Causes of Flooding
- Flooding occurs when the capacity of the river channel is exceeded
- The water flows over the banks and onto the surrounding area
- Flooding is usually the result of heavy or prolonged rainfall
- The key factor is the time taken for the precipitation to reach the river from where it falls - the lag time
- The shorter the lag time, the higher the risk of flooding because the discharge of the river increases faster
- The lag time depends on some human and physical factors
- These lead to increased overland flow which shortens the lag time
Human and Physical Factors which Increase the Risk of Flooding
| Human Factors | Impact |
| Deforestation |
Lack of trees reduces interception and infiltration, increasing overland flow |
| Urbanisation |
Impermeable concrete and tarmac increase overland flow Water flows into the drains reaching the river rapidly |
| Agriculture |
Bare soil and ploughing increase overland flow |
| Climate Change |
Rising global temperatures may increase storm frequency and intensity |
| Physical Factors | Impact |
| Relief |
Steep slopes reduce infiltration and increase overland flow |
| Rock Type |
Impermeable rocks reduce percolation and increase overland flow |
| Soil |
Frozen, saturated or compacted soil reduces infiltration and increases overland flow Some soil types such as clay reduce infiltration and increase overland flow |
| Weather |
Heavy or prolonged rainfall means that the rate at which water reaches the surface exceeds the infiltration rate leading to increased overland flow After a period of snow rising temperatures can cause rapid melting which increases overland flow |
| Seasonal Variations |
Flooding in Northern Europe tends to occur in the autumn and winter when rainfall is more frequent In areas affected by monsoons, much of the annual rainfall occurs in a few weeks saturating the ground and increasing overland flow Higher temperatures in spring lead to snow melt in mountainous areas increasing overland flow |
| Drainage Density |
Where drainage density is high there are many tributaries taking water to the main channel causing a rapid increase in discharge |
| Vegetation |
Where there is little natural vegetation there is reduced interception leading to increased overland flow |
Exam Tip
In the exam, you may be asked to explain the causes of flooding. Remember this means that you need to give connections between factors such as deforestation and the increased flood risk. Rather than simply stating that deforestation increases flood risk you need to explain the reasons - decreased interception and infiltration, leading to increased overland flow and shorter lag time.
