Causes of flooding

A flood occurs when a river bursts its banks and the water spills onto the flood plain. Flooding tends to be caused by heavy rain: the faster the rainwater reaches the river channelriver channel: The river channel is the part of the river that holds the water, 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:

Helicopter carrying British aid across flood water, Mozambique

• A steep-sided channel. A river channel surrounded by steep slopes causes fast surface run-off [surface run-off: water running over the surface of the land ].
• 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 basindrainage basin: A drainage basin is the area of land that is drained by a river and all its tributaries. consisting of mainly impermeableimpermeable: When a substance is described as impermeable it means that fluids, especially water, are unable to pass through it. 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. Urban areas 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.