It may sound obvious but flooding occurs due to a sudden increase in the amount of water travelling down a river, and can happen for a number of reasons: a rapid snowmelt, a prolonged period of heavy rain, a sudden and intense fall of rain, and human interventions such as deforestation, which cause the water to run-off faster than previously.
The discharge of a river is shown on a graph called a flood or storm hydrograph. It shows the rainfall amount and then the discharge of the river. Most of the rain falls onto the land rather than directly into the river.
The water will then make its way into the river and you can use a hydrograph to see how rapidly this occurs. In fact you might have to look at one of these in the exam, but by looking at the peak rainfall and comparing it with the peak discharge you can work out the lag-time (the time between the two peaks).
To prevent the impact of flooding schemes have been introduced in many places. These methods can be very successful, or can cause greater problems further downstream. In Exeter, flood relief channels and raised riverbanks have been used to diminish the flood risk. The scheme in place is aimed at countering a “once in one hundred years” flood, and has been severely tested a couple of times.
As humans have increasingly used and abused river basins, the management and planning of them has become increasingly important. Flooding is the most common thing to have to plan around. In many cities the floodplain has not really been built on. Oxford and Salisbury are all good examples of where this is the case. Other schemes include damming rivers, building flood defenses along channels and creating flood relief channels.
However, building dams across rivers can also cause problems. Obviously there are the advantages of creating a large reservoir, which can be used for drinking water or as the source of water for a hydroelectric power scheme. The reservoir will often also be used for recreational purposes.
But it will come as no surprise that the construction of a large dam can cause problems by affecting the flow of water further down the river, by flooding areas of farmland and even towns or villages and by affecting entire ecosystems.
The Tucurui Dam in the northern rainforest of Brazil did just this, flooding an area of 2875 sq. km. It was built to provide power for local industries, but at quite a price. The lake it created displaced 40,000 people, and is estimated to have destroyed hundreds of species of animals and plants, some of which may never have been actually known about.
This being said, the impact of a flood can be catastrophic and it is important that people are aware of both the advantages and disadvantages of implementing food defenses!