Evaluate the extent to which flooding is caused by humans PDF

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Imogen Goode

‘Evaluate the extent to which flooding is caused by humans.’ (20 marks) There are significant factors in which prove that humans play a role in causing flooding. One important factor is the concept of deforestation. This is the removal of trees and vegetation, meaning a reduction in evapotranspiration and an increase in surface run off. This will have an obvious effect on a storm hydrograph, if there is no vegetation to intercept water then surface run-off will reach the river much quicker, the short lag time means the river will reach peak discharge very quickly, and in result cause flooding. An example of the effects it has is the Amazonia rainforest. In this location, over 20% of the forest has been cleared, for reasons such as cattle ranching, large scale commercial agriculture for biofuels, soya beans, the development of roads and legal and illegal logging. The amazon contains 60% of the world’s rainforest. 75% of intercepted water is returned by evapotranspiration to the atmosphere, this is reduced to 25% when the forest is cleared. As more water runs off into the Amazon drainage system, this exacerbates the possibility of severe flooding. So, deforestation clearly has a negative impact for flooding as it leads to a reduction in evapotranspiration and an increase in surface run-off, speeding up the cycle. This closely links to the problem of urbanisation. A combination of economic and population growth throughout the twentieth century have caused many flood plains to be built on and many natural landscapes to be modified for agricultural, industrial and urban purposes. Urbanisation causes a number of factors which increase flood risk; first being the creation of impermeable surfaces, this means that not interception can take place so more surface run-off will reach the river in larger volumes and more quickly, for example, in London around 60% of gardens have been paved over in order to create parking spaces, this proves how much ground is being converted into impermeable surfaces and is increasing with time. Secondly, the speeding up of drainage of water in built-up areas via artificial conduits, such as drains and sewers, means that more water is getting to rivers faster, so in a heavy rainfall event the lag time of a river would be very short and would be likely to flood fast. Also, straightening channels, or realignment, aims to increase the flow of a river, this will result in flooding downstream. Another process that increases risk of flooding is irrigation of groundwater, which is used for extensive cereal farming and has led to declining water tables, such as the Texan aquifer). Recent reductions in water using manufacturing activity has now led to less water being abstracted and groundwater levels have risen, this has caused surface water flooding, flooding of cellars and basements, and leakages in tunnels used by the London underground. These factors clearly suggest that humans do influence flooding to a considerable extent, although there are also significant physical factors that naturally occur that will influence flooding too. The primary causes of floods are either meteorological, such as short-term weather events, or longer-term climatic causes such as rainfall patterns. In the UK, the usual cause of flooding is prolonged or heavy rain associated with the passage

Imogen Goode

of low-pressure systems or depressions. The degree of flooding can depend on precise depression sequence, sometimes a succession of very intense storms has a cumulative effect on the drainage system, like in October-December 2015. In other areas, in particular southern and eastern Asia, intense seasonal monsoon rainfall can result in widespread, damaging flooding. For example, Bangladesh is at risk due to being in a low-lying location, 80% of their people are exposed to flood risk and half of the country is less than 12.5m above sea level. This proves that the location of a region can immediately expose them to flood risk, regardless of if there are any human factors. Another factor that can encourage floods physically is through the processes of snow and ice. This usually occurs in higher latitudes or mountainous areas. Melting snow in the late spring regularly causes extensive flooding in the continental interiors of Asia and America. The quick transition from winter to spring upstream causes rapid snow melting, while their lower reaches remain frozen, with very limited infiltration. Flood water is often held up by temporary ice dams. Sometimes rain falls on melting snow when a rapid thaw occurs and this combination can cause heavy flooding. In addition, other physical factors that can affect flood risk include the amount and type of vegetation, the slope, the rock type, drainage density and soil depth. All these factors show that even without human interference, physical landscape can naturally increase flood risk. In conclusion, there is sufficient evidence to suggest that humans do increase flooding, such as the rapidly increasing urbanisation and activities such as deforestation that speed up the flooding cycle and can ultimately increase the amount of river discharge too. However, there are physical factors which show that whether human disturbances are there or not, some regions are naturally more at risk to flooding due to factors such as rock type and soil depth. So, whether human factors or physical factors are more responsible for the flooding depends on the location and landscape....