floods Flashcards
definition of floods
Occur when land that is usually dry is submerged by large amounts of water.
Considered as NATURAL HAZARD as it can endanger lives and destroy property.
*EXAMPLE: In 1999, a flood hit Venezuela in South America. About 650 people died and 150 000 people became homeless and 7 000 people went missing.
types of floods
- coastal floods
- river floods
- flash floods
what are coastal floods
-Occur in areas along the coast e.g. low-lying coastal areas, such as river mouths
-Caused by intense storms such as tropical storms and tropical cyclones (a.k.a. hurricanes and typhoons).
described as to have tall, large waves
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what are river floods?
- Occur when water in the river rises and overflows its banks.
- Occur on the flood plain which is the relatively flat land stretching on either side of the river.
How are flood plains formed?
When there is heavy rain, water overflows the banks and floods the surrounding areas. Sediments are deposited on the banks. repeated flooding cause deposits to accumulate forming flat land also known as flood plains
what are flashfloods?
- A sudden, localised flood of great volume
and short duration which usually affects a small area. - Caused by unusual heavy rainfall.
- Occurs when the ground becomes saturated with water that has fallen too quickly to be absorbed.
- Can be caused by coastal and river floods.
- Recognised by its quick rise in water level.
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causes of flooding (natural causes)
- Excessive rainfall (river/ flash)
- Storm surge (coastal)
- Snow melt (river)
- El Nino / La Nina (coastal)
- Movement of the Earth’s
surface (tsunamis) -> (river/ coastal)
causes of flooding (human causes)
- Deforestation (river)
- Urban development
- Dam failure
- Climate change (coastal)
Natural causes of floods -> excessive rainfall
** Excess rainwater is unable to seep into the ground - increase in surface runoff when ground is saturated.
- Especially in urban areas as ground is covered with concrete - little infiltration – high surface runoff.
- Surface runoff occurs quickly within a short span of time.
- Water flows swiftly into drains; if the urban drainage system cannot cope with sudden increase – it overflows.
- Need a good system of artificial drainage to prevent floods e.g. drains/canals.
** Rivers overflow their banks when river channels can no longer contain the excess water.
* E.g. Yangtze River and the Yellow River in China frequently overflow their banks after heavy rains because the rain washes large amounts of sediments into the rivers. These sediments are then deposited on the river beds, causing the rivers to become shallower. With time, flooding occurs.
Natural causes of flood -> snow melt
ST John’s
- Places that experience cool temperature climate.
- Melting snow in spring releases large amounts of waters into rivers - sometimes cause rivers to overflow their banks.
- E.g. In 1986, the St John River in Canada overflowed its bank as it could not hold the excess water from melting snow in its channel.
Several homes became submerged in floodwaters and families had to be evacuated.
natural causes of flood -> storm surge
bangladesh
- Cause floods in coastal areas.
- Occurs when strong winds raise the waves in the ocean to exceptionally high levels.
- Gigantic waves crash onto the coast and flood the land.
- E.g. More than 80% of Bangladesh sits on low-lying floodplains of the Brahmaputra and Ganges Rivers. About 35% of the country is less than 6m above sea level. As such it is frequently subjected to regular flooding caused by storm surges. When strong winds blow towards Bangladesh from the Bay of
Bengal, flooding occurs.
*Storm surge brought about by gales (strong winds) and tropical storms; resulting in increase of sea level and flooding of low-lying coastal areas subsequently.
natural causes of flood -> el nino
alternate patterns
Alternate patterns of heating & cooling between eastern & western South Pacific Ocean:
-El Niño is an abnormal warming of ocean at the southeastern part of the Pacific Ocean. It occurs every 3 to 7 years and lasts for several weeks.
-Surface temperature of the ocean off the coast of South America heats up causing heavy rains and floods at southeastern part of the
Pacific Ocean.
e.g. Peru while droughts occur in the southeastern part of the Pacific ocean
e.g. Australia / Indonesia.
natural causes of flood -> movement of the earth’s surface
indian ocean tsunami
- During an earthquake, soil may loosen along the hill or mountain slopes and cause a landslide. Loosen soil when deposited into a river may cause water in the river to overflow.
- Earthquakes that occur under the ocean displaces the ocean floor which generates tsunamis which may bring floods to coastal areas.
E.g. The 2004 Indian Ocean Tsunami, which is triggered by a strong earthquake in the Indian Ocean in Sumatra. Banda Aceh, Indonesia was badly affected as the raging waters of the tsunami engulfed most of the infrastructure of this coastal town (Dec 2004).
natural causes of flood -> la nina
-La Niña follows after El Niño and it occurs every 3 to 5 years.
- Southwestern part of the Pacific Ocean
experiences abnormal warming thus bring heavy rains and floods to Australia & Indonesia.
- Drought like conditions are experienced in Peru in South America (Southwestern part of the Pacific Ocean).
what are the human causes of floods
- deforestation
- urban development
- dam failure
- climate change
human causes of floods- deforestation
- Reasons include creating land for agriculture, urban development, wood for fuel, etc.
- Little / no interception of rain - soil and mud are washed into the river beds causing the rivers to become shallower.
z Little infiltration of rain water into soil - more surface runoff enters river, increase in volume of water flowing into rivers
human causes of floods- urban development
- In urban areas, concrete pavements and tarred roads prevent rainwater from seeping into the ground.
- Clearing of vegetation to construct settlement – less vegetation to intercept the rain resulting in an increase in amount of surface runoff.
E.g. In London, areas around the River Thames experience regular flooding as he natural vegetation around the river has been replaced with concrete pathways and buildings. This has led to increase surface runoff flowing into
the River Thames, causing it to overflow.
human causes of floods- dam failure
- A dam is a barrier built across a river to retain and store water with the purpose of generating hydropower and regulating the amount of water flowing in a river.
- When continuous heavy rain occurs, reservoir overflows or when sediments pile up behind dam to destabilise it.
- Catastrophic flooding will occur.
E.g. Banqiao Dam failure in August 1975 in China, caused an estimated 26 000 deaths when villages downstream were flooded. An additional 204 000 people died from famine and diseases caused by the floods.
human causes of floods- climate change
- Long term change – global warming
(enhanced greenhouse effect) - Temperature increased by 0.5 degrees celcius per decade
- Polar ice caps melt – sea level rises
z Flooding of low-lying cities, countries, islands e.g. London, Bangladesh, Maldives, etc.
what are the 3 types of impacts of floods?
- social
- economical
- environmental
what are the social impacts of floods?
- injuries and loss of lives
- loss of property*
- spread of diseases
social impacts- injuries and loss of lives
- People may be injured by objects and debris washed by fast-moving floodwaters.
- Floodwaters may cause buildings to collapse and injure or kill people.
- People caught in the flood for a long period of time, they can suffer from hypothermia with symptoms such as constant shivering, confusion and loss of co-ordination.
- Floodplains around coastal areas and rivers provide fertile alluvium, making them very attractive places for cultivation of crops– densely populated. Impact is higher when flooding occurs.
- E.g. In 2003, heavy downpour led to rising floodwaters in the north- central Huai River Basin, located in the provinces, Anhui, Jiangsu and Henan in China. The flood claimed 298 lives and forced people to evacuate from the affected areas.
