Discuss the Extent to Which You Believe the Carlisle Floods of January2005 Were a Result of Intense Rainfall Rather Than the Physical Characteristics of the Drainage Basin

The rapid rise in discharge in the River Eden in 2005 was a result of many different factors, in particular physical, which, according to the AQA AS Geography textbook, came together to produce flood conditions that killed three people and left thousands homeless. However, it could be argued that some of the physical factors had more of an effect than others. On one hand, precipitation could have been the most important factor, due to its significant intensity, duration and volume. But the characteristics of the drainage basin, such as its size, shape, geology, vegetation and relief may have also had an important part to play in the 2005 floods.

Firstly it is undoubtedly evident that the volume and intensity of precipitation were two of the most, if not the most important factors. Carlisle is located in North West England, and is south of the Scottish border, and north of the Lake District, which is known as the wettest part of England. Therefore, it is not unusual for there to be heavy rain in Carlisle. A few days before the floods, Carlisle experienced a period of intense rainfall, where 201,286,000m3 of water fell. According to the Geographical Association website, rainfall was very high for the period 6 to 8 January, during which two months' worth of rainfall was released in 24 hours. As a consequence of this, the floods were very ‘flashy’, as the high intensity of the rainfall in such a short space of time produced a short lag time between the peak rainfall and peak discharge (see hydrograph). According to the gauging station at Carlisle, the peak rainfall was 48mm, and occurred at just before midnight on 07/01/05. This lead to 147,205,000m3of overland flow, which meant there was 73% runoff in the River Eden at Carlisle. The speed of overland flow is very fast, so such a high percentage of precipitation travelling as overland flow meant that precipitation was a very important factor in causing the discharge levels to rise and exceed the bankfull level of the river Eden.

But as well as precipitation, it is fair to say that the type of flow, i.e. overland flow, was also important to the flooding of Carlisle, as not all of the rainfall would have instantly gone into the channel flow. So a fast flow like overland flow would be ideal in helping to increase the river’s discharge. In addition to overland flow, channel flow could be argued as having affected the duration and intensity of the floods. This can be supported by the fact that Carlisle is in the lower course of the River Eden (i.e. near its mouth), and in the lower course there are several tributaries flowing into Carlisle and joining onto the River Eden. For example, the rivers Caldew, Petteril and Irthing have a confluence with the Eden just before or in Carlisle itself; so the area of Carlisle has a high drainage density. Therefore, on the 8th January, the river level rose 6m above its normal channel flow, as due to the intense rainfall,