What are floods?

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Types of floods

floods -- overflows of large amounts of water onto land that is normally dry

Coastal floods

where?

areas along the coast (especially low-lying coastal land - river mouths)

how?

caused by:

tropical storms

tropical cyclones

> strong winds during storms generate large waves (storm surges - bring about higher water levels and cause coastal floods)

tsunamis -- huge ocean waves sometimes reaching heights of over 30 metres

River floods

when?

water in the river rises

water overflows its banks

where?

on the floodplain - the river, and the low-lying land on either side of it

levees -- raised banks on both sides of the river

how?

snowmelt

heavy rainfall

dam failure

example

Mississippi River flood, the United States of America: In 2011, heavy rain and snowmelt upstream caused the Mississippi River to swell to many times its usual size and result in floods

Flash flood - takes place very suddenly and lasts for a short period of time

how?

heavy rainfall

coastal floods

river floods

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Causes of floods

Rainfall -- droplets of water that fall to the ground

how?

condensation on atmospheric water vapour leads to the formation of water droplets

water droplets form clouds

if the water droplets become too heavy, they fall as rain

characteristics

rainfall intensity

rainfall duration

rainfall frequency

example

Hurricane Sandy, New Jersey, United States of America

Snowmelt -- surface runoff that results from melting snow

where?

in temperate countries during the spring season

how?

during winter, ice and snow form on the ground and on the mountains

when spring arrives, the temperature rises and the ice and snow melts

the snowmelt flows over the land and into rivers

therefore, the amount of water in the river increases

river overflows its banks, floods the surrounding areas

example

City of Minot, North Dakota, United States of America: flash flood caused by heavy spring snowmelt and rain in July 2011

Storm surge -- sudden rise of the sea level in which water is piled up against a coastline beyond normal high-tide

how?

during the storm, strong winds push water towards the coast

creates huge waves, resulting in a storm surge

Failure of man-made structure - dams and levees

dam -- barrier built across a river to retain and store water

generates hydropower

helps to regulate the amount of water flowing in a river

example

Banqiao Dam failure, China: one of the biggest dam failures in history because of its high number of casualties