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Identify and explain how human intervention in the drainage basin can increase the risk of flood

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Identify and explain how human intervention in the drainage basin can increase the risk of flood David Mothersole 12BS Introduction There are many human factors that can increase the risk of flooding. These include the deforestation of trees, urbanization, global warming and dam building. People also believe that building flood defences such as those along the Mississippi, USA, can also increase the risk of flooding, as can human intervention of a rivers course. Flood risk is increased when there is more surface run off and fast surface water heading towards and into a river channel. Many physical factors can affect flood risk, but more common in the 20th and 21st century is the effect of humans on the land which is increasing the risk of flood. Deforestation by humans leads to less interception and more surface runoff etc. Human causes of increased flood risk The first and most obvious cause of a human intervention that has increased flooding in the drainage basin is the urbanization and building of large towns and cities on a river flood plain or in the river drainage basin. The problem is the size of the urban area increases it makes more of the land surface impermeable, and increases the amount of surface run off which goes directly into the river and greatly reduces the lag time of the river. ...read more.


As well as urbanization playing a huge role in increasing flood risks another human factor that greatly affects the flood risk in deforestation. The removal of trees reduces interception and transpiration from the tree leaves, resulting in an increase in surface run off and soil erosion. The soil erosion may not sound like a big deal but, if it finds its way into the water course it gets aggraded into the river bed, reducing the channel capacity and increasing the likelihood of flooding. In Nepal and Tibet an increasing in population has create a dramatic increase in deforestation. The forest used to play a major role in the hydrology of the upland drainage basins absorbing water from the ground, binding the soil particles and reducing the impact of rain droplets on the ground surface. Now without this protection, there is a reduction in interception, increased landslides and overland flow, which results in more flooding downstream in Bangladesh. The silt and soil is deposited in the river channels reducing the capacity of the rivers as it raises the river beds. Soil is said to be lost 400 times faster in deforested areas and is raising the Brahmaputra riverbed by 5cm every year. ...read more.


Also due to heavy machinery much of the soil is heavily compacted, this prevents infiltration which leads to a greater amount of surface runoff. Pollution in the flood basin can also cause an increase of flood risk. Pollution can kill off the trees in the basin via poisoning them with Sulphur Dioxide and other harmful gases; the removal of trees removes the interception which increases the surface runoff Conclusion There are many ways in which humans have interfered with the drainage basin causing an increase in flooding, including deforestation, building dams, urbanization, compacting ground, shortening rivers. There are also larger problems such as Global Warming which are affecting low lying flood proned land. There are different ways of dealing with such problems many people in MEDCs believe that we should leave the rivers to run a natural course, and leave the floodplains as we can afford to move to other places. This is not possible in LEDCs such as Bangladesh where there is such a high population that people are forced to live on the flood plain, especially since 90% of the country is floodplain. Unless all the population of Bangladesh is moved, which is totally impossible, and then there is no other option for the Bangladeshi government but to create flood defensives. David J Mothersole Page 1 04/05/2007 ...read more.

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