<< Chapter < Page Chapter >> Page >

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.

Questions & Answers

how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
what is system testing
what is the application of nanotechnology?
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
anybody can imagine what will be happen after 100 years from now in nano tech world
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
silver nanoparticles could handle the job?
not now but maybe in future only AgNP maybe any other nanomaterials
I'm interested in Nanotube
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
can nanotechnology change the direction of the face of the world
Prasenjit Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Berger describes sociologists as concerned with
Mueller Reply
Got questions? Join the online conversation and get instant answers!
QuizOver.com Reply

Get the best Algebra and trigonometry course in your pocket!

Source:  OpenStax, Geography grade 7. OpenStax CNX. Sep 09, 2009 Download for free at http://cnx.org/content/col11021/1.1
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Geography grade 7' conversation and receive update notifications?