Hydrological Cycle - Water Surpluses (Flooding)

Introduction:
  • Flooding is a major surplus in the water cycle,
  • Flooding occurs when too much water exceeds the capacity of the river channel i.e the river overflows onto the land
    • Mainly caused by prolonged rainfall
    • Snow and ice melt in the warmer seasons also contributes to flooding.
  • Flash floods occur very quickly due to a large rapid input such as rain storms and rapid runoff due to sleep slopes, little infiltration, and no vegetation.
    • This causes very rapid peak discharge and can even cause a surge of water in the river channel.
  • Coastal flooding can also occur, but this is not hugely important (see Coastal Landscapes).
Causes Of Flooding:
Physical:
Human:
Intense precipitation
Changes in land use
Prolonged rainfall
Urbanisation
Snow melt or ice thaw
Climate change
Storm surges
Poor dam construction
Landslides
Poverty
Volcanic eruptions
Causes Of Flooding In Bangladesh:
  1. Monsoon Climate: Brings very heavy rain and snow. Soils are leached and heavy runoff results in soil erosion.
  2. Spring Snow Melt: Results in soil erosion and a rapid increase in river discharge.
  3. Deforestation In Headwater Areas: Due to an increasing population in Nepal, trees are cleared for fuel and grazing land. There is less evapotranspiration, more runoff and faster soil erosion. Landslides also occur.
  4. Rivers slit due to increased soil erosion. This raises the river bed and reduces the capacity of the channel resulting in an increased likelihood of flooding.
  5. 80% of Bangladesh lies on a huge floodplain and delta, most of which is only 1m above sea level.
  6. The Ganges has been diverted for irrigation purposes, this removes some of the silt and prevents the floodplains further downstream from being built up.
  7. Cyclones are very frequent in Bangladesh.
Case Study 1: Pakistan Floods 2010:
Background:
  • Mainly built on the Indus River plain.
  • The climate is generally arid whilst most of the rainfalls during the monsoon season with over half of the annual rainfall falling in July & August.
Causes:
  • A monsoon depression formed over the Bay of Bengal and crossed Pakistan on the 27th July 2010.
    • The rainfall became intense as another depression formed and then merged with yet another depression from the west. This meant that 203 mm of rain fell in three days.
  • Climate change was another cause as officials suggest that hundreds of trees was deforested illegally which would lead to less infiltration
  • Snowmelt from Mount Godwin also contributed as the weather became hotter as warm fronts crossed the area.
Impacts:
  • 23% of the crops were completely destroyed. Farmers rely on the weather to make money in the summer and the crops were gone so they could not make any money during this time.
  • Reviving Mangroves: The mangroves trees were reviving themselves and in 2010, the reviving ones were completely lost.
  • Habitat loss: 80% of habitats for reptiles were gone in the Swat river catchments.
Human Impacts:
  • 1781 dead, 2966 injured
  • 20 million people affected, £1.5 billion agricultural loss, 1.9 million homes damaged
  • Infrastructure was damaged
  • Food shortages and the spread of disease.
Human Actions To Reduce Or Increase Flood Risk:
  • Deforestation in the Amazon, Himalayas and UK means that there is less interception and evapotranspiration leading to more runoff, water will reach the river faster and flash flooding is more likely.
  • Hard engineering can change the flood risk by ‘mis-managing’ by actually increasing the flooding risk.
    • Straightening river channels to get the water through an area fast. There will be more flooding further downstream as water backs up. This also removes natural banks and vegetation increases the speed of flow which leads to more flooding downstream.
  • Land use changes in the catchment area changes runoff speed and reduces lag time.
    • Urbanisation is the biggest human factor as humans are building on land which is not very permeable which would speed up runoff and reduce soil moisture storage as well as reducing interception, infiltration.
Case Study 2: UK Floods 2012:
Background & Causes:
  • After a long and hot summer, September showed the first signs of what was going to come in the winter – lots of weather fronts which were already bringing lots of rain.
Impacts:
  • Loss of infrastructure. The A83 road in Scotland was partially destroyed
  • 40 people died – many old people died.
  • Over 500 homes flooded across the UK
  • 300 homes flooded just in Cumbria.
  • People in Worcestershire criticised the new flooding defence which was put into place after they thought that they were not working.
  • 816 homes affected
  • Landslides leading to some homes having to be evacuated
  • Some farmers fields were affected because of the vast amount flooding leading to a surplus of soil moisture storage.
at