G1 – River Regime




Figure 1 represents the average monthly flow characteristics of the River Severn (UK). Its source is in the mountainous region of Mid-Wales – the mountain is called Plynlimon – where it rains on average 2 out of every 3 days. The discharge in the river measured in cumecs reaches over 800 cubic metres per second.




(Hint: A small stream such as the Alderbrook River (Solihull) would record an average monthly discharge of approximately 0.5 cubic metres per second – just to set the 800 cubic metres per second in context – this the equivalent of 800 big shopping trolleys of water passing you standing at a set pint on the river bank, every second – here is a sample answer.)




(a) Use figure 1 to outline the characteristics of this river regime. (5 marks)

The river regime in Figure 1 is an Atlantic Regime in the Northern Hemisphere. The discharge of the River Severn is highest in the winter months, when there is more rain and lowest in the summer months when it is dryer For instance, the wettest month is the winter month of February, with the discharge measuring at around 850 cumecs. The driest month however is the month of August with the discharge at about 350 cumecs. The discharge in the winter is 2.43 times larger than the discharge in the summer. The discharge declines in half a year ten takes half a year to increase.


(b) Explain how human factors can influence the severity of flooding. (10 marks)

Human factors can greatly increase the severity of flooding. For instance in Lynmouth, Devon, when the River Lyn flooded in 1952, it was partially the fault of the town and the buildings. Firstly, the man-made bridges trapped boulders and trees causing temporary dams. These dams later broke due to the pressure of the water, which caused a wall of water 12 metres high to travel downstream at 30km/yr, causing yet more flooding. Also the river had been straightened and re-directed through a narrow culvert. Because of this, the West Lyn River went back to its old course, which in turn caused a triangle of destruction. The river was also made narrower due to hotels and amenities on the river bank. This meant that the river had no floodplain and nowhere to go when the discharge increased ver bankfull.

Another flooding example is the Boscastle Flood of 2004. Here, there was poor communication between the helicopters that were rescuing people. This meant that each helicopter did not know who the other helicopters were rescuing; therefore valuable time was wasted in deciding who to rescue. There was a delayed rescue, putting more lives in danger. Also, new houses were built on the floodplain, so they flooded very quickly.

The third example is the River Severn Flood of July 2007. Here, the man-made dam (the Clywedog Dam) in the Plynlimon Mountains, overtopped due to the heavy rain that fell at the end of June and than again at the beginning of July (the so called Double Deluge). The River Avon flowing from the east was also in flood too and when the River Avon joined the River Severn in Tewkesbury the town suffered severe flooding as it is located at the confluence of the two major rivers.

The final example is the flooding of Bangladesh in 1998, located at the confluence of the Meghna and Brahmaputra Rivers, two mighty rivers draining the Himalayas. Here the number of people living in the Himalayan country of Nepal had increased, so trees had been cleared for farmland. Also the people had cut dowm trees for firewood to cook with. The deforestation has reduced interception increased surface run-off, and caused soil to be washed into the river channel reducing the volume of water it can carry. The end result is major flooding downstream in Bangladesh.


(c) Outline why the knowledge of a river’s regime might be necessary in implementing effective management strategies. (10 marks)

The river regime tells you when the average monthly discharge is highest and when it is lowest. When the discharge is highest (in winter) there is more water, therefore there is more chance of a flood. This means that there is more need for flood management. In the summer the discharge is lower, so management strategies often switch from holding back flood waters to ensuring there is enough water flowing down the river for farmers to take out for irrigation, and that there is sufficient water to allow for the dilution of river pollutants.

The river regime also helps us to manage the amount of water in the basin dams. The Clywedog Dam that holds water back from entering the River Severn. The dam contributed to the floods as if the rain water from the end of June had been released, the dam may not have overtopped when the second deluge occurred at the beginning of July.

The river regime also helps to predict floods. If the discharge is highest in the winter, then you know that it is more likely to flood during the winter. If you find out the bankfull discharge, you can see from the river regime how often a river floods in a year. In the case of the River Severn at Bewdley, river managers can decide whether to put up the temporary barriers or not to prevent flooding of this riverside town.

Overall, a river's regime helps river engineers decide what river management should be put in place and at what time of the year they should be implimented, from releasing dam water, installing temporary barriers or evacuting flood risk homes in Tewkesbury and bringing in the army to help save the Mythe Pumping Station that provides drinking water to most of Gloucestershire.