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Rivers 3


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Rivers 3

  1. 1. Topic: Rivers 3River Management Strategies• Two types: 1. Channelisation Strategies: • River re‐alignment • River re‐sectioning 2. Bank protection strategies: • Dykes/ Embankments • Vegetation planting/  clearing• Effectiveness of strategies
  2. 2. River Channel Management Why manage river channels?
  3. 3. Flooding occurs when:1. Clearance of vegetation  increase surface  runoff  sudden increase in river volume2. Period of heavy and continuous rainfall  river exceeds carrying capacityThese may be triggered by:• Global warming – Extreme weather phenomena called El Nino and  La Nina effect
  5. 5. River Channel Management  StrategiesTwo types of strategies 1. Channelisation Strategies: Change the river  • River re‐alignment channel shape/  • River re‐sectioning length, etc. 2. Bank protection strategies: • Dykes/ Embankments Make changes to  • Vegetation planting/ clearing the river banks
  6. 6. 1. Channelisation  Strategies • River re‐alignment• River re‐sectioning
  7. 7. 1.1 River Re‐alignment• Straightening of river channel to increase  speed of river  water to flow away from an  area more quickly & wash away sediments  accumulated on river bed  reduce flooding  in that area Source: n/fcd/policy/mrcomp/ f
  8. 8. River re‐alignmentE.g. MississippiRiver (USA) has Straighteningbeen shortened of river = riverby 240 km to re-alignmentreduce threat offlooding.
  9. 9. 1.2 River Re‐sectioning• Widening and deepening of river channel.• Increase channel’s ability to hold water decrease likelihood of floodsOr,• Smoothening of river beds and banks with  cement and granite  less friction allow  water to flow away more quickly  reduce  likelihood of floods • E.g. Singapore River ay Boat Quay
  10. 10. River re‐sectioning Widen & Deepen river = River Re- Sectioning
  11. 11. 2. Bank Protection  Strategies • Dykes/ embankments• Vegetation planting/ clearing
  12. 12. 2.1 Bank Protection (Embankments)1. Dykes/ Artificial levees : Walls of sand, stone  or concrete built along rivers  increase  river capacity to hold water  reduce  flooding 2. Protects banks from erosion by force of  running water  reduced sediments flowing  into river  increase river capacity  less  flood• E.g. Artificial embankments along river of  Pasir Ris Park, Singapore 
  13. 13. Embankments:  Dykes/ LeveesDykes
  14. 14. Dykes
  15. 15. 2.2.1 Vegetation Planting• Roots of trees and plants holds soil together  stabilise river banks• Vegetation also slows down rate of surface runoff  prevent large amount of water entering river  at any one time• Reduced surface runoff  less sediments  transported  river channel does not become  shallower  reduce flood occurrence• E.g. Joint effort to protect mangroves along  Mekong River.
  16. 16. Vegetation Planting
  17. 17. 2.2.2 Vegetation Clearing• Weight of vegetation may add stress to river  banks and cause them to collapse• Fallen trees or branches in the river bed cause obstruction to river flow – Reduce speed of flow of river  – Increase likelihood of flooding• E.g. Vegetation clearance done on Geylang River near Tanjong Rhu, Singapore.
  18. 18. Vegetation Clearing
  20. 20. Limitations of River Channel Management  Strategies1. Causes flooding downstream2. Accumulation of sediments3. Loss of marine life and wetlands4. High cost of building and maintenance
  21. 21. Effectiveness of River Channel  Management Strategies1. Flooding downstream  Increase in speed of river due to  channelisation may result in flooding  downstream Hence, flooding is not reduced. They can still  occur in another part of the river.
  22. 22. Recall: River re‐alignment strategy Increase in speed of flow due to re- alignment upstream may cause flooding downstream.
  23. 23. Recall: River re‐sectioning strategy Increase in speed of flow due to re- sectioning upstream may cause flooding downstream.
  24. 24. Effectiveness of River Channel Management  Strategies2.1 Accumulation of sediments• Channelisation allows rivers to flow at higher  speed. • Sediments, previously deposited on river bed  are carried downstream into sea/ ocean.• Result: less sunlight reach seabed, affect  growth of marine life such as coral reefs
  25. 25. Recall: River re‐alignment strategy Accumulation of sediments
  26. 26. Effectiveness of River Channel Management  Strategies2.2 Accumulation of sediments• Bank protection measures such as dykes  causes sediments to accumulate behind these  structures • Sediments build‐up over time  river channel  become shallower still flood with  continuous rain• Higher dykes need to be built/ money spent to  dig out accumulated sediments
  27. 27. Recall: Building of dykes Sediments accumulated behind structures makes the river shallower E.g. Build up of sediments along lower course of Yellow River, China, making the channel shallower over the years.
  28. 28. Effectiveness of River Channel Management  Strategies3. Loss of marine life and wetlands1.Too much vegetation prevents sunlight from  reaching river bed  kill aquatic life  affect  aquatic food chains2.Changing river course, lining of banks with  concrete destroys plants and animal habitats3.Channelisation leads to disappearance of  wetlands. i.e. river flows over smaller area with  shortening of river course
  29. 29. Recall: River re‐alignment strategyLoss of Loss ofwetlands wetlands
  30. 30. Effectiveness of River Channel Management  Strategies4. High cost of building and maintenance• Expensive to build and maintain river  defenses• Resources and manpower needed for river  realignment and re‐sectioning.• Aesthetically unappealing
  31. 31. In summation… Name of strategy/  Flooding  Accumulation  Loss of  Expensive to  Limitation(s) Downstream of Sediments  wetlands &  build &  marine life maintain Re‐alignment √ √ √ √Channelisation (downstream) Re‐sectioning  √ √ √ √ (downstream) Building of  √ (behind  √ embankments structures)Bank protection Clearing Vegetation √ Planting Vegetation √