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Flooding
The Thames Barrier holds back the sea and so protects London from flooding.

Flooding

In GCSE Geography students will look at rivers. One aspect of this topic is flooding. This is one of two quizzes on flooding and it focusses in particular on the causes and the effects of floods.

Floods can have devastating effects. In 2014, across Europe the estimated cost of flood damage was around £3.2 billion. It’s thought that by 2050 the cost could have risen to £16 billion as more extreme weather conditions lead to more extreme and more frequent flooding. It is also estimated that the formerly once in a century extreme events could become annual occurances.

A river is normally seen as just being the channel that it flows through, but in reality the flood plain either side of the river, as well as the bogs, marshes and pools that flank the river channel, not forgetting the tributaries and distributaries, should all be considered an integral part of the river.

What does this more inclusive view of a river mean? Well, we need to accept that flooding is a natural part of any river cycle. The flood plains are carved by rivers and sculptured by the floods themselves.

What exactly are the causes of flooding? A huge range of factors influence how much time the water takes from reaching the ground to entering the river. These factors include the speed of drainage into parts of the river channel, and what is there to slow and prevent the water reaching the channels immediately. Studies show that some modern rivers can carry water from the land near their source to the estuary in a matter of hours, when before deforestation, channel straightening, drainage and changing land use it would have taken days. The faster the water reaches the channel after raining the higher the risk of flooding in high risk areas.

Try this quiz to see how much you have learned about the causes and the effects of floods.

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1.
In Britain millions of homes are required to keep up with housing needs. How does building houses increase the risk of flooding?
Increased water consumption increases the amount of necessary drainage
More lawns, trees and other vegetation leads to more water soaking into the ground after people water their plants
Increased pollution leads to more rain in the immediate area
Impermeable surfaces, better drainage and less vegetation all lead to water reaching the river channels faster
Housing estates increase the amount of impermeable surfaces in the form of house roofs, concrete, tarmac, and membranes. These solid surfaces drain water into drainage systems that carry the water into the streams far quicker. Modern housing estates attempt to allow more natural drainage to increase the time it takes for water to reach the main river channel
2.
Which of the following river features are not influenced or created by flooding?
Waterfalls
Levees
Ox-Bow Lakes
Flood plains
Waterfalls are created by the river channel passing over layers of soft and hard rock, rather than catastrophic flood events. Levees can form naturally as a result of the slowing down of water flow as the flood water leaves the main channel. The quantity of water in ox-bow lakes can be altered by flooding and flood plains are the area of land either side of the river channel that floods naturally
3.
In 1993 the Mississippi river overtopped its banks and flooded a huge proportion of the USA. How did hard engineering lead to this catastrophic flooding?
Dams were built to hold back the water levels. Due to the soft geology on either side these failed. The extra pressure of the water from upstream dam failures contributed to failures further down stream
Levees had been constructed to raise the banks, as the river bed silted these were raised and the river flowed higher than the surrounding land. When they failed the water drained across the land
Straightening of the rivers lowered the bed level, meaning the channel was able to hold more water. The extra water created a greater flood when the banks burst
The width of the river valley was increased, allowing more water to build up in the valley before the banks burst and the river flooded the area
The water levels were so high that 6 million acres of farm-land was flooded. The human causes of the flood include the poorly built levees, the development of unsuitable areas and large amounts of channelisation
4.
How do river floods occur?
When the volume of water in the river is more than the channel can contain
When it rains heavily, increasing the amount of water in the river channel
When dams release water, giving a sudden large amount of water in the river channel
During spring melt, leading to a large amount of water suddenly entering the channel
The technical definition of a flood is when the water leaves the main river channel or defined edges. Whilst spring melt, dams bursting and heavy rains will increase the amount of water in the river channel, many river channels can cope with this
5.
Rather than putting expensive flood defences in place, why are some areas being abandoned to flooding?
The areas abandoned are industrial areas, reducing the economic cost and risk to residential areas
Council planning and zoning means that it is illegal to build on the floodplain. Older properties that have already been built on the flood plain do not represent a cost-effective area to protect
Allowing rivers to flood is cheaper in the short term, but may be more expensive in the long term as it encourages flooding down river over time
Allowing rivers to naturally flood areas that are undeveloped reduces the risks of flooding further down river. The cost of protecting the entire length of every river would be prohibitively expensive
Flooding undeveloped areas or sparsely developed areas is a cost effective solution. Expenisve hard engineering projects often lead to more problems down stream
6.
Once raised the Thames Barrier stops water movement along the river. How do barriers such as this reduce the risk of flooding?
The barrier prevents the high tide increasing the water level in the river
The barrier holds back the outflowing river discharge
The barrier prevents groundwater entering the river
The barrier stops tributaries entering the main channel
Rising sea levels and the land level falling in the south of England both put London at a high risk of flooding. The Thames Barrier is meant to hold back the sea but it has been suggested that low-lying land either side of the barrier may allow the sea to pass round it
7.
Each year the Amazon River and several tributaries over top their banks and flood the surrounding area. Which of the following is not a positive of these seasonal floods?
The floods deposit alluvial silt on the thin infertile soils of the rainforest floor
The flooded areas provide a unique habitat for certain species of fish during the flood season
Increased pollution levels in the river may lead to an increase of pollution in the flooded soils
Seasonal pools either side of the river are flooded and the water refreshed after the drier periods
Some fish, such as discuses and other cichlids, have adapted to these yearly flooding events. Some fish live in the pools either side of the river and thousands die during the dry season. Others use the flooded plants to raise their fry before retreating water levels force them back into the main river channel.
8.
How do impermeable rocks influence the risk of flooding?
Impermeable rocks increase the flood risk
Impermeable rocks decrease the flood risk
Impermeable rocks don’t influence flood risk, but they do force rivers to rise faster
Impermeable rocks are never associated with rivers
Water can't penetrate the surface of impermeable rocks. Numerous igneous and metamorphic rocks are impermeable, forcing the water to run over their surface and rapidly into the drainage system
9.
In Ancient Egypt they welcomed the floods each year with ceremonies and celebration. How are floods beneficial to the land either side of the river?
The flood destroys predators of domestic animals
The silt deposited by the flood can make the ground more fertile
The flood removes debris from the land
The flood drops stones and blocks on to the land which are used as building materials
Alluvial soils can contain organic material, but also the minerals and other nutrients that might be depleted by plant growth and rainfall
10.
Deforestation can increase the risk of flooding. Which of these is not a way trees, vegetation and forests decrease the chances of flooding?
The uptake and storage of water into the leaves and stems of plants removes it from the ground
Less water gets to the river as transpiration via the leaves means water is returned to the atmosphere
Water falls on the trees and so does not reach the ground
River banks are made more stable by vegetation
Vegetation is one of the leading ways that water can be slowed as it passes through the drainage basin. Trees draw up the water from their roots and pass it straight into the atmosphere, or store it themselves. They also allow natural drainage through the soil rather than over land across hard packed earth
Author:  Ruth M

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