case studies

• The Mississippi is 3800 km long, goes through 10 states, and has a drainage basin covering 1/3 of the USA.
• Causes of 1993 Mississippi floods:

  • heavy rain in April saturated upper basin
  • thunderstorms in June caused flashfloods
  • 180 mm of rainfall in 1 day in July
  • levees in nearby towns collapsed
• Effects of 1993 Mississippi floods:

  • 43 deaths
  • 50, 000 evacuated
  • 26, 000 km of land flooded
  • $12 billion damages
  • buildings destroyed
  • disease from sewage
  • insurance claims high
  • stagnant water attracted rats and mosquitoes
• management of Mississippi:

  • 6 dams
  • 105 reservoirs
  • afforestation
  • levees
  • concrete mattressing
  • straightening course
  • less construction on floodplains
• The Chang Jiang River is 6380 km long, with 400 million people living in its valley. It floods every year due to monsoon rain and snowmelt from the Himalayas.
• Human causes of Chang Jiang flood:

  • increasing population forces more people to live on the floodplain
  • industrialisation put factories on the countryside floodplain where the houses are less protected
  • less space on floodplain for river due to urban areas
  • deforestation and overgrazing
  • straightening with canals decreases lag time
  • levees
  • lakes are much smaller for agriculture
  • global warming increased monsoon intensity
• 1998 Chang Jiang floods:

  • 1 in 100 year flood due to extra month of monsoon
  • 240 million people affected
  • 4000 people drowned
  • crops and cattle destroyed
  • 14 million people homeless
  • factories destroyed so millions of unemployed cost the economy greatly
  • flood defences failed
• The Three Gorges Dam is the largest dam in the world, as it is 2.3 km long, with a hydroelectric power plant that generates 18000 MW. It cost $38 billion.
• Advantages of TGD:

  • 50 million people protected from floods
  • land protected and irrigated
  • produces 10% of energy, all renewable
  • 10 000 tonnes ships can now sail
• Disadvantages of TGD:

  • 150 towns, and 1300 villages were flooded by the reservoir
  • 1.2 million people had to relocate with no compensation
  • pollution from flooded factories entered ecosystems, threatening Siberian crane
  • earthquakes could cause dam to collapse
  • water from reservoir causes more frequent landslides
  • silt from landslides reduces reservoir capacity
  • lack of silt in soil means more fertilisers needed
  • Over 1000 cultural sites were flooded, including the Zhang Fei temple
• A forest between Sichuan and Yunnan was cut down in the 1960s for farmland and timber, but it is being replanted to increase interception, evapotranspiration, infiltration, and to decrease soil erosion. The trees planted are not native so they don't help the ecosystem.
• Soft Engineering - River Severn:

  • flood zoning - areas to be flooded in Aberystwyth, 100 m from channel
  • peat bogs - natural sponges in Cors Caron
  • afforestation - planting trees to increase infiltration, interception, evapotranspiration
  • log jams - slow river flow and make wetlands with beavers in Devon