1993 Floods in Mississippi
For an incredible 144 days, the people of Mississippi, but more specifically St - 1993 Floods in Mississippi introduction. Louis lived in fear. The gigantic Mississippi River was above flood stage between the 1st of April 1993 and the 30th of September later on the same year. The events between created the largest floods in the USA and devastated an area larger than the UK.
The Midwestern streams that were flooded were the Mississippi, Missouri, Kansas, Illinois, Des Moines, and Wisconsin. These streams rose higher and higher by the day, but the National Weather Services had already predicted below normal precipitation for the summer, but above average rainfall meant flooding. Despite this, the first heavy rainfalls started in Dakota, Wisconsin, and Minnesota. This was followed by the upper 200 miles of the Mississippi River to be closed to traffic. Around 100 homes were under water, as the first dams burst.
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Between the 5th of June and the 16th of June, the Keokuk and Fort Madison bridges in Iowa had to be closed. As were the Quincy II, Alton and Burlington bridges. By now the river had spread an impressive seven miles inland. However, even more remarkable, was the amount of water flowing under the Eads Bridge. It was eleven times the volume of Niagara Falls, and could fill the world famous baseball Busch Stadium every 65 seconds. Despite this and several warnings, the Mississippi locks were reopened for traffic to flow.
At the end of this tragedy, around 3 billion cubic meters of water had been overflowed from the river channels to the floodplain downstream at St. Louis. This covered 17000 square miles of land covering nine states: North Dakota, South Dakota, Nebraska, Kansas, Missouri, Iowa, Wisconsin, Minnesota and Illinois. Around 50 people in total died as a result. 26000 people had to be evacuated, and around 56000 homes were destroyed. Around $11,000,000,000 worth of damage was created as a direct result, but loss of wages, business and production, could not be easily calculated.
The greatest affected area, were the cities directly in the floodplain, Des Moines and Iowa. These cities had lost its drinking water supply after being cut off from their water treatment plant that became flooded. Over 250,000 people lost drinking water for 19 hot summer days. Water pipes, contaminated by floodwaters carrying sewage and agricultural chemicals, had to be flushed out before the municipal water supply was reconnected.
The flooding submerged eight million acres of farmland and production of corn and soybeans were down 7%. As a result, corn prices rose by $0.15 per bushel. Floods deposited thick layers of sand in some fields and the U.S. Soil Conservation Service spent $25 million to buy flood-prone farmlands for conversion to natural conditions (e.g. wetlands). The Mississippi River itself is a crucial part of the Midwest’s economic infrastructure. Barge traffic normally moves goods through 29 locks between Minneapolis and St. Louis carrying 20% of the nation’s coal, a third of its petroleum, and half it exported grain. Barge traffic was halted for two months; carriers lost an estimated $1 million per day. Some power plants along the river saw their coal stocks dwindle from a two-month supply to enough to last just 20 days.
Hundreds of miles of roads built on the flat, wide floodplain were closed. Flooding is estimated to have cost approximately $500 million in road damage alone.
But why did this flood occur? Was there not a way to stop or predict it?
There are four main reasons why they did occur:
1. The flooded region received higher than normal precipitation during the first half of 1993. Much of the area received over 150% of normal rainfall.
2. Individual storms frequently dumped large volumes of precipitation that could not be held by local streams. The map shows rainfall in Iowa over a two-day period.
3. The ground was saturated because of cooler than normal conditions during the previous year less rainfall was absorbed by soils and more ran-off into streams.
4. The river system had been altered over the previous 100 years by the draining of riverine wetlands and the construction of levees.
These four reasons were the main causes and the flood could have been prevented. The abnormal rain conditions were a cause that could not be prevented.
This diagram shows that the air from Canada collided with the air from the Gulf of Mexico and this caused the Mexican air to make its moist into rain. Heavy thunderstorms were a result.
The river system was something that could be dealt with to prevent the risk of floods. The Mississippi River is divided into two parts. The Upper Mississippi that runs from it source to Thebes, southern Illinois, where the Ohio river meets the Mississippi. The Lower Mississippi runs downstream from Thebes to the Gulf of Mexico. Flooding was confined to the Upper Mississippi because the river channel widens south of Thebes, and the Lower Mississippi received lower than average inflow from tributaries.
The modification made to the river over the last 100 or so years have been immense and sometimes even pointless. The Mississippi River is one of the most heavily engineered natural features in the USA. The floodplain has changed to hold agriculture and urbanisation. Approximately 80% of the original wetlands along the river have been drained since the 1940’s. These wetlands acted as natural storage reservoirs for floodwaters. They absorbed water during heavy precipitation and released it slowly – reducing run-off to streams and decreasing flood volumes.
The river channel itself had been artificially constrained by levees and floodwalls. These were the biggest cause. These structures increased the volume of water that could be held in the channel and increased the size of the flooded area if the levee breaks. That is what happened.
There are two ways that engineers have attempted to control the flow of rivers in the Mississippi River basin:
1. Build levees to contain rising stream levels
2. Build reservoirs on tributary streams to store floodwaters for later release.
Steps taken to adjust to flood events included:
1. Purchasing flood insurance though the National Flood Insurance Program
2. Seek assistance from the Federal Emergency Management Agency (FEMA) following a flood event.
The U.S. Army Corps of Engineers were given directions to construct levees on the Mississippi River following flood in the 1930’s. However the levees were bound to fail because the flood water rises over the top of the structure or the levee collapses under the weight of the water. Levees and floodwalls protect people on the floodplain from most floods. However, they may not protect against the largest floods with recurrence intervals of more than 100 years.
Over 9,300 km of levees were damaged following the 1993 flood. Only 17% of federal levees were damaged, but up to 77% of locally constructed levees failed. Most of these occurred south of St. Louis despite being protected by a massive floodwall. The wall developed a leak but held up over the length of the flood. Over 50 propane tanks containing over a million gallons of gas in south St. Louis gave the threat of a massive explosion. A levee break south of the city allowed the river level to drop around St. Louis and reduced pressure on the propane tanks. Many of the smaller levees in rural areas, however, failed.
Once the flood did occur, people also used sandbags. An estimated 26.5 million sandbags were used weighing about 331000 tons in total.