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Causes of flooding

Causes of flooding

  1. 1. Why do we manage River Basins?• Irrigation for agriculture• Constant clean drinking water• Steady water supply for industry• Better navigation• Tourism (lakes/boating/recreation)• Hydro-electricity• Flood Prevention/alleviation
  2. 2. Focus On Flooding...and how to alleviate it
  3. 3. Causes of flooding Physical Human Intense precipitation  Changes in land use Prolonged rainfall  Urbanisation Snow Melt or Ice Thaw  Climate Change Storm Surges Landslides  Poor dam construction Volcanic Eruptions  Poverty
  4. 4. Vaiont River Valley, Northern Italy In 1963, a majorrock slide resulted inthe deaths of over2600 people. Theslide block movedsuddenly into thenewly filled VaiontReservoir, flushinglake water up & overthe dam. The wall ofwater was over 200 fthigh as it swept intonearby villages,wiping out everythingin its path.
  5. 5. Direct/Hard engineering strategies:2. Storage - Dams/Reservoirs3. Channelisation – enlargement, extra channels linked by sluice gates, permanent diversions (often straight).4. Levees5. Dredging
  6. 6. Indirect/soft/behavioural strategies1. Land Use Regulation (washland/wetland – land left deliberately to flood)2. Warning Systems (allows for evacuation)3. Afforestation (usually on slopes of upper sections of the river basin…sometimes called ‘flood abatement schemes’ along with modified agricultural practices)4. Flood Proofing measures – houses on stilts. Or better built to withstand flood conditions.5. Flood damage insurance
  7. 7. Cost/benefit analysisSTRATEGY BENEFITS COSTSRegulations Low cost, promotes Possible losses of livestock suitable use of landReservoirs Reduces flooding & costs. Cost of construction and Multipurpose use maintenance. EnvironmentLevees Reduces flooding by Expensive to strengthen. holding more water Catastrophic if breached.Dredging More water can be Costly & needs to be repeated contained. Affects local ecosystem.Channelisaton Reduces flooding in area May increase flooding of strengthening downstream.Insurance Spreads cost Reliance on public moneyWarning system Less damage, loss & Little use if ignored. death Response not solution
  8. 8. Flooding in the Tees Valley• Heavy rainfall & steep relief in the upperPennines. Low infiltration, impermeable rock• Elongated river basin, with many tributaries.• It is a ‘flashy’ river system. Almost instantresponse to a rainfall event, with a small lag time.• High tides (coastal flooding)• Draining of peat bogs & new drainage systemsfor intensive agriculture had decreased soil storage& increased run off• Urbanisation - building on flood plains beforestrategic land use planning.
  9. 9. Tees Valley River Management• Reservoirs e.g. Cow Green reservoir (1970) – regulates river flow by storing excess water, so reducing flooding and the water can be released in times of low flow.• The Tees Barrage• Tunnel used to add water from the Kielder at times of low flow• New developments discouraged in low lying areas• Yarm flood defence system (gabions, meander cut)
  10. 10. Tees Valley River Management: Other options• Improved flood warning system & emergency reaction procedures• Diversionary spillways• Re-routing of rivers• Embankments/levees• Afforestation of the catchment• Dredging• Cutting of meanders
  11. 11. Bangladesh: coexisting with floods
  12. 12. Why Bangladesh floods: Physical causes• The country is a giant flood plain - 70% of Bangladesh is less than 1m above sea level• Rivers, lakes & swamps cover 10% of the land area• Tropical cyclones & monsoon rain bring heavy rain & storm surges• Snowmelt from the Himalayas increases discharge
  13. 13. Why Bangladesh floods: Human causes• Global warming• Deforestation in Nepal• Dam building in India• Urbanisation
  14. 14. Bangladesh:management strategies pre-87• Hard’ embankment building schemes, such as the Meghna-Dhonagoda irrigation project• Dredging• River diversion & channelisation
  15. 15. Current proposals• Stream storage by building 7 huge dams taking 40 years to complete• 12 to 15 floodplain retention basins to absorb excess flow diverted from the main rivers and released after the main floodwaters subside• Lowering of water table in the Himalayas• Avoid all ‘hard’ schemes & advocate better warning systems, improved flood shelters & emergency services
  16. 16. New focus on ‘softer’ strategies• High–tech weather data analysis & forecasting systems• New & improved flood shelters• Education & advance warning system (volunteers on bikes with megaphones)• Better organised emergency services & aid teams• Embankments around major urban areas, whilst letting low lying rural areas flood
  17. 17. Future trends & issues• In 2003, disasters stemming from natural hazards affected seven times more people than those tied up with conflict.• More than two billion people around the world could be at risk of flood devastation by 2050, according to United Nations University forecasts. Today floods happen mainly in Asia, but climate change and overpopulation will make them a real and constant threat in many other regions.
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