human impact

• ways of reducing flood effects:

  • sandbagging
  • flood proof housing
  • insurance
  • evacuation
  • aid
• Flood proof housing can involve sealing entrances, moving valuables to higher floors, or building on stilts.
• Insurance is often only an option in MEDCs, but sometimes they refuse to cover buildings in high flood risk areas
• Aid can come from the local/national government, from other countries, or from charities.
• soft engineering to prevent floods:

  • afforestation/reseeding sparsely vegetated areas
  • recreating wetlands and lakes
  • terracing
  • protecting against wildfires
  • banning clear cutting(deforestation) and over-grazing
  • removing sediment from rivers
  • building flood water and sediment holding areas
  • managed retreat
  • building regulations on floodplain
• hard engineering to prevent floods:

  • large scale dams
  • levees (but they can exacerbate floods if they fail)
• humans reduce interception:

  • deforestation - removes forests that intercept
  • crops, especially when harvested which leaves fields bare
  • urban areas - water goes into drains, which take it straight to the river
• humans reduce evapotranspiration:

  • deforestation - trees can't transpire water from the soil into the atmosphere
  • urban areas - there aren't enough plants
• humans reduce infiltration:

  • compacted soil on farms - cattle and ploughing compacts the soil, reducing pore space causing impermeable soil
  • urban areas - replace permeable soil with impermeable tarmac
• humans reduce storage:

  • natural stores in wetlands are drained for farmland or urban areas
• humans cause overland flow:

  • smooth surfaces in urban areas increase overland flow
  • gutters and drains take water straight into the river channel
• humans decrease the river channel size:

  • building concrete channels
  • more eroded silt from soil without tree roots to hold it together
  • bridges cause blockages
• humans create artificial stores that fail:

  • dams - collapse under flood conditions, leading to surges
  • levees - increase water volume, so when they fail the floods are more severe
• Humans often build on the flood plain to take advantage of the flat fertile land, but this puts them at higher risks of being flooded.
• Humans induced climate change which leads to more intense storms, which increases flood frequency and intensity.
• Floods need an input of excess water to first force a river to bankfull conditions, then over-topping and inundation of the flood plain.
• Causes of flooding:

  • rainfall over a long period of time (mid-Atlantic depressions)
  • intense rainfall (hurricanes)
  • summer snow melt
  • earthquakes and landslides push rubble into lakes
• Flood causes are normally natural, because nature provides the excess water, but humans can cause floods from dam failure, although this tends to happen due to a natural high input of water. Humans can exacerbate floods due to changes in the drainage basin.
• Flood recurrence interval - how often, on average, a particular magnitude of flood is likely to occur, calculated using data from previous floods, plotted on a semi-log graph
• Flood forecasting:

  • estimates of snowpack levels
  • estimates for rainfall levels
  • gauging rivers
  • mapping river channels
  • mapping soil and rock type
  • mapping land use, especially human changes
  • mapping past rainfall and flood patterns
• effect of urbanisation on groundwater:

  • solid waste
  • heavy metals
  • liquid effluent
  • sewage
• effect of agriculture on groundwater:

  • pesticides
  • irrigation depletes sources
  • nitrates
  • salination
  • over-irrigation e.g. centre pivots
• effect of industry on groundwater:

  • acids
  • oils
  • fuels
  • solvents
  • heavy metals
• hard engineering - changing the river channel by building large solid structures, often made of concrete, to change flow intensity or quantity of a river
• soft engineering - working with nature to enhance the processes that naturally reduce flood risk, without directly interfering with the river channel