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Social sciences: geography

Grade 7

Natural hazards

Module 2

Geographical phenomena

. Floods

1. The occurrence and cause of floods

1.1 What is a flood?

A flood occurs when there is excess water. Soil which is not usually under water becomes covered by water during a flood.

It is a general rule across the world that a yearlong drought is suddenly ended by persistent heavy rain. Such disasters then lead to questions such as: Is the climate changing?

Activity 1:

To hold a certain opinion about a geographical phenomenon

[lo 2.1]

Give your opinion on the statement: The climate of the earth is changing

1.2 Where do floods occur?

Have a look at figure 3 (a world map showing where the most serious floods have occurred) on p. 10.

You will notice that the most floods occur in India, China and North and South America. Yet floods can occur in practically any environment where water is found. Serious floods can even occur in deserts. Remember that there are various possible causes of floods, and these causes determine WHERE the flood occurs.

Figure 3

1.3 What causes floods?

Water that floods certain parts of the earth can occur as a result of various reasons. Here are a few causes:

  • Melting snow which increases the amount of water in rivers.
  • Exceptionally high rainfall in a specific area causes rivers to flood their banks. This kind of rainfall is usually associated with storms, cyclones and monsoon rains (India). Examples of this are the Laingsburg flood and cyclone Demoina, which hit Natal.
  • Dams overflow or their walls break and/or rivers burst their banks.
  • Landslides, earthquakes and volcanoes can disturb rivers and other bodies of water and lead to floods.
  • Floods along the coast are the result of enormous sea waves (tsunamis) which are caused by earthquakes on the ocean floor. More about this when we study earthquakes.

2. The effect (consequences) of floods on the lives of people and their socio-economic activities

Human activities cannot cause floods, but people can worsen the extent and damage indirectly by:

  1. Careless use of the soil as a result of
  • deforestation
  • overgrazing
  • soil erosion

Thus natural vegetation and the roots of the plants are destroyed. This means that the plants cannot hold the topsoil during heavy rainfall. Also, rainwater is not optimally absorbed by the soil. The rainwater flows away rapidly and takes the surface soil with it. As a result, rivers and dams are silted up.

  1. The construction of roads, buildings and channels also destroys the natural vegetation, and increases the draining effect.
  2. Houses are built on less appropriate or even dangerous terrain where floods can occur.

An example of a flood in South Africa.

On September 30th, 1987, a violent tropical storm broke out over Natal. At least 180 people died. Thousands of shacks collapsed under the heavy rain. Roofs were ripped off and houses were seriously damaged. Seven thousand families were left cold and hungry with no roof over their heads, in a single day. Most of the bridges and roads were washed away and emergency personnel could not reach victims by truck. The electricity failed and there was no sewerage or tap water. All the pipelines were destroyed by the floodwaters. Businesses came to a standstill and severe financial losses were felt in the economic field. Farmers lost their harvests and their cattle. All the rich topsoil was washed away and would take years to recover. The walls of farm dams broke and fences were swept away. These alone would bring about serious financial implications if they were to be fixed.

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Source:  OpenStax, Geography grade 7. OpenStax CNX. Sep 09, 2009 Download for free at http://cnx.org/content/col11021/1.1
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