River Floods

A flood is when a river bursts its banks on to normally dry land.

Causes

The physical causes of flooding are:

1.Prolonged precipitation which saturates the soil until the water table (the level of saturated ground in the soil) reaches the ground surface.

2.Intensive precipitation over a short period of time. This is more likely to lead to after a long period of drought when the soil will be baked hard and it is more difficult for the rainfall to infiltrate the soil.

3.Rapid snow melt when the ground below is still frozen makes it difficult for the excess moisture to infiltrate the soil.

In addition the shape and nature of the drainage basin (the area drained by a river) influences the likelihood of a flood event. Steep sided drainage basins with impermeable rock and little vegetation are the most likely to flood.

The human causes of flooding are:

1.Urbanisation means more surfaces of concrete and tarmac which are impermeable. Precipitation cannot infiltrate these surfaces and is channelled into drainage networks and gets to the river very quickly, making it more likely to flood. Also as urban populations increase houses are built on more marginal land including flood plains.

2.Deforestation can cause flooding as trees are excellent at intercepting rainfall and storing water and when they are removed the precipitation will reach the river channel more quickly.

3.The building of levees (an embankment built to prevent a river overflowing) and dams can reduce flooding but if these protection measures fail they can cause catastrophic flooding.

River Management

The likelihood of flooding can be reduced with hard engineering (large man made solutions) or soft engineering (natural sustainable) techniques.

Further Reading & Research

Flooding of the Somerset Levels in 2014 (Telegraph)

Somerset floods crisis: How the story unfolded (BBC)