prolonged rainfall and heavy rainfall are the main causes of flooding
1)it occurs when the discharge of the river is so high that the river spills over the banks into the floodplain.
2) a major cause of flooding is prolonged rainfall as after a long period of rain, the ground becomes saturated and so can't absorb anymore water (can't infiltrate). which causes surface runoff which increases discharge.
3)heavy rainfall can lead to rapid surface runoff if the rainfall is too intense for infiltration to occur. this can lead to a sharp rise in river discharge(flash flood)
4)melting snow and ice can lead to an increase in river discharge.
- means little rainfall is intercepted, so more rain reaches ground
- Deciduous trees have same effect as little rain is intercepted.
- clay soils and some ricks such as granite and shale and impermeable- they don't allow infiltration of surface water. this increases surface runoff which increases discharge.
- if the ground has been baked hard by the heat of the summer or it is frozen the same thing happens- water can't infiltrate, increasing surface runoff and discharge.
Physical factors- continued
circular drainage basin
- water draining into the main river channel will all arrive in a short space of time because all points in the basin area are a similar distance from the river. this increases discharge.
high drainage density
- drainage basins with a high drainage density(lots of streams) drain quickly so have short lag times. lots of water flows from steams into the main river in a short space of time, increasing discharge.
- if the drainage basin has steep sided valleys water will reach the river channel much faster as water flows more quickly on steeper slopes. this increases discharge.
- urban areas have large areas of impermeable tarmac and concrete, so when it rains surface run off is very rapid.
- gutters and drains quickly take run off to rivers
- both of these things reduce lag time and so increase discharge.
- clearing tree's and plants reduce interception and evapotransporation. this increase the volume of water that reaches the channel, which increases discharge.
- deforestation leaves the soil loose. The soil is eroded by rainwater and carried to the river, which raises the river bed, this reduces the channel capacity so it takes less water for the river to flood,
Human Factors - continued
Flood management strategies
- can make flooding worse. e.g. if dams fail they release a huge volume of water at once.
- overgrazing leaves area with less vegetation so has the same effect as deforestation. overgrazing and ploughing also increase soil erosion.
Social impacts of flooding
- 1) people and animals can be killed.
- 2)floodwater is often contaminated with sewage , this can lead top a lack of drinking water.
- 3)contaminated water can put people at risk of diseases.
- 4)possessions can be damaged by floodwater or lost.
- 5)people can be made homeless as their properties are inundate or damaged.
- 1)businesses often have to shut down as premises are inundated and power supplies are affected.
- 2)rescue work and repairs are usually costly. insurance premiums go up after the flood.
- 3)unemployment levels often rise as businesses shut down as they can't recover from flooding.
- 4)public transport, roads and bridges can be destroyed.
- 5)crops can be destroyed. this can lead to a rise in the price of food.
- 1)floodwater contaminated with sewage and rubbish can pollute rivers.
- 2)river banks are eroded
1)river sediment is deposited on the flood plain, this makes the land more fertile.
2)wet lands can be created e.g.marshland and ponds