Physical causes of flooding
Flooding occurs when a river's discharge exceeds the capacity of its channel to carry that discharge. The river over flows its banks. Flooding may be caused by a number of natural causes or physical factors:
- excessive levels of percipitation occuring over a prolonged period of time. This eventually leads to the saturation of the soil. When the water reaches the ground surface, there is increased overland flow or runoff.
- intensive percipitation over a short period of time, particulary when the ground surface is baked hard after a long period without rainfall. In such circumstances the infiltration capacity is such that the ground cannot soak up the rainfall quickly enough, so more water reaches the river than would normally be the case.
- the melting of snow, particulary when the subsoil is still frozen, so the infiltration capacity is reduced.
- climatic hazards such as cyclones in Bangledesh, hurricanes in the Gulf of Mexico or deep low-pressure weather systems in mid-latitudes bringing abnormally large amounts of percipitation.
The nature of the drainage basin has an influence on the likelihood of flooding. Some drainage basins are more likely to flood than others. Relief, vegetation, soil type and geology all have a part to play. In areas of the world vegetated by dense forest, interception ammd uptake by plants reduce the risk of flooding during times of heavy rainfall.