A flood on the surface can appear to caused by only one thing: a lot of rain, which is a physical factor, but there is so much more to a flood than that. firstly, other physical factors such as the relief of the land, the amount of vegetation, the shape of the river, whether the ground is already saturated, as well as the intensity and the duration of the rainfall will affect how and when a river floods and to what extent it will pose a risk to people and property surrounding the river. however, human factors also play a part here, and they can often influence the importance of the physical factor. human factors can include things like urbanisation, increasing population density, building property on a known floodplain and infrastructure such as bridges that can slow the river velocity down. It is hard to generally say what causes a flood as it will also depend on that particular flood; often one flood may have completely different causes to another.
The central European flood that occurred in June 2013 was mainly caused by physical factors. it effected 7 countries in central Europe and resulted in the deaths of 25 people. the main cause of this flood was a 1 in 100 low pressure system that was trapped by the jet stream and so stayed in the same area for 5 days, causing almost continuous rainfall on already saturated ground, as there had been an unusually wet spring that year too. Also, due to this unusual weather, the rainfall was concentrated in one area of Europe; if it had been more evenly distributed than the extent of the flooding may not have been as bad. To make things worse, a lot of the rain fell over high ground which caused a lot of water to drain of the higher ground into the rivers, which mean the river levels increased as the river went downstream. some human factors did have an influence on the risk of flooding in central Europe, such as increased urbanisation and population densities as if there are more people and properties then there is more that can be effected by the flood. these were the main causes of the flood and almost all of them are physical, as physical factors were the primary cause in this case, however in some cases human factors can be the main causes of a flood.
A flood that had many more human than physical causes was the flood that occurred in the lower areas of the town of Helston in Cornwall, back in December 2012. this flood occurred when many other floods were taking place all over the UK due to unusually high levels of rainfall, however, Helston’s floods were influenced by many human factors as well. The flood received the first severe flood warning in the country and the river Cober’s level rose to 2.14 meters which was at least 1.5 meters over the normal level of the river Cober, effecting 200 people. the town is built on the side of a valley and this urbanised scene means that most water will run down along the impermeable surfaces towards the river that is below the town, and amongst a mainly residential area of St. Johns. this meat that the amount of water entering the river Cober was a lot higher and so the river levels grew quicker than they would if they didn’t have the surface run off water to deal with. furthermore, many of the properties affected by the flooding were situated on a floodplain and in some cases houses were built on the river banks. this increased the level of damage that could be expected as the houses were built in the area that would naturally flood if the river Cober burst its banks. Also, at the point where the river Cober enters the lower area of Helston, there are a lot of obstructions in the river that can increase the risk of flooding such as out of date double arched bridges. these cause the velocity of the river to slow down meaning that in a flood the river cannot move the water through the bridge quick enough, causing the water to back up and flood. to make things worse, this occurs at two points in the town and so the impact of these obstructions is even greater.
Human factors were the main cause of the Helston floods in my opinion, however physical factors did also influence the flooding such as the long duration of the rainfall, as in the previous months the rainfall for that area was particularly high, which meant that the ground was already over saturated when the rain fell on it in late December and therefore even a small amount of rainfall would change the river levels significantly.
As these examples have shown, it is hard to say if a flood is caused by physical or human factors alone as often it is a combination of the two. human factors can increase the significance of physical factors and vice versa. there is rarely a flood that is caused due to purely a physical or human factor alone. therefore it is only possible to say that the primary cause of flooding is a combination of human and physical factors, although the extent to which each one is important will vary with each individual flood.
To see some images that i took of the Helston floods, visit a previous post of mine on the effects and responses of floods here.
- information on both the central European floods and the Helston floods was sourced from what i learnt in my A level geography lessons.
- the effects and responses to a flood disaster (vamoswearegolden.wordpress.com)
- Environment Agency warns of increased flood risk this winter (northdevongazette.co.uk)
- More wet weather ‘brings flood risk’ (bbc.co.uk)