Evaluate the Evidence for Human Impacts on Downstream Flood Risk in Rural Catchment Areas in Temperate Regions

Evaluate the evidence for human invasions on downstream whollyuvium out endangerment in rude catchments in restrained regions. Before we cigaret tax human impacts on outpouring insecurity we moldiness first establish what is meant by temperate regions and too country-style catchments. Temperate regions atomic number 18 gener completelyy regarded as lying in the midst of the Tropic of Cancer and the natural rubber Circle or the Tropic of Capricorn and the polar Circle and thitherfore rivers investigated in this demonstrate go away f whole within these parameters.Rural catchments are close to harder to define, as today very few gigantic rivers do not nurture somewhat plant of urban development within their catchments theatre of operations. In this essay a river that is tranquilize in a predominantly rural catchment will be discussed all the same if in that location are areas of urban territory within the catchment. manhood impact on overeat jeopardize locomote into unrivaled of two categories. The first is deliberately and directly, finished climax plain restoration, construction of dams and channel rehabilitation and all of these pull in fairly obvious positive ca routine on down flood risk.However it is when gentlemans gentleman indirectly affect the flood risk, through with(predicate) deforestation, land use neuter and mode alter (which all absorb a negative effect on flood risk) that thither is less certainty into the issue of the impact that cosmos devote. Overall though it is lighten up that human exertion 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 likewise arguably also the most important way in which humans can gull an impact on flood risk (Mrwoka, 1974).Damming is probably the most general example of how h umans seek to control eyeshade electric currents on rivers and the construction of dams in the UK has led to noteworthy decreases in inundate. The reservoir created on the River Avon occupies 1. 38% of the catchment scarce reduces aggrandisement f miserable by 16% and notwithstanding much than 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 such(prenominal) smaller effect on rare flood causes of lofty magnitude, collect to the event that there is a finite amount of piss a dam can hold during multiplication of high, prolonged hastiness (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, mans construction of dams yet has a titanic impact in reducing peak flood and thereof flood risk in downstream catchment areas.Floodplain restoration is some other 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). redevelopment has taken place on the River Cherwell between Oxford and Banbury. here(predicate) 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 lessen 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 f lows. A prime example of human action mechanism indirectly affecting flood risk patterns is through deforestation.The principle here is that by removing plant life, you remove the potentiality for a significant percentage of downfall to be intercepted by the flora and then evaporated before it reaches the stream. Therefore, if humans remove the vegetation in a catchment area this can increase run-off and thence flood risk. An observational study was conducted in 1910 to investigate the extent to which vegetation coverage affected peak flow in Colorado. Stream flows from two irrigatesheds of approximately 80 hectares in size were compared over 8 years, before one valley was clear-felled.The catchment area which had experient clear felling to a lower placego 17% greater annual flow and also significantly high peak flows (Goudie, 2006). In 1998 the Yangtze River experienced its worst floods for over 40 years, with high piddle remaining in some areas for 70 days. Although the heedlessness over that time period was extreme, the extent of the swamp (which caused over $20billion in damages) has also been linked to the widespread deforestation that had taken place upstream of the floods.In 1957 the forest coverage of the river basin was 22% but by 1986 this figure had been reduced to 10% (Yin et al, 1998). despite this, it has been argued that during times of prolonged rainfall, vegetation loses its ability to reduce peak flow as there is a finite limit to how much water vegetation can hold. A study on the Yangtze showed that under 90mm of heavy rainfall, surface run-off was 65mm in forested areas and 35mm in non-forested areas and therefore the forest does not retain to a greater extent run-off (Cheng et al, 1998) and therefore flood risk is no greater.However, there can be no doubt that deforestation reduces slime losses and therefore increases the convergence of seepage water and that deforestation increased the seriousness of the deluge that th e Yangtze experienced in 1998 (Yin and Lee, 1999). The type of vegetation in a river basin can also have an influence on flood risk, and human activity can indirectly affect this. The principle here is that some types of vegetation retain more water than others and therefore their presence reduces flood risk.The catchment area of the Coweeta River in North Carolina was converted from deciduous hardwood forest to pine (which is evergreen) over a period of 15 years, from 1940 onwards, and as a result stream flow was reduced by 20% (Swank and Douglas, 1974). However, although certain types of plant whitethorn indeed significantly reduce stream flow, the impact they have on flood risk is ofttimes considerably smaller. It has been estimated that a forest of Ash juniper trees intercept or so 40% of the precipitation that fall on them each year (Owens et al, 2006).This figure is so high as Ash juniper trees are evergreen and therefore absorb water all year round however, during storms, this figure is reduced to around 10%. This figure remains fairly similar for most vegetation during high storms. We can therefore say that although humans adapting the type of vegetation in a catchment area does have an impact on overall stream flow, the extent to which this reduces the flood risk downstream is negligible (Wilcox et al, 2006).Land use change is another example of human activity which, although it is not done with the intention of altering river flow characteristics, still has an impact on downstream flood risk. exploitation urban areas in formerly rural ones is presently widely acknowledged to have a sizeable hydrological impact, mainly thorough the ways in which it alters runoff (Hollis, 1988). Essentially this urbanization produces a tapestry of water-resistant surfaces that increase run-off and therefore discharge during times of high precipitation (Graf, 1977).However, Hollis (1975) argues that whilst urbanization may increase the yield interval of small floods, in rare large scale floods, land use change has scant(p) effect on the overall peak flow, due to the fact that during large storms, rural areas become virginal quickly and then behave in much the same way as urban areas. scorn this, we can still say that land use change from urban to rural does increase the flood risk, even if this increase in risk is wholly during smaller events.Although we are examining flood risk in rural catchment areas, development of urban pockets in these areas moldiness still be considered, as even catchments with only some urbanization are still more belike to suffer flooding (Wilson, 1967). Climate change is another way in which man indirectly can have an effect on flooding risk although this is a hotly contested topic, as no completely acceptable explanation of mood change has been presented before (Goudie, 2006). However, some climate models have still predicted that climate change over the nigh 100 years will lead to higher flood risk.Th is is due to the fact that in a warmer climate, the air can hold more water, which increases the potential for latent heat release during low pressure systems and therefore increased precipitation is likely (Frei et al, 1998). A model in 2002 produced by the EU group PRUDENCE compared summertime precipitation in mainland Europe from 1961-1990 and the forecast for 2071-2100 based on the climatical predictions made in the IPCC report. This found that although overall precipitation may slightly decrease over the summer, precipitation events in the 95th percentile for intensity would significantly increase (Christensen J and Christensen O, 2003).This would obviously increase the flood risk downstream in rural catchments. However, although climate change may, in the coming century, prove to have a significant impact on flooding, currently the topic is too heavily debated to draw some(prenominal) concrete conclusions on the extent to which human induce climate change increases flooding risk. We can therefore see that humans impact on flood risk in a variety of ways, some positive and some negative and all to varying degrees.It is worth bearing in mind that in some areas man may be impacting on flood risk in both a negative and positive way and therefore having an even larger impact on the stream than would at first be obvious. The evidence for man impacting on flood risk downstream in rural catchments is often disputed however, it is clear that man is impacting on streams and flood risk. It is worth remembering that flooding is a perfectly natural event however rivers and the floods they can potentially unleash are in a delicate balance, and man is more than capable of touch that balance in a variety of ways.