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Factors Affecting Flood Risk

When the amount of water in a river is greater than the river's capacity, flooding will happen. Increasing river level is caused by increasing river discharge. There are human and physical factors that can increase discharge, and therefore cause flooding:

Precipitation and storms

Precipitation and storms

  • Heavy rainfall causes high runoff because the water arrives so quickly that there isn't enough time for it to infiltrate the soil.
  • Prolonged rainfall makes the soil become saturated, and so any more rainfall will run off instead of infiltrating.
  • If storms are more frequent have more rainfall, then the risk of flooding will rise.
Geology

Geology

  • Runoff is higher in areas that are mainly made up of impermeable soil (e.g. clay) or impermeable rock (e.g. granite).
Relief

Relief

  • Rainfall will reach the river channel quicker in a steep-sided valley.
Land use

Land use

  • Buildings and roads are usually made of impermeable materials. More impermeable surfaces mean more runoff and more discharge.
  • Trees intercept and store water that they take up from the ground. Removing trees will, therefore, increase flooding risk as more water will be finding its way into the river channel.
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