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
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
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
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
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
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
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
Modify the river channelUse engineering structures to control floodsHard engineering to carry large discharge quickly downstream
Protect the whole basin Use methods that work with nature rather than against it Soft engineering To reduce overland flow hence river discharge
Straightening Resectioning Building levees Bank stabilisationEffects: Reduce friction Increased velocity Increased water temperatures: › No shading, no cover for fish life, rapid daily & seasonal fluctuations in temperatures – reduced leaf material input
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
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