The likelihood of a river bursting its banks and flooding is determined by factors in the surrounding landscape, such as steepness of the river valley, the amount of vegetation and the prevailing rock-type. The short-term impact of floods can be catastrophic, but they can have positive long-term effects as well.
Causes of flooding
A flood occurs when a river bursts its banks and the water spills onto the floodplain. Flooding tends to be caused by heavy rain: the faster the rainwater reaches the river channel, the more likely it is to flood. The nature of the landscape around a river will influence how quickly rainwater reaches the channel.
The following factors may encourage flooding:
- A steep-sided channel - a river channel surrounded by steep slopes causes fast surface run-off.
- A lack of vegetation or woodland - trees and plants intercept precipitation (ie they catch or drink water). If there is little vegetation in the drainage basin then surface run-off will be high.
- A drainage basin, consisting of mainly impermeable rock - this will mean that water cannot percolate through the rock layer, and so will run faster over the surface.
- A drainage basin in an urban area - these consist largely of impermeable concrete, which encourages overland flow. Drains and sewers take water quickly and directly to the river channel. Houses with sloping roofs further increase the amount of run-off.
Flood management techniques include river engineering, afforestation and planning controls to restrict urban development on floodplains.
The impact of flooding
Floods can cause damage to homes and possessions as well as disruption to communications. However, flooding can also have positive impacts on an area. Flooding deposits fine silt (alluvium) onto the floodplain, making it very fertile and excellent for agriculture. People living on or near floodplains may rely upon regular flooding to help support their farming and therefore provide food.
LEDCs tend to be affected more than MEDCs by the effects of flooding. This is partly because LEDCs have more farms, and farming communities are attracted to fertile flood plains. LEDCs often do not have the resources to prevent flooding or deal with the aftermath of flooding.
Case study: causes and effects of flooding in Mozambique (2000)
The Mozambique floods of 2000 show that what happens in one country can very often affect another. The flooding was triggered by exceptionally heavy rain in South Africa, lasting for five weeks in early 2000. Botswana was particularly badly hit, receiving 75 per cent of its yearly rainfall in three days. On 22 February, Cyclone Eline hit, bringing more heavy rainfall. The rain from Botswana and other Southern African countries ran into the Limpopo, Zambezi and other rivers which flow through Mozambique to the sea. These rivers eventually burst their banks, causing severe flooding in Mozambique.
In addition, the loss of grassland and draining of marshland for farms contributed to more rapid surface run-off. The results were disastrous: services were cut off and many people were stranded, homeless or had died through drowning or disease. Urbanisation in South Africa may have contributed to the large quantities of surface water run-off swelling the rivers.
Steps can be taken to manage flooding. Often these steps involve trying to lengthen the amount of time it takes for water to reach the river channel, thereby increasing the lag time. Flood management techniques can be divided into hard- and soft-engineering options.
Hard options tend to be more expensive and have a greater impact on the river and the surrounding landscape. Soft options are more ecologically sensitive. The tables summarise the main flood management techniques.
-Settlements and agricultural land may be lost when the river valley is flooded to form a reservoir.
|River engineering||-The river channel may be widened or deepened allowing it to carry more water. A river channel may be straightened so that water can travel faster along the course. |
-The channel course of the river can also be altered, diverting flood waters away from settlements.Altering the river channel may lead to a greater risk of flooding downstream, as the water is carried there faster.
Different interest groups have different views about flood management techniques:
|Afforestation||-Trees are planted near to the river. This means greater interception of rainwater and lower river discharge. This is a relatively low cost option, which enhances the environmental quality of the drainage basin.|
|Managed flooding (also called ecological flooding)||-The river is allowed to flood naturally in places, to prevent flooding in other areas - for example, near settlements.|
|Planning||-Local authorities and the national government introduce policies to control urban development close to or on the floodplain. This reduces the chance of flooding and the risk of damage to property.|
-There can be resistance to development restrictions in areas where there is a shortage of housing. Enforcing planning regulations and controls may be harder in LEDCs.
- Governments and developers often favour large hard engineering options, such as dam building. Building a dam and a reservoir can generate income. Profits can be made from generating electricity or leisure revenue.
- Environmental groups and local residents often prefer softer options, such as planting trees. Soft options cause little damage to the environment and do not involve the resettlement of communities.
- Effective flood management strategies should be economically, environmentally and socially sustainable. Sustainable strategies allow management without compromising the needs of future generations.
Case study: coping with flooding in Bangladesh
Bangladesh is an LEDC. The land is densely populated. Most of the land forms a delta from three main rivers - Ganges, Brahmaputra and Meghna - and 25 per cent of Bangladesh is less than 1 m above sea level. Flooding is an annual event as the rivers burst their banks. This seasonal flooding is beneficial as it provides water for the rice and jute (two main crops in the area) it also helps to keep the soil fertile. Bangladesh also experiences many tropical cyclones. The low-lying land means it is easily flooded. Half the country is less than 6m above sea level. The snowmelt in the Himalayas adds water into the main rivers. There are human causes too - building on the floodplains and cutting down trees both increase the effects of flooding.
There are advantages to living here:
- The flat floodplains of the delta are very fertile. Rice is grown.
- The area can also be used for shrimp farming.
- The low-lying islands are very vulnerable and flood easily. It is difficult to protect them.
- There are poor communications. Many locals do not own their own telephone or television so it is difficult to give successful flood warnings.
How can the risk of flooding be reduced?
Bangladesh is an LEDC and therefore does not have money to implement large schemes.
It is always going to be threatened with flooding, so the focus is on reducing the impact.
The Flood Action Plan is funded by the world bank. It funds projects to monitor flood levels and construct flood banks/artificial levees.
More sustainable ways of reducing the flooding include building coastal flood shelters on stilts and early-warning systems.
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