Bangladeshi pedestrians affected by flooding

Floods have a variety of causes including heavy rainfall, tidal surges and the failure of manmade structures such as dams. During a flood an area of land that is normally dry is covered by water.

Heavy rainfall over a short time period caused the 2004 Boscastle flood in north Cornwall, UK. The village of Boscastle, which is located in a steep-sided valley, was overwhelmed when rivers running through the area broke their banks.

Some floods are seasonal. Large areas of Bangladesh, for example, are submerged each year when the monsoon rains cause that country's rivers to break their banks. Monsoon floods in Bangladesh and other parts of the world take lives, make thousands homeless and spread disease.

Image: Pedestrians attempt to stay dry in Dhaka, Bangladesh, during flooding on 28 July 2009 (credit: AFP/Getty Images)

Introduction

Bangladeshi pedestrians affected by flooding Floods

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Floods

A flood is an overflow of water that submerges land which is usually dry. The European Union (EU) Floods Directive defines a flood as a covering by water of land not normally covered by water. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Flooding may occur as an overflow of water from water bodies, such as a river or lake, in which the water overtops or breaks levees, resulting in some of that water escaping its usual boundaries, or it may occur due to an accumulation of rainwater on saturated ground in an areal flood. While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, these changes in size are unlikely to be considered significant unless they flood property or drown domestic animals.

Floods can also occur in rivers when the flow rate exceeds the capacity of the river channel, particularly at bends or meanders in the waterway. Floods often cause damage to homes and businesses if they are in the natural flood plains of rivers. While riverine flood damage can be eliminated by moving away from rivers and other bodies of water, people have traditionally lived and worked by rivers because the land is usually flat and fertile and because rivers provide easy travel and access to commerce and industry.

Some floods develop slowly, while others such as flash floods, can develop in just a few minutes and without visible signs of rain. Additionally, floods can be local, impacting a neighbourhood or community, or very large, affecting entire river basins.

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