Causes of Flooding
There were four principal reasons why flooding was so extensive:
- The region received higher than normal precipitation during the first half of 1993. Much of the area received over 150% of normal rainfall and parts of North Dakota, Kansas, and Iowa received more than double their typical rainfall.
- Individual storms frequently dumped large volumes of precipitation that could not be accommodated by local streams. The map at the bottom shows rainfall in Iowa over a two-day period. Over six inches of rain fell in parts of southern Iowa;
- The ground was saturated because of cooler than normal conditions during the previous year (less evaporation) so less rainfall was absorbed by soils and more ran-off into streams.
The draining of riverine wetlands and the construction of levees had altered the river system over the previous century.
Weather Conditions
The abnormal rainfall was attributed to a weather system formed when warm moist air from the Gulf of Mexico collided with cold, dry air from Canada over the Midwest. When the warm Gulf air cooled it lost the moisture it carried as rain. Normally this rainfall would have been distributed throughout the northeastern states but a stalled high-pressure system over the southeast blocked the flow of the jet-stream bringing a constant stream of storms over the Midwest. For nearly two months (June, July) weather patterns in the U.S. were dominated by this stationary high-pressure system (see diagram below).
Mississippi River System
The Mississippi River is divided into two parts. The Upper Mississippi 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 considerably south of Thebes, and the Lower Mississippi received lower than average inflow from tributaries.
Discharge increases steadily downstream as more tributaries add water to the river. The map on the left shows the locations of four gaging stations and the graph above illustrates how discharge increases on the river between its source and St. Louis. The large increase near St. Louis is caused by the addition of the flow of the Missouri River.
Modification of the Mississippi River
The Mississippi River is one of the most heavily engineered natural features in the U.S. The character of the floodplain has changed to accommodate agriculture and urbanization. Approximately 80% of the original wetlands along the river were drained since the 1940’s. Wetlands act as natural storage reservoirs for floodwaters. They absorb water during heavy precipitation and release it slowly thus reducing run-off to streams and decreasing flood volumes. The river channel itself has been artificially constrained by levees and floodwalls. These structures serve to increase the volume of water that can be held in the channel and thus increase the size of the flooded area if the levee breaks.
Upper Mississippi River in July, 1992 Upper Mississippi River in July, 1993
We can either attempt to stop natural hazards from occurring (prevention) or recognize that they will happen and modify our life styles to deal with them (adjustment).
Prevention
There are two principal ways that engineers have attempted to control the flow of rivers in the Mississippi River basin: 1) Build levees or floodwalls to contain rising stream levels. 2)Build reservoirs on tributary streams to store floodwaters for later release. Steps taken to adjust to flood events include: 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 flood control structures (dams, reservoirs, levees) on the Mississippi River following flood events in the 1930’s. Levees may 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. Floodplain residents may experience a false sense of security that can lead to more extensive development of flood prone lands (the "levee effect").
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 levee breaks occurred south of St. Louis.
St. Louis was 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 presented 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 failed.
Large dams and reservoirs on the Mississippi River would hinder transportation. Therefore, the majority of flood control structures in the Upper Mississippi River basin are located in the Missouri River basin. The Missouri basin can be further subdivided and most of the dams are located in a southern sub-basin, the Kansas River basin. Approximately 85% of stream flow in the Kansas basin is controlled, i.e. it flows through dams.
Locations of dams on tributary streams in the Kansas River basin.
Adjustment
Approximately 10% of Midwest residents who lived in flood-prone areas had flood insurance prior to the 1993 floods. Over eleven million buildings are located in flood-prone areas in the U.S. but only 19% purchased flood insurance.
FEMA was created in 1979 to provide financial assistance to people and areas affected by natural disasters. Over 400 counties in the flood region were declared disaster areas, making them eligible for disaster relief. Not all areas affected by flooding are declared disaster areas. Residents who don't buy flood insurance must gamble that they will receive disaster aid from FEMA to help cover clean up costs.
People and the Flood
Nearly fifty people died as a result of the flooding, 26,000 were evacuated and over 56,000 homes were damaged. Economic losses that are directly attributable to the flooding totaled $10-12 billion. Indirect losses in the form of lost wages and production can not be accurately calculated.
The consequences of flooding were determined by land use patterns.
- The greatest economic losses occurred in cities on the floodplain. Des Moines, Iowa, located in the center of the flood region, became the largest U.S. city to lose its water supply when its water treatment plant flooded. More than 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. Economic losses in Des Moines totaled approximately $716 million.
- The flooding submerged eight million acres of farmland. Production of corn and soybeans were down 5-9% as a result and corn prices rose by $0.15 per bushel. Floods deposited thick layers of sand in some fields. The U.S. Soil Conservation Service spent $25 million to buy flood-prone farmlands for conversion to natural conditions (e.g. wetlands). Conversion of natural lands to farmlands has resulted in greater run-off and exaggerated the effects of flooding. Modern farming methods leave plant residue on the surface and reduce run-off.
- The Mississippi River itself is a crucial part of the Midwest’s economic infrastructure. Barge traffic normally moves goods through a system of 29 locks between Minneapolis and St. Louis. Barges carry 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 floodplains were closed. Flooding is estimated to have cost $500 million in road damage.
By: Tadas Balnys 10A
