1.4 The human impact

• cloud seeding


  the process of introducing freezing nuclei or condensation nuclei into a cloud in order to cause rain to fall (silver iodide)

  - could lead to less rainfall in other areas
• reducing dam's evaporations

  - shallow = more evaporisation e.g. Aswan dam loses 1/3 of its water

  reduce?

  - chemical sprays

  - sand fill dams

  - covering them in plastic sheets
• flood recurrence interval

  average time between flood events of a similar size
• how does interception vary?

  - determined by vegetation density and type

  - cereals intercept less than broadleaves

  - row crops leave soil bare

  - Mississippi basin = woodland areas are 1 unit of sediment, pasture fields 30, corn fields 350 units are washed into the river
• deforestation leads to

  - reduce in evapotranspiration

  - more surface run off

  - decline in surface storage

  - decline in lag time
• afforestation leads to

  - more overland flow

  - firebreaks make channels

  - less ground cover

  - On the river Severn, sediment in afforested areas is 4x higher
• flood


  - when a rivers discharge exceeds its bankful capacity
• environment agency


  responsible for flood warnings and risk analysis
• how should areas be protected?

  - urban areas protected up to a recurrence level of 100 years

  - shows how high to build defences

  - flood risk maps
• channel straightening


  - removes meanders to make water flow faster
• levees


  embankments built along rivers to increase it's capacity
• dispersion spillways


  channels that take the water elsewhere of the water level is too high, often having gates
• hard engineering

  - expensive, ugly, more hazardous, natural processes disrupted

  - man made structures
• soft engineering


  - cheaper to maintain, more natural, recreational opportunities
• land use management

  restrictions on where things can be built e.g. parks and football pitches on high risk land
• wetland and river bank conservation

  stores floodwater, increases lag time, increases interception
• river restoration

  Removing hard engineering from a river and returning the river channel back to its natural shape.
• alteration to urban surfaces

  porous pavements and soak ways reduce overland flow
• infiltration and soil water

  infiltration is 5x higher in forest than farmland

  grazing = less infiltration

  ploughing soil means more inflitration

  poor drainage leads to salinisation

  over abstraction leads to falling water levels and saltwater intrusion in coastal areas e.g. in Florida
• changing groundwater

  pivot schemes in high plains go texas where groundwater is over extracted
• Lubbock, Texas

  groundwater has fallen by 30-50m
• when industrial activity decreases in an area

  - springs and rivers re emerge

  - reemergence of flows

  - surface water flooding

  - flooding of basements

  - leakage in tunnels

  - reduced stability of slopes
• groundwater recharge


  e.g. in Israel, to avoid saltwater intrusion
• Dam negatives (Aswan)


  - seepage leads to increased groundwater levels, causing secondary salinisation

  - people are displaced

  - ruin of archaeological sites

  - seismic stress

  - infilling causes 100 milion tonnes of deposition per year in Aswan dam

  - channel erosion

  - erosion of nile delta

  - decrease in sardine catches 95%, 3000 jobs lost

  - spread of waterborne diseases

  - 1/3 less crops in areas irrigated by the Aswan dam due to salinisation
• Dam positives (Aswan)


  - allow good crops in dry years e.g. 1972-73

  - 60% od water in the dam is used for irrigation, 4000km of desert is irrigated

  - tourism and transport

  -$500 million worth of the Egyptian economy