Thomas Healy L6G

With reference to a flood event you have studied assess to what extent it was the result of intense rainfall rather than physical characteristics of the drainage basin or human factors? (15)Carlisle is a large city situated on the flood plain of the River Eden, with three rivers meeting the in the city (confluence). The catchment area of the River Eden is comprised mainly of rural areas, making up 99%, this is in stark contrast to the 1% of urban areas that withhold Carlisle’s 244,000 people. The catchment covers an area of 2400km2 and has a vast history of flooding dating back to the 1700’s. The flooding of much of Carlisle in 2005 affected 2700 homes and killed and killed 3, injuring many more. After the flood had ceased in late 2007 the damage was clear – a startling £400 million. It was obvious the clean-up could take months, even years due to the severity of the flooding.In regard to the causes it is important to distinguish whether the flooding was the result of intense rainfall rather than physical characteristics of the drainage basin or human factors. Human factors involved human interaction for example the construction of man-made dams. Physical factors could refer to the geology of an area, for example the im/permeable nature of rocks within an area. In the case of the flooding of the river Eden, rainfall was extremely high – between a 24 hours period in January, rainwater exceeded the normal precipitation of 2 months. Intense rainfall – 201,286,000m3 lead to 147,205,000m3 of surface runoff with 73% of that runoff in the River Eden at Carlisle.This overwhelming amount of rainfall was too much for the surrounding area which, despite mainly being comprised of rural areas could not in fact cope with the water that precipitated. This is due for a number of physical reasons. Firstly, the area had flooded many times before, leaving the soil saturated and unable to withhold much more water. When the excessive amount of water reached the soil, it simply could not hold it and therefore it existed as surface run-off, transferring most of the water to the channel causing a rise in discharge. The geology of the area did not allow the rainfall to infiltrate as much as was necessary – Impermeable slate and volcanic rocks in the Skiddaw area, along with sandstone and limestone in the Vale of Eden. These rock types did not allow the water to percolate down through the rocks and thus through flow transfers water to the channel.The steep slopes increase the amount of runoff, along with the fact that many of them were comprised of thin soil as time available for infiltration is reduced. This explains the short lag time of the hydrograph taken of the River Eden, the high relief also may have triggered orographic rainfall. The lower gradients around Carlisle also explain why the water settled there for so long. Sparse vegetation and cooler temperatures in upland areas means relatively little evotranspiration occurred. These physical factors had a large effect in regard to the Carlisle flood although if the rain had been less heavy, the damage would have been much less widespread.The human factors involved are fairly minimal as only 1% of the catchment area was made up of urban areas. Local drainage systems were overwhelmed and as a result water could not be channelled towards the river, causing flooding within the towns. The flood defences were inadequate for this type of flood and preparation prior to the event was poor. If these human aspects had have been improved the magnitude of the flooding would have been to a much lesser scale.To conclude it seems that the collaboration of intense rainfall, physical and human factors were deadly concoctions that ultimately lead to the flooding of Carlisle. Despite this each played a different role and had a different responsibility for the disaster. I believe the intense rainfall resulted in the flooding; this was the major problem, although the other two factors amplified the situation. My assumption is in line with data collected after the flood with states that:‘67% of the flooding resulted from rivers and watercourses, 25% of flooding was caused by surface water, 8% was due to flooding from sewerage and infrastructure.’The fact that the soil and rock could let the water infiltrate them was a deciding factor but the sheer amount of water that caused the surface runoff, despite flood defences was overwhelming for the area.

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