2.  Human attempts to modify drainage systems
to prevent flooding
 But sometimes these efforts have adverse
effects and actually helps to cause flooding
at other areas
 Any modification has the potential to cause
changes in the drainage system and these
changes have severe consequences

3. Direct Indirect
• River regulation e.g R. Nile • Deforestation e.g Amazon
• Channelisation e.g R. Thames • Afforestation – a problem
in the early period when
• Water abstraction e.g R. much of the ground is bare
Colorado egTennessee Valley
• Irrigation e.g Syr Darya, Aral • Urbanisation e.g floodplain
sea development in Oxford
• Dam Construction e.g Three • Mining – impact of water
Gorges Dam
quality
• Waste disposal e.g R. Ganges
• Agricultural practice e.g
New Bedford River22
• Drainage

10.  The water in the basin is used for a variety of
purposes:
› Irrigation – there is a vast network of canals helping to
irrigate India’s rice crop
› Domestic use – for drinking water and sewage disposal
› Industrial use – there is a large concentration of tanneries,
petrochemical and fertiliser complexes, rubber, jute and
textile industries
› Disposal of waste water – from industrial, residentials and
agricultural areas
› Navigation
› Religious practices – disposal of dead bodies

18.  Water quality is low
 Biochemical Oxygen Demand
(BOD)levels are very high
 Riverquality decreases during
periods of low flow because there is
less water to dilute the polluting
materials

19.  Excessive
extraction of groundwater has
caused the water table to drop rapidly
 Three
sectors consuming the largest
amount of water are:
› Agriculture
› Industry
› Domestic use

20.  Land subsidence – Bangkok is sinking by 10cm
per year – lead to major structural damage to
buildings and roads
 Shortage of safe drinking water – the shallow
aquifers in Bangkok have become
contaminated with salt water from the nearby
ocean
 Increased risk of flooding – aggravated by
land subsidence

25.  It
is estimated that many parts of the city
will be under water all-year round in 17
years’ time.
 To alleviate its problem, Bangkok would
have to reduce its groundwater extraction
rate by at least one-half – a formidable
challenge, because water demand is
expected to grow rapidly in the coming
decades

27. Modify the river channel
Use engineering structures to
control floods
Hard engineering to carry large
discharge quickly downstream

28.  Protect the whole basin
 Use methods that work with nature rather than
against it
 Soft engineering
 To reduce overland flow hence river discharge

29.  Straightening
 Resectioning
 Building levees
 Bank stabilisation
Effects:
 Reduce friction
 Increased velocity
 Increased water temperatures:
› No shading, no cover for fish life, rapid daily
& seasonal fluctuations in temperatures –
reduced leaf material input

30.  Increased flood defense – additional flood
storage provided by the enlarged floodplain
 More opportunities for recreation – public
perception is largely favour of natural landscape
 Improved water quality – due to natural settling
of sediments on the floodplain and river bed,
increased interception of pollutants by
vegetation
 Greater nature conservation of wildlife in the
riverine environment