The Need For River Management

AS Geography

MISSISSIPPI FLOODS 1993Case Study at a National Level

Learning Objectives

• To understand the history behind flooding on the Mississippi River

• To appreciate that flood events occur often on the river and control methods have been implemented

• To draw a graph representing the amount of flooding that has occurred on the river in the past 300 years

• Understand the main reasons behind the flood of 1993

• Distinguish between the issues and the reasons behind what went wrong

Mississippi River

This looks big!

• It is the Mississippi has two main tributaries the Missouri and the Ohio

• Together they drain a third of the USA and a small part of Canada

How often does it flood?

• Left to it’s own devices the flooding frequency would be almost annually with the events mainly occurring in late spring

How big are these floods?

• Major floods (Those causing major damage, major loss of life or major disruption) occurred every 5-10 years

• There were 6 between 1879- 1889• Serious or extreme floods occur once every 40

years.

What causes these events?

• Usually they come about as a result of heavy rain (January – May) in the Appalachian mnts.

• This can be made worse when it coincides with snowmelt

How bad are the Consequences?

• The first majorly recorded event happened in 1927. Development on the flood plain cost 217 lives

• 700,000 people were evacuated• The river was 150km at it’s widest (usually only

1km)

Fatal Flood 1927

What attempts have been made to reduce the flood hazard?

• Until 1927 ‘Hold the Levee’ was the main form of defence

• By 1993 some of these were 15m high• After 1927 6 Major dams and 105 on the Missouri

were built• Afforestation to reduce or delay runoff• Diversion spillways• Meanders were cut through to straighten and

shorten it’s course

Success...

• By 1973 it appeared that the river had been tamed, that was until 1993.

• Had human intervention made it worse

Major Flood Dates

1718 1735 1770 1782

1785 1791 1796 1799

1809 1811 1813 1815

1816 1823 1824 1828

1844 1849 1850 1851

1858 1859 1892 1893

1903 1907 1908 1912

1913 1916 1920 1922

1923 1927 1929 1932

1936 1937 1945 1950

1957 1958 1973 1974

1975 1979 1983 1984

1993 1997 2005 2008

Activity

• Draw a graph that shows the number of floods per decade from 1718 – 2008

• Explain why you think there were more floods between 1900 and 2000 than between 1700 and 1900

• Do you feel that the development of management has done anything to stem the floods?

Graph of Floods

1650 1700 1750 1800 1850 1900 1950 2000 20500

1

2

3

4

5

6 Number of Floods

Number of Floods

1920’s floods

1970’s

1850’s

1810’s

Management

• Prior to 1993 it was perceived that the controls in place were working

• Heightening of Levees to upto 15m in places• Cutting through meanders over 1750km of artificial

channels• Large spillways built to take excess water during

floods• Dams were controlling the flow of the major

tributaries (Ohio, Missouri and Tennessee Rivers)

So why did it flood?

• Mid- West was already having a wet year when record spring and summer rains hit

• Run off increased on the already saturated ground• Several parts had already 200% more rain than

normal• The ferocity, location and timing that took everyone

by surprise

Why?

• At that time of year it is usual for river levels to be receding

• Upper Mississippi was not believed to be a flood risk then due to controls

• Flooding in St. Louis reached an all time high• Major wetlands had been drained and some

urbanised

Satellite Images of Mississippi flood 1993

Floods of 1993

Levees

• Since 1718 engineers have been trying to channel the river to protect farmland and towns

Does this work

• 2 schools of thought• 1 school believes that rivers should be allowed to

run free and are a natural ecological event• The other argues for defence.

Damage

• Estimated to be around $10 billion• 50 deaths – would have been higher if not for the

good early warning devices

Responsibility

• Flood defences are the responsibility of the US army Corp of engineers. They have been using hard engineering methods (Dams, levees, reservoirs, barrages)

• Some of the levees are set back from the river to allow it to expand but still be channelled and it had been effective up until 1993 however there are issues

Issues

1. With Each successive flood Levees grow higher and wider increasing the amount of land and cost to build

2. River flow confined to levees rises above the level of the surrounding floodplain, if a levee breaks this increases the damage done

3. Levees increase flood peaks and the large storage capacity of the floodplains is lost.

Homework Due 14th October

• For a River Basin you have studied outline the impacts of flooding and examine how effective the measures to control flooding are. (15 marks)

• (750 Words)

• Please use all the following sources to help you with this.

References and Links

• Floods on the lower Mississippi: An Historical Economic Overview

• The Levee Break at Mounds Landing• The Mississippi River and Tributaries Project • Discussion forum• Integrated Flood Mismanagement on the Mississip

pi River• Flood Analysis 1993 Mississippi Floods• Should I trust that Levee?