Levees

(to protect existing high value dvlpmts)

• Raised banks along rivers that are usually mounds of sand&gravel (so water can percolate into them too) that artificially raises bank height so channel depth is ↑ , allowing for greater discharge during floods

  
  

  🏴 In low-lying Bangladesh, 2500km of tall embankments were built along major rivers, protecting 15mil hectares of high value areas.

  
  
  

  ❌

• However, levees were unsustainable, not designed to cope with catastrophic floods that would overtop the embankments
• Levees are expensive, costing $10bil to build and $500mil/yr to maintain in Bangladesh
• If levees fail, impacts are amplified (levee effect: false sense of security)
• Prevents backflow into river so areas of stagnant water are created, increasing likelihood of cholera like in Bangladesh

Channel Management

• Aims to reduce flooding by increasing either channel capacity (resectioning / dredging) or efficiency (changing channel shape to ↑ HR)
• Designs a new channel which can carry a flood's peak flow through channelization (straightening, deepening, widening, cleaning or lining existing streams)
• Essential to select the right flood values to design the new channel
  🏴 In India, discharge ended up cutting into existing material as canals were under-designed, leading to erosion
• Overdesigning can ↑ deposition and governments have to dredge often (due to concretized streams being unable to adjust) to not impede flows which can lead to floods
• Canals designed bazed on a specific value may not cope with unexpected weather conditions
  🏴 In the 2010 flood in SG, the Stamford Canal designed to handle a 5yr return period event overflowed when there was 2 consecutive rainfalls within 30mins (intensity well above the designed value)
• Meanders might be cut off to improve flow, resulting in straight concrete lined channels which efficiently transport water
  ❌ Largely for HICs/CHLDs as its costly

Dams

• Large scale structures built to control floods by water storage in the reservoirs

  
  
  

  Impacts:

  
  

  ❌

• Dams result in ↑ sedimentation behind dam walls when river sediments can no longer be deposited on floodplains during regular floods

• This ↑ river energy and affects downstream areas

  
  

  🏴 The 2335m Three Gorges Dam across the Yangtze River in China stores about 60% of the sediments behind its walls

  
  

  🏴 1.3mil ppl displaced from >150 cities in towns in Chongqing, triggered 3k EQs

  
  
  

  ✅

• Built with long life spans where long recurrence intervals of >100years must be considered for the amount of funds in construction to be justified
  🏴 3Gorges Dam has a 22m3 storage capacity that can reduce frequency of major downstream flooding from 1 in 10 yrs to 1 in 100yrs
• Only brings significant impacts to small scaled basins, as for large basins, there must also be a large storage and a huge dam in proportion
  🏴 66mil m3 of storage needed to mitigate floods in Bangladesh

Flood Abatement: Afforestation

(manages runoff: sustainable, low $, X immediate)

• Slows down the rate of stormwater reaching the river through delayed runoff generation & greater infil
• Use of upstream land use management plans to ↑ capacity of water storage by planting vegetation on floodplains
• Afforestation and farm land terracing slows down OF downstream & reduces soil erosion & sedimentation in the channel that increases channel carrying capacity & reduces flood risk
• Requires widespread planning for the whole catchment so conflicts arise across political boundaries & need for consideration of relevant stakeholders: The Mekong River (SL)
• Not effective for arid tropics where rainfall is highly localised & abatement cannot be adopted on a basin wide scale
  🏴 Loess Plateau Project in China Shaanxi & Shanxi where replanting and grazing bans allowed the perennial vegetation cover to ↑ from 17-34% and sediments from the Plateau into the Yellow River ↓ by >100mil tons/yr, reducing flooding around Yellow River

River Restoration

(regulate flows & flooding)

• Restoring channelized rivers to natural states
• Reinstatement of river features like meanders, bars, riffles to allow for regular occurrence of floods

• Hard engineering protections are removed so large flood events are less likely to occur, allowing for natural occurrence of low magnitude floods

  
  
  

  ✅

• Low cost and low impact

• Aesthetically appealing and environmentally sensitive

  
  
  

  ❌

• Most projects only restores short stretches of a river
  🏴 Bishan Park, Singapore and Cheonggyecheon Stream in SK Seoul in 2013

• Criteria:
• Time
  
  • reversibility
  • duration for recovery
  • does the impacts amplify over time (floodplain encroachment where more people move to floodplains and are put at risk)
• Space
  
  • CHLDs mostly economic impacts
  • CLLDs human AND economic

• Scale

  
  
  • extent of damage

  • results in other processes? connectedness

    
    
    

    Factors affecting impacts:

• Characteristics of flood
  
  • Seasonal (higher magnitude and duration, more impacts)
  • Flash (shorter duration, less impact)

• Magnitude: size of flood event and in turn, severity of impacts

• Frequency: how often an area experiences flood events of a specific magnitude

  
  

  [Low magnitude events have high frequency and vice versa, however, with climate change, floods might have greater magnitude and frequency]

  
  
  

  ☀ Global Warming

• Increases risk of flooding and flood magnitude and frequency
• Rainfall amounts increase exponentially as temperatures rise
• More ghg in the atmosphere traps more moisture resulting in more rainfall

Evaluation:

• Most bring immediate aid to flooding & are reliable but
  ❌
• Unsustainable: Provides false sense of security so dvlpmt of floodplains would continue (The Levee Effect)
• Requires constant maintenance and are costly
• Ineffective in extreme situations
• Brings unintended environmental impacts like loss of wetland environments or disturbance of habitats and decreased water quality
  

• Floods cause 40% of all deaths worldwide
• Risk: Probability of a hazard creating loss of lives and livelihoods (can be altered by human measures to reduce vulnerability)
• Vulnerability: Exposure to hazards and inability to cope (reliant on economic status)
• Impacts of flooding differs across CHLDs and CLLDs