Impact of human activities on flooding

What are the impacts of human activities on flooding? Includes Urbanisation, Deforestation, River Management and Climate Change.

Refers to AQA AS Geography (Chapter 1 - Rivers, floods and management)

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  • Created by: Bethany
  • Created on: 19-04-13 16:21
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  • Impact of human activities on flooding
    • Deforestation
      • Rapid deforestation had taken place over recent years in many less developed countries.
        • Rainforests of South America, Africa and Asia at particular risk as new land is opened up for farming, settlement etc.
        • e.g. Nepal in the Himalayas (timber is a valuable and profitable resource for building and firewood.)
      • If trees are removed there is a greater risk of soil erosion and sediment finds it way into rivers, obstructing them which further adds to flood risk.
        • Wosened by the fact that trees are no longer available to intercept water, so in deforested areas, more water reaches channel as runoff.
      • CASE STUDY - Bangladesh lies downstream from Nepal and most of the land is low lying (less than 1m above sea level.) During Spring, snowmelt occurs and once Summer monsoon rains occur there is a natural rise in volume of river water.
        • In recent years, it has been claimed that flooding in Bangladesh has been made even more severe by deforestation in nearby Nepal
    • Urbanisation
      • Coupled with natural population growth, urbanisation has led for increasing demand for space to build houses. Floodplains are an obvious choice (flat) but naturally susceptible to flooding.
      • Urban surfaces: Impermeable concrete/ tarmac so precipitation is unable to infiltrate through.
        • Would naturally infiltrate soil before urbanisation
        • Uptake from plants/ trees also reduced.
        • So, higher proportion of rainfall makes it to river in urbanised areas
      • Water travels quickly through drains/ sewers in urban area. Water reaches river quickly.
        • Reduced lag time between peak rainfall and discharge.
      • Bridges slow down discharge and reduce carrying capacity. Can be deposited directly behind the supports in times of spate. Exaggerate effects of a flood.
        • e.g. Boscastle 2004 debris blocked culverts upstream of the town.
    • Climate Change
      • Average sea temperature has risen. Blamed on increase in tropical storms, which bring heavy rainfall and storm surges in countries in their path (Caribbean)
      • Warmer temperatures = increased evaporation of seas and oceans = greater precipitation leading to increased river flooding.
      • Increased El Nino events : Low pressure dominates bringing increased rainfall and flooding to the west Coast of South America.
      • Melting of polar ice caps. Rise in sea level so floodplains lying close to sea level. Built up deltas at risk (e.g. Nile, Mississippi)
    • River management
      • Aim to reduce likelihood of flooding but can increase the risk
        • Bangladesh: Embankments have been built along the river channels in some places. Designed to increase capacity of river, but have prevented flood water raining back into rivers.
      • Mississippi River USA: Case study
        • One of most managed rivers in world
        • Artificial levees have been built in lower reaches to protect the heavily populated floodplain.
        • New Orleans; levees and diversion channels BUT Hurricane Katrina (2005) caused river to rise dramatically when coupled with the heavy rainfall brought by the storm. Major damage to the embankments as they were breached in several places.
      • Channelisation
        • Lining river with concrete and strengthening it
          • Water directed through urban area more rapidly
        • Leads to greater flood risk downstream as water is delivered more rapidly. Unmanaged river channel is unable to cope with rapid increase in discharge.

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Josh747

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