopping occurs, there is nothing left to stop the flow into the flood plains.  The risk of flooding becomes more dangerous as water travels downstream. The water passes by the north most levees and travels downstream without being able to drain into the flood plain. The water level rises as it travels further, until the levees at some location cannot accommodate for the buildup.  Also adding to the system is urban runoff, the flood plain acts as a sponge during rainfall. Runoff now enters the streams and rivers instead, adding to the rising level of the Mississippi.  Research after the 1993 floods indicated that $2 billion in wetland recovery would have contributed to the prevention of $16 billion dollars in damages due to the flooding since.

This article, from scientific America, is a local’s account of the levees in her hometown and the illusion they bring to society: http://blogs.scientificamerican.com/guest-blog/2011/05/20/levees-and-the-illusion-of-flood-control-explainer/

Figure 9: Photos Cairo, Illinois before and after the Birds Point Levee demolition.(9)

The Federal Army Corps of Engineers has already begun to correct the damage done to the flood plains; however, it has come at a great cost to the agriculture industry.  The corps has demolished levees in states along the Mississippi to prevent flooding in towns located on the river. The 2 largest and most controversial demolitions took place in Birds Point, Illinois and Baton Rouge, Louisiana.  An article in St. Louis Today- http://www.stltoday.com/news/local/metro/article_3c73c9f8-74ff-11e0-a74d-0019bb30f31a.html – explains the demolition of Birds Point levee in order to protect the town of Cairo from an eminent flood.

The demolition saves the 2800 residents of Cairo from the waters destroying their homes, but the flood water covered and destroyed 130,000 acres of farmland; an estimated $1 billion in agricultural losses. a satelite view of the controlled flooding can be seen in figure 9. This demolition was an emergency act, but further wet land repair is needed. The government must figure out a plan to restore Mississippi wet lands with minimal damage to farmers and residents. Otherwise floods will continue to get worse in the future, and the damages will be greater than what it will take to repair the wetland.

Cites:

(1) http://www.mvn.usace.army.mil/pao/bro/misstrib.htm

(2) http://www.mvn.usace.army.mil/bcarre/pastfloods.asp

(3) http://www.sciencenewsforkids.org/2005/09/unnatural-disasters-2/

(4) http://www.geology.wisc.edu/~maher/air/air13.htm

(5)http://aquadoc.typepad.com/waterwired/2008/06/after-1993-floods-call-for-change-ignored.html

(6) http://casehistories.geoengineer.org/volume/volume2/issue2/IJGCH_2_2_3.pdf

(7) http://water.weather.gov/ahps/index.php

(8) http://www.moenviron.org/water-program/floodplains/

(9) http://newswatch.nationalgeographic.com/2011/05/03/mississippi-floods-can-be-restrained-with-natural-defenses/

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One Response to “Methods in Geotechnical Engineering- Group 1”

  1. Mr WordPress October 20, 2011 at 2:16 am #

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