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Firstly, it occurs due to heavy rain and storms during the monsoon season that leaves a lot of rainwater behind on land. This may in turn cause flash floods, where floods move at great speeds, causing destruction and casualties. This is partly because there is too much rainwater to be drained out of the area using pipes and partly because of the height of the land above sea level. For example if the piece of land is below sea level, it would be greatly affected whereas those above sea level would not encounter the same situation as the water from the land will flow downwards either to a lower piece of land or a water body. Poor sewage/drainage systems may also contribute to flooding.
Secondly, it occurs due to natural disasters such as cyclones and hurricanes when they sweep across the sea or ocean, bringing a lot of water with them and as they touch land, the water is washed onto land by its powerful high-speed winds. A tsunami may also cause floods by washing seawater onto land when its storey-high waves reaches land. Earthquakes can also contribute in a way that it blocks the movement of a river, clogging up the river and causing an overflow of water in the river, flooding the lands near it. Melting of snow may also be another reason, as when the snow thaws after winter, the water might not be drained off or evaporated that quickly.
Lastly, global warming may be another cause of floods. Due to global warming, sea levels are rising as the ice caps at the north and south poles are melting significantly. Sea levels are increasing at an alarming rate of 20+mm/year! If this continues, many more islands and lands below sea level such as Netherlands and Bangladesh may vanish under the seas forever. Currently, more than 30 small islands have been covered totally by seawater. These islands are usually small islands "stranded" in the middle of the oceans or seas, making them vulnerable to the situation. Others are submerged under sea levels due to the effects/aftermath of tsunamis.
Flooding occurs most commonly from heavy rainfall when natural watercourses do not have the capacity to convey excess water. However, floods are not always caused by heavy rainfall. They can result from other phenomenon, particularly in coastal areas where inundation can be caused by a storm surge associated with a tropical cyclone, a tsunami or a high tide coinciding with higher than normal river levels. Dam failure, triggered for example by an earthquake, will result in flooding of the downstream area, even in dry weather conditions.
Other factors which may contribute to flooding include:
- volume, spatial distribution, intensity and duration of rainfall over a catchment
- the capacity of the watercourse or stream network to convey runoff
- catchment and weather conditions prior to a rainfall event
- ground cover
- tidal influences.
Number of ways that the flood effect in our life.
- Economic loss
- The flooding in Jiangxi of China in 1998 caused great damage. The economic loss was HK$156 billion, 400 buildings surrounding the lake were inundated, leaving more than 1 million people homeless.
- After flooding, government has to input many resources for aiding or reconstruction, e.g., police force, fire control, aid worker, resources used for resisting flood, etc. This also brings loss to society.
- Flooding will lead to the damages of roads, collapse of bridges or traffic congestion, which may affect the daily operation.
- Flooding brings too much water which will cause damage to farmland.
3. Human Beings
- People died and lose their homes
- Flooding will cause death and injuries. In year 1998, 0.23 billion of people were affected in the flooding of Xian, including 3 thousands people dead, 1 million of people lost their homes. In 1996, the monsoon flood in India affected more than five million people in the northern and eastern part of the country. Severe floods have also killed some 200 people in India and Bangladesh and left millions homeless and starvation.
- Flooding usually brings infectious diseases, e.g. military fever, pneumonic plague, dermatopathia, dysentery, common cold (type A), breakbone fever, etc. And for those areas which have no electric supply due to flooding, food poisoning may occur as food may not be properly frozen.
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