Nan Hua High School Secondary Three Express

Pure Geography Chapter 8: Causes of Floods - Notes

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Introduction/definition: Floods are natural hazards and their occurrences are difficult to predict. A flood occurs when land that is usually dry is submerged by large amounts of water. This water may come from excessive rainfall, melting snow, an exceptionally high tide or when a river

or lake overflows. Floods can be caused by natural processes in the atmosphere and/or human activities which

influences the environment. Natural Causes:

a. Excessive rainfall i. Continuous heavy rainfall will cause an increase in surface runoff, increasing the volume of

flow in a river as it exceeds its carrying capacity and overflows, resulting in flooding. ii. For example, The Yangtze River in China experiences flooding regularly because of large

amount of sediments carried into the river by the frequent heavy rain. iii. Urban areas are more prone due to no vegetation and flash floods often occur when urban

drainage system cannot cope with sudden surge of water.

b. Storm Surge i. The strong winds that bring about tropical storms can create a storm surge with gigantic

waves rising at high levels and bring massive floods which destroy coastal settlements. ii. For example, Bangladesh is frequently flooded when strong winds blow over the Bay of

Bengal as 80% of its land sits on low-lying areas and about 35% of the country is less than 6 meters above sea level.

c. Snow Melt

i. Floods can also be brought about by the melting of vast amounts of snow in spring when temperatures rise.

ii. This occurs more in temperate areas and the increase in volume of flow can overflow rivers and cause floods.

iii. For example, the recent flood in 2011 in the Mississippi River, USA, is a result of a wet spring and huge snow melt from an unusually stormy winter.

The natural causes of floods in an area are often determined by its location. For example, areas experiencing tropical monsoon climate are more likely to be flooded due to excessive rainfall, while areas experiencing cool temperate climate are more likely to be flooded due to melting snow. Similarly, storm surges will mostly likely flood coastal areas more severely than inland areas.

d. Global atmospheric Processes: El Nino and La Nina i. Global atmospheric processes such as the El Niño causes floods in the eastern Pacific while

the La Niña causes floods in the Asia and western Pacific. ii. El Niño and La Niña are alternate patterns of heating and cooling between the eastern and

western parts of the South Pacific Ocean. iii. El Niño, abnormal warming of the water surface at the southeastern part of the Pacific Ocean

causes the surface temperature of the ocean off the coast of South America to heat up. This

 

causes trade winds to push warm surface water eastwards, resulting in extreme weather like heavy rain in this area. La Niña usually follows an El Niño and occurs on average every 3 to 5 years. The southwestern part of the Pacific Ocean experiences heating and causes trade winds to push warm surface waters westwards, causing floods to occur in the western Pacific coast such as Indonesia & Australia.

e. Movements of the Earth’s surface

i. Movements of the Earth’s surface such as an earthquake will lead to landslides and the loosen soil will be deposited into the river, causing overflow and flood as the capacity of the river is reduced.

ii. Earthquakes occurring under the ocean also trigger tsunamis which can flood and destroy coastal settlements.

iii. For example, the recent March 2011 disaster in Japan caused extensive flooding on its coast and thousands of people perished along with towns.

Human Causes:

a, Clearing of forest

i. Clearing of forests/deforestation for resources to meet human demands causes floods to occur more frequently.

ii. Deforestation reduces the trees and vegetation cover and causes an increase in the volume of water flowing into rivers. As less vegetation intercepts the rainwater, more surface runoff flows into rivers and increase the tendency of flooding.

iii. With lesser vegetation to reduce the impact of falling rainwater, more soil/sediment will be displaced into the river, resulting in shallower river bed and lessen the capacity of the river to hold water, making it easier to flood.

iv. In addition, there will be lesser absorption of rainwater by vegetation roots, causing more runoff, and resulting in flooding.

v. For example, the clearing of forest in Bangladesh has increased the amount of sand and mud being washed into the Ganges and Brahmaputra, causing floods to occur.

b, Urban Development

i. The increase of urban development has also caused flooding in many cities. ii. As more people move from villages to cities, causing the population in cities to increase, more

land is cleared for the development of housing, industries and roads to meet the needs of the urban population and as a result built-up areas increase.

ii. Concrete pavements and tarred roads lead to increased surface runoff flowing into rivers causing them to overflow.

iii. For example, in London, the River Thames experiences regular flooding as concrete pathways and buildings replaced natural vegetation, causing an increased in runoff.

c, Impacts of Enhanced Greenhouse Effect

i. Human activities such as burning of fossil fuels in industries, fumes from vehicles and use of aerosol sprays increase the emissions of greenhouse gases and bring about enhanced greenhouse effect which leads to global warming.

ii. Global warming will cause climatic changes such as higher rainfall in wetter regions, which may in turn result in higher incidences of floods and severe storms.

iii. The increase in global temperatures has also caused the reduction of ice cover in the polar regions resulting in rising sea-levels and increasing the possibility of flooding especially in low-lying areas.

- Floods occur naturally and thus may not be prevented in some cases. - Human activities contributed to the occurrence of floods, increasing the frequency and

severity of floods.