Why does flooding occur?

A flood occurs when a river bursts its banks and the water spills onto the floodplain.

The causes of flooding are usually a result of both physical and human factors.

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Physical factors are those which are caused by nature. Human factors are when human intervention and interference create the conditions for flooding.

Physical factors

Illustration showing four phyiscal causes of flooding.Physical causes of flooding

Climate

Flooding is caused by heavy rainfall. The faster the rainwater reaches the river channel, the more likely it is to flood. The nature of the landscape around a river will influence how quickly rainwater reaches the channel.

A steep-sided channel

A river channel, surrounded by steep slopes, causes fast surface run-off.

A lack of vegetation/woodland

Trees and plants intercept precipitation, ie they catch or drink water. If there is little vegetation in the drainage basin then surface run-off will be high.

Impermeable rock

A drainage basin, consisting of mainly impermeable rock will mean that water cannot percolate through the rock layer, and so will run faster over the surface.

Human factors

Drainage basins in urban areas

A drainage basin in an urban area consists largely of impermeable rock and concrete, which encourages overland flow. Drains and sewers take water quickly and directly to the river channel. Houses with sloping roofs further increase the amount of overland flow.

Deforestation

In areas where trees and forests have been purposely cut down and destroyed by humans, flood risk dramatically increases. This is because trees intercept and absorb water as part of photosynthesis. Without trees, rainfall directly hits the surface and may quickly saturate soils or run-off into nearby rivers, causing the rivers to rise quickly.