5.5A - Meteorological Causes of Flooding
Meteorological causes of flooding include: intense storms leading to flash flooding, unusually heavy or prolonged rainfall, extreme monsoonal rainfall and snowmelt.
Surplusses within the hydrological cycle more often than not mean flooding. The meteorological causes of flooding are:
- intense storms, which lead to flash flooding (short lag time), as in semi-arid areas but more common in mountainous areas
- prolonged, heavy rain, such as during the Asian monsoon and with the passage of deep depressions across the UK
- rapid snowmelt during a particularly warm spring, as on the plains of Siberia.
Bangladesh is a particularly food-prone country mainly because it is a land of floodplains and deltas built up by mighty rivers such as the Ganges, Padma and Meghna. These rivers are swollen twice a year by meltwater from the Himalayas. and by the summer monsoon. Hilly tracts between the rivers and behind Chittagong are often victims of flash floods.
There is also tidal flooding, often a result of storm surges or when high river flows meet particularly high spring tides in estuaries. A storm surge is caused by very low air pressure which raises the height of the high-tide sea. Strong onshore winds then drive the 'raised' sea towards the coast, often breaching coastal defences and flooding large areas.
The likelikhood of flooding is also increased by other physical circumstances:
- in low-lying areas with impervious surfaces, as in towns and cities
- where the ground surface is underlain by impermeable rocks
- when ice dams suddenly melt and the waters in glacial lakes are released
- where volcanic activity generates meltwater beneath ice sheets that is suddenly released (jökulhlaups)
- where earthquakes cause the failure of dams or landslides that block rivers