A flood occurs when a river bursts its banks and the water spills onto the floodplain. Flooding tends to be caused by heavy rain: 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.
The following factors may encourage flooding:
A steep-sided channel – a river channel surrounded by steep slopes causes fast surface run-off.
A lack of vegetation or 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.
A drainage basin, consisting of mainly impermeable rock – this will mean that water cannot percolate through the rock layer, and so will run faster over the surface.
A drainage basin in an urban area – these consist largely of impermeable 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 run-off.
Flood management techniques include river engineering, afforestation and planning controls to restrict urban development on floodplains.