Flooding occurs when the discharge of a river is so high that the river spills over its banks onto the floodplain.
- Prolonged rainfall - after a long period of rain, the ground becomes saturated so any further rainfall can't infiltrate, increasing surface runoff which increases discharge.
- Heavy rainfall (e.g. thunderstorms) - can lead to rapid surface runoff if the rainfall is too intense for infiltration to occur. Leads to a sharp rise in river discharge called a flash flood.
- Melting snow and ice - leads to huge increase in river discharge
Physical Factors that increase the risk
Sparse vegetation/deciduous trees
- Little rainfall is intercepted so more rain reaches the ground, increasing the volume of water reaching the river, increasing discharge
- Deciduous trees have no leaves in winter - same effect as sparse vegetation
- Impermeable rock/surfaces don't allow infiltration of surface water - increase surface runoff - increase discharge
- If the ground is too hard due to heat or freezing, infiltration cannot occur - increase surface runoff - increase discharge
Circular drainage basin
- Water draining into the main channel will all arrive in a short space of time as all points in the basin are a similar distance from the river - increases discharge.
Physical Factors that increase the risk cont.
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 the streams into the main river in a short space of time - inc. discharge
- If the drainage basin has steep-sided valleys, water will reach the channel faster because water flows more quickly on steeper slopes - inc. discharge
Human Factors that increase the risk
- Large areas of impermeable tarmac and concrete - rapid surface runoff
- Gutters and drains rapidly take runoff to rivers
- Both of these reduce lag time so increase discharge
Flood management strategies
- If dams fail they release a huge volume of water all at once, giving a huge increase in discharge
Deforestation and Agriculture
- Clearing trees and plants (like overgrazing by animals) reduces interception and evapotranspiration - increases volume of water reaching the channel, increasing discharge
- Deforestation leaves the soil loose - the soil is eroded by rainwater and carried to the river, raising the river bed - this reduces the channel capacity so it takes less water for the river to flood.
Human Factors that increase the risk cont.
- Could cause an increase in rainfall and more storms in some areas, increasing the risk of flooding
- The social impact is usually higher in poorer countries as flood defences are of poorer quality, people are less able to evacuate, sanitation systems aren't as good and buildings are of poorer quality
- The absolute economic impact is usually higher in richer countries as they have more high value buildings and infrastructure
- The relative economic impact is usually higher in poorer countries as the buildings and crops that are damaged are worth less money but this affects the economy more because they have less money to recover from it
Social and Economic impacts
- People and animals can die
- Floodwater is often contaminated w/sewage, which can lead to a clean drinking water shortage
- Contaminated water can also put people at risk of disease e.g. diarrhoea and dysentery
- Possessions can be damaged by floodwater or lost
- People can be made homeless as their properties are destroyed or damaged
- Business often have to shut down due to damage and power supplies are affected
- Rescue and repairs are costly - insurance premiums go up after floods
- Unemployment levels rise as business shut down
- Public transport, roads and bridges can be destroyed
- Crops can be destroyed - can lead to a rise in food prices
- Floodwater contaminated with sewage and rubbish pollutes the rivers
- River banks are eroded
- River sediment is deposited on the flood plain making the land more fertile
- Wetlands can be created which are habitats for many species
Floods of a very large magnitude are not very frequent. Small floods are more frequent. Large floods usually occur due to unusually heavy or prolonged rainfall.
By keeping records over many years, people can predict how often a flood of a certain magnitude may occur - this is the flood return interval (RI).
A small flood may have an RI of one or two years.
A large flood may have an RI of 100+ years.
RI = number of years of observation + 1
the rank order
The size of the largest flood for every year is placed in rank order, with 1 being the largest for all available records for any given river.
Case study - Bangladesh 2007 - Primary Effects
- 2000+ deaths - many people reluctant to leave land and livestock, and children drowned as they couldn't swim
- Poor transport meant evacuation was slow
- Approx. 25 million made homeless
- 44 schools destroyed
- 112 000 houses destroyed
- Many factories closed
- Lots of livestock killed - cattle farmers lost their livelihoods
- 10 000km of raods destroyed - access to Dhaka severely affected
- Rivers polluted with rubbish and sewage
- Floods caused river bank erosion, especially on embankment areas close to the main channels
Case study - Bangladesh 2007 - Secondary Effects
- Floodwater placed population at risk of water-borne diseases e.g. diarrhoea, dysentery, typhoid, cholera
- Children lost out on education - approx. 4000 schools affected by floods
- Flooded fields reduced basmati rice yields - prices rose 10%
- Many farmers and factory workers became unemployed