R - Causes of flooding Flashcards
What are the main causes of flooding?
Prolonged/heavy rainfall
Snow melt
Explain how prolonged rainfall causes flooding
After a period of rain, the ground becomes saturated - any further rainfall can’t infiltrate which increases surface runoff which increases discharge
Explain how heavy rainfall causes flooding
It 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 called a flash flood
What physical factors can increase the risk of flooding?
- Sparse vegetation or deciduous trees
- Impermeable ground
- Circular drainage basin
- High drainage density
- Steep slopes
How does sparse vegetation or deciduous trees increase the risk of flooding?
- Sparse vegetation means little vegetation is intercepted so more rain reaches the ground - this increases the volume of water reaching the river, which increases discharge
- Deciduous trees have no leaves in winter - which has the same effect as sparse vegetation
How does Impermeable ground increase the risk of flooding?
Clay soils and some rocks, eg granite and shale, are impermeable which means 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 is frozen the same thing happens
How does a circular drainage basin increase the risk of flooding?
Water draining into the main river channel will arrive in a short space of time because all points in the basins are similar distance from the river - this increases discharge
How does high drainage density increase the risk of flooding?
Drainage basins with high drainage density (lots of streams) drain quickly so have short lag times
Lots of water flows from the streams into the main river in a short space of time, increasing discharge
How does steep slopes increase the risk of flooding?
If the drainage basin has steep-sided valleys, water will reach the river channel much faster because water flows more quickly on steeper slopes - this increases discharge
What are the human factors that increase the risk of flooding?
- Urbanization
- Deforestation
- Flood management strategies
- Agriculture
- Climate change
How does urbanization increase the risk of flooding?
- Urban areas have large areas of impermeable tarmac and concrete, so when it rains surface runoff is rapid
- Gutters and drains quickly take runoff to rivers
- Both of these things reduce lag time and so increase discharge
How does deforestation increase the risk of flooding?
- Clearing trees and plants reduces interception and evapotranspiration - this increases 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
How do flood management strategies increase the risk of flooding?
They can actually make flooding worse eg if dams fail they release a huge volume of water all at once - giving a huge increase in discharge
How does agriculture increase the risk of flooding?
Overgrazing areas leaves areas with less vegetation - so has the same effect as deforestation
( reduces interception and evapotranspiration - this increases the volume of water that reaches the channel, which increases discharge
& 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)
-Overgrazing and ploughing also increase soil erosion
How does climate change increase the risk of flooding?
Could cause an increase in rainfall and more storms in some areas, which could increase flooding
