Classnotes: 15th December: Flooding in an MEDC

This lesson is an important case study:

The Mississippi Floods 1993
There is lots of extra information here, here and here (the last link is the best).

Causes
1. Heavy rain during the spring months due to a frontal rain system (see here for an explanation of frontal rainfall - you will need to scroll down to number 2).
2. Natural vegetation in the Mississippi basin had been removed for farming. Less vegetation = less interception = more overland flow
3. The river naturally floods but is prevented from doing so by flood management. In this year, the river levels rose so high the flood defences gave way allowing huge amounts of water to enter the basin.

Effects
1. 17 people killed
2. $2 billion of damage (about £1 billion)
3. Harvests fell by between 10%-70%
4. 7000 people evacuated in Missouri
5. Water at 4 ft deep meant heavy silt deposits so couldn't farm effectively

How the river is managed
The river is a 'managed' river meaning it has been changed by people for one purpose or another. There are several ways to alter a river - see here for some examples. On the Mississippi there are several things to be aware of:
1. River straightening
River has been straightened. This gets rid of meanders in the river. No meanders = ships can get through more easily.
AND
the river water moves faster and is removed from the area of potential flooding more quickly
BUT
it also means that there is 250km less river channel for the water to sit in
SO
less space for the water means the water level will rise
SO
a flood!!!

2. Levees
Levees - raised banks on a flood plain next to the river - occur naturally. Along the Mississippi, artificial levees have been built to keep the water in
SO
the river levels can rise quite high without flooding the surrounding land
BUT
when the river level rises too high the pressure on the levees increases
SO
the levees burst
SO
the land floods
BUT
the water can't get back over the wall into the river when the river levels have gone down
SO
the flood water stays where it is for a lot longer

Notice the link words here - try to do more of that in your writing.