River management strategies
Land-use zoning(£0.5 million if the business agree, could be more if affects trade) / Land use zoning involves dividing the floodplain into areas which experience different degrees of flood risk. e.g
Red Zone = High chance of experiencing a flood
Amber Zone = Flood possible but rare
Green Zone = Flooding very unlikely.
Once the floodplain has been divided into the different zones, an appropriate landuse is chosen for each zone e.g
Red Zone = Golf Course, Parkland, farmland
Amber Zone = Car parks, Sports facilities, some houses but these should have flood protection considered in their design
Green Zone = Residential housing, flood protection does not need to be considered.
This mean the river can flood naturally but will cause minimal damage and disruption to people. Floodplain zoning is an effective method to manage floods because it is cheap and effective at slowing the river down. Unfortunately it is not realistic if the area is all ready built on because people would have to be moved and homes destroyed. This would be extremely expensive and very unpopular for those businesses or people that are forced out of their homes.
Channel enlargement (deepening and widening)
(£0.5 million per year) / Flooding can be reduced by increasing the size of the channel so that it can hold more water. This means that in times of very heavy rainfall the river is less likely to flood over its banks. This has the advantage of protecting the land either side of the river. It also does not greatly affect the look of the river or spoil the natural habitat for animals. Channel deepening and widening will need to occur regularly as it will quickly silt up.
In order to increase the depth and width of the river heavy machinery will have to be used, which is expensive. Although it may not drastically alter the look of the river, it may disrupt the natural ecosystem. Some also argue that the natural process of flooding helps to slow the river down. If you prevent this by making the channel deeper and wider it will result in very high river speeds (velocity) that could be extremely dangerous and make things worse downstream where it has not been enlarged.
Afforestation
(£0.5 million) / Planting trees in the upper course of the drainage basins helps to reduce flooding because they can intercept the rain and store it. Trees store water on their branches, on their leaves and in their roots. This helps to stop some of the water from going into the river and slows down the rest of it. This reduces the risk of flooding.
Afforestation does not prevent flooding from occurring and so houses could still be in danger, but it does help to reduce the likelihood of flooding.
Afforestation is only possible if there is a lot of spare land that trees can be planted on. It is also only possible if farmers or other land owners agree. A benefit of afforestation is that new woodlands can improve the natural environment by providing homes for animals. The timber can also be harvested generating extra income.
Widened bridge
(£1 million per bridge) / The bridges would be widened so that debris such a trees and boulders can not be trapped behind them creating a dam that lead to the large wall of water. The new bridges may not be as attractive as the old arched bridges
Washlands
(£0.25 million) / Washlands are areas of land in the lower course that the river can ‘wash’ or flood into in times of flood. They are a bit like a storage area for the extra water. When the river floods, it spills over into the washland, this not only stores the water but also increases the amount of friction, which slows the speed of the river down. To create a washland, a large area of land should be cleared and left empty near to the river. This land should not be used and must be large enough to store the floodwater.
Washlands can be effective in helping reduce the impact of flooding, but it can be difficult to set aside enough land especially in a city. Another problem can be that the washlands can become a dumping ground for rubbish and become an unpleasant sight.
Straightening the River
(£0.25 million per meander) / A meandering river can be straightened. This helps to increase the speed (velocity) of the river and therefore reduce flooding because the water drains away from the flood plain more quickly. Removing the meanders also makes using the land either side of the river much easier.
e.g for farmers, it means that their fields no longer change size because the meanders keep changing its position. This is a particularly problem if you farm only one side of the river
e.g for property developers it is easier to build a long straight river than one that keeps changing its course.
Straightening is not always successful, frequently straightened rivers quickly return back to their natural meandering shape. There is also a large environmental impact; fish tend to lay eggs in parts of the river where the flow is slower. Without meanders the river flow is a lot faster and so fish are unable to lay their eggs. Some also argue that the increase in speed just creates bigger floods and problems downstream.
Levees and embankments (flood walls)
(£0.5million per 1km) / Levees and embankment walls can be built either side of the river. The walls mean that even if the river floods above its natural riverbanks the water cannot spread over the flood plain and damage property and land. They can be very effective in preventing flooding.
Building the levees and embankment walls can be quite expensive although it can be carried out without disturbing the river too much. (Especially if the walls are set back from the river).
Some argue that the walls can spoil the look of the river, although this may depend on the material used and how well they are landscaped. One major criticism of embankments is that they can increase the speed (velocity) of the river and so if the flood does break the embankments, the flood can often be much more destructive because the water is travelling so fast. When a river naturally floods it also deposits alluvium or silt on the floodplain, this makes excellent farmland. Levees prevent this natural deposits occurring. Also if the river does break the embankments the water cannot easily drain back into the river after the flood.
Dams or reservoirs
(£3 million) / Dams and reservoirs could be built in the upper course of a river to hold back the floodwater. This could then be slowly released after the rains have stopped. Dams could ensure the risk of flooding is nearly completed removed. Dams could also be used to generate hydro-electricity power, which is a cheap and renewable way to produce electricity. The lakes behind the dam could also be used for recreational activities such as sailing or canoeing. However there are serious questions over the use of dams to help prevent flooding. They are extremely expensive. They would flood large parts of the valley. They will have a large impact on the natural environment and finally some argue there is the potential danger from collapse in earthquakes or terrorist attacks.