The likelihood of a river bursting its banks and flooding is determined by factors in the surrounding landscape, such as steepness of the river valley, the amount of vegetation and the prevailing rock-type. The short-term impact of floods can be catastrophic, but they can have positive long-term effects as well.
Causes of flooding
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 fastsurface 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 mainlyimpermeable 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.
The impact of flooding
Floods can cause damage to homes and possessions as well as disruption to communications. However, flooding can also have positive impacts on an area.
Flooding deposits fine silt (alluvium) onto the floodplain, making it very fertile and excellent for agriculture. People living on or near floodplains may rely upon regular flooding to help support their farming and therefore provide food.
LEDCs tend to be affected more than MEDCs by the effects of flooding. This is partly because LEDCs have more farms, and farming communities are attracted to fertile flood plains. LEDCs often do not have the resources to prevent flooding or deal with the aftermath of flooding.