3. Why do floods occur?

Floods mainly occur as a result of a rise in water levels above the normal level or obstruction of the water flow. The speed of water movement and the time taken to reach the threatened properties are decisive for the scale of the impact.

The reasons for increases in water levels above the normal level are as follows:

  • Heavy or prolonged rainfall or intensive melting of ice (of glaciers)
    • The main reason for the occurrence of floods along river valleys, lowlands and plains, changes in the water levels of lakes, etc.;
  • The effect of a strong wind
    • The main reason for coastal floods and floods along river deltas; particularly dangerous when the high wind is combined with high tides (as in the Philippines). The causes of the strong wind are sea storms, hurricanes and monsoons, etc.;
  • Obstruction of riverbeds
    • Obstruction of rivers beds due to vegetation or blocks of ice from glacier or landslides, etc.
  • Destruction or malfunctioning of hydraulic structures
    • Dam walls, dykes and drainage pipes, etc.
  • Levees failure due to erosion, alterations produced by wildlife and alterations produced by wild vegetation.
  • Underwater earthquakes (underwater or island volcanoes)
    • Resulting in waves/tsunamis which are the main cause of catastrophic floods;
  • Climate change
    • The reason for the increase in the intensity and consequences of floods.¬† Contributes to the combination of a number of flood causes, resulting in a significant increase in the scale of severe flooding and the damage caused;
  • Human activity
    • Human activity is a key cause of flooding and especially increases in the damage caused. Clearance of forests, changes to riverbeds (e. g. meanders cut off) causing¬† more intense hydrologic dynamics, building work in areas at risk of flooding and poor maintenance of hydrotechnical installations are some of the causes of flooding. Land use change (including urban, industrial, agricultural, forestry, and wildlife issues).
Fig.3. Aerial survey carried out by the Civil Protection of the area flooded after the levee failure (visible in the SW side of the image) occurred along the Secchia River, North of Modena (Po Plain, Northern Italy) on 19 January 2014. The hydraulic and geotechnical mechanisms were triggered by the activities of burrowing animals leading to levee failure (Orlandini et al., 2015).