When we experience bad flooding in the UK, as we have done recently, there are always areas that are worst hit than others. This is down to many different factors both physical and human, meaning some rivers flood regularly and some very rarely.
the size of a river channel influences its capacity to hold large volumes of flood water. If a river has a small cross-sectional area then it will have a lower ability to cope with the extra volumes of water that it will experience during a period of heavy or long-lasting rainfall. there are often small rivers like this on moors where the river is young and not far from its source. These rivers, such as those found on Dartmoor, are shallow and narrow, but will have to cope with most of the surface runoff from a storm. They struggle to do this and inevitably, they burst their banks. moorland rivers tend to flood quickly too. I experienced this last year on holiday on Dartmoor, when driving over the moors I drove over the East Dart river, whilst still in its upper course, it had only been raining for a few hours and the river was inches away from bursting its banks. So small rivers are more likely to flood than large rivers due to their lack of capacity to contain large volumes of surface runoff. However, if the period of rainfall is long, lasting months like it did earlier this year, then even large rivers, which usually are able to cope with surface run off, would be at high risk of flooding.
length/ intensity of rainfall
The length and intensity of rainfall prior to flooding can make some areas more prone to flooding than others. If an area receives long periods of almost uninterrupted rainfall, lasting weeks or even months then this will lead to flooding in that area, no matter what size the river is. This uninterrupted rainfall will saturate the ground and cause the groundwater level to rise. Once this occurs then most water will end up as surface runoff and will either become trapped by the relief of the surrounding landscape or will fill up the nearest river channel. If this continues over a long period of time then rivers will be unable to cope with the gradual increase in volume of water they have to carry and will flood the surrounding area. It can take months for the groundwater level to become low again, meaning any more rainfall could cause a sudden or flash flood, as the rain would be landing on already saturated ground.
However a sudden intense period of rainfall, perhaps only lasting a few hours can cause a river to flood. This is due to the relief of the land; a steep valley will have a faster surface runoff, filling up the river channel below quicker, and if the area was saturated from a previous period of prolonged rainfall, then the surface runoff would also cause the rainfall to enter the river quicker. This was one of the main causes of the 2004 Boscastle flood in Cornwall.
Rivers that have large areas of wooded vegetation surrounding their upper courses, will be less likely to flood as large vegetation such as trees act as interception by reducing the amount of water that falls directly on the ground by capturing it in their leaves and also by reducing the levels of saturated ground, as result of heavy rainfall, by absorbing it though their roots. This reduced surface run off in areas with large areas of vegetation. Therefore when large areas of forest, such as in Northern Bangladesh, are destroyed then the levels of interception are dramatically decreased which will lead to an increase in surface runoff and therefore more water will reach the river channel, increasing the rivers’ chances of flooding.
channel straightening is actually a type of river management aimed at reducing flooding along certain parts of rivers that are prone to flooding. It works by making the river channel straighter allowing the velocity of the river to increase, moving any extra flood water down its course quicker, meaning water is less likely to back up and cause flooding in that part of the river. whist this is very effective for that area of a river, it can cause extra flooding downstream of the straightened channel as the volume of water that the downstream area receives will increase as water reaches that area quicker. This could lead to water backing up downstream and could cause flooding in that area, the exact same problem that the channel straightening management system was trying to solve.
rivers that flow through urban areas are more likely to flood as the process of urbanisation has increased the amount of impermeable surfaces near rivers and also has placed obstacles like bridges in the rivers themselves. impermeable and therefore any rainfall cannot naturally be absorbed by the ground and so will enter the river as surface runoff. This will increase the volume of water the river has to carry and along with other factors can lead to a higher risk of that river flooding. Obstacles such as bridges that result from urbanisation, slow the velocity of the river down, this can cause a river that is already in flood to build up behind a bridge and flood around that bridge and onto the land. bridges with two or more arches are most prone to causing this type of flooding as they create more of a barrier in the river channel.
- information on factors that influence river flooding sourced from: ‘OCR AS Revise Geography’ textbook