Evaluate the evidence for human impacts on downstream flood risk in rural catchments in temperate regions. Before we can evaluate human impacts on flood risk we must first establish what is meant by temperate regions and also rural catchments. Temperate regions are generally regarded as lying between the Tropic of Cancer and the Arctic Circle or the Tropic of Capricorn and the Antarctic Circle and therefore rivers investigated in this essay will fall within these parameters. Rural catchments are slightly harder to define, as today very few large rivers do not have some form of urban development within their catchments area. In this essay a river that is still in a predominantly rural catchment will be discussed even if there are areas of urban land within the catchment. Humans impact on flood risk falls into one of two categories. The first is deliberately and directly, through floodplain restoration, construction of dams and channel rehabilitation and all of these have fairly obvious positive effects on reducing flood risk. However it is when humans indirectly affect the flood risk, through deforestation, land use change and climate change (which all have a negative effect on flood risk) that there is less certainty into the extent of the impact that humans have. Overall though it is clear that human activity has resulted in ‘major changes’ (Goudie, 2006) in downstream flood risk in temperate regions and rural catchments. The most obvious way in which humans impact downstream flood risk is through direct adaptation of the river itself and this is also arguably also the most important way in which humans can have an impact on flood risk (Mrwoka, 1974). Damming is probably the most widespread example of how humans seek to control peak flows on rivers and the construction of dams in the UK has led to significant decreases in flooding. The reservoir created on the River Avon occupies 1.38% of the catchment but reduces peak flow by 16% and even more impressively the reservoir on the Catcleugh in the Cheviots occupies 2.72% of the catchment and reduces peak flow by 71% (Petts and Lewin, 1979). The creation of dams clearly reduces the flood risk overall, however, dams have a much smaller effect on rare flood events of high magnitude, due to the fact that there is a finite amount of water a dam can hold during times of high, prolonged precipitation (Goudie, 2006). On the River Avon the ratio of pre-dam discharges to post-dam discharges is a mere 1.02 in a once-in-10 year event (Petts and Lewin, 1979). However, despite this, man’s construction of dams still has a large impact in reducing peak flood and therefore flood risk in downstream catchment areas. Floodplain restoration is another example of humans deliberately impacting on flood risk. It has been calculated that the flood reduction function of 3800 hectares of floodplain storage on the Charles River, Massachusetts saved US$ 17 million worth of downstream flood damage each year (US Corps of Engineers, 1972). Restoration has taken place on the River Cherwell between Oxford and Banbury. Here the embankments were removed and the channels restore to their pre-1900 dimensions. As a result of the rehabilitation of the channel peak flow was reduced by between 10-15% and the embankments which had been removed were shown to have been increasing peak flow by between 50-150% (Acreman et al, 2003). This clearly shows the extent to which humans can actively work to reduce the flood risk in a rural catchment area, and shows how important the role of floodplain restoration and channel rehabilitation is when reducing peak flows.
A prime example of human activity indirectly affecting flood risk patterns is through deforestation. The principle here is that by removing vegetation, you remove the capacity for a significant percentage of precipitation to be intercepted by the vegetation and then evaporated before it reaches the stream. Therefore, if humans remove the vegetation in a catchment...
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