Rivers, Floods and Management: Flood Management

Approaches to Flood Management
1. Basin Management: employs schemes that aim to reduce the amount and speed of water flowing towards a river. Large-scale tree planting (afforestation) increases interception and uses up some of the water. The construction of a reservoir to store and regulate water flow is another effective form of basin management.
2. Channel Management: Makes changes to the river channel itself, e.g. increasing the height of the banks or straightening the channel.
3. Hard Engineering: Intervenes directly with the river's natural process. It usually involves construction e.g. concrete embankment alongside a river channel.
4. Soft Engineering: Aims to work with the river's natural processes. Tends to have less of an impact on the environment than hard engineering. One example is the creation of wetlands on a floodplain to store water.
5. Warning and Forecast: By careful monitoring of precipitation and river levels, it's possible to issue warnings to people to prepare themselves for a possible flood, e.g. move furniture upstairs and use sandbags to seal the outside doors.
6. Do Nothing: This approach is often adopted in locations where there is a low level of risk. Some people get used to regular, small floods.
Making Decisions
1. Choosing the right option for a particular place depends on many factors, including construction costs, maintenance costs, social, economic and environmental impacts, the views of local people, government policy, and so on.
2. Very often, the main factor is cost, which relates to the potential severity or magnitude of a particular flood risk.
3. Most flood defence schemes aim to protect places from a 1 in 100 year flood. This is a flood with a magnitude likely to occur, on average, only once every 100 years
Hard Engineering Options
1. Dams and Reservoirs
  1. Used to regulate river flow and reduce risk of flooding.
  2. Most dam projects are multi-purpose which means they serve several different functions, e.g. flood prevention, irrigation, water supply, HEP (hydroelectric power), and recreation (fishing, sailing etc)
  3. During periods of high rainfall, water can be stored in the reservoir: It can then be released slowly during low flow conditions. This allows steady flow of water to be maintained throughout the year.
  4. They cost a huge amount of money to build, and the reservoirs behind them often flood large areas of useful land.
2. River Straightening
  1. Engineers apply the same method that happens when a river becomes straighter due to oxbow lakes being created, but do it artificially. This is called river straightening. Water flows faster and is transferred downstream more rapidly.
  2. While this might relieve the flood risk in one location, it increases it further downstream. So the problem isn't dealt with, just shifted somewhere else.
  3. Channelisation is when straightened sections of the river are lined with concrete, which then increases flow along the river and reduces bank collapse, which can cause the channel to silt up.
  4. Creates unnatural-looking river environment, and damages local wetlands and wildlife habitats.
3. Levees and Embankments
  1. A raised riverbank is usually called an embankment in the UK, but in the USA the term levee is more common, and this allows to hold more water before flooding occurs.
  2. A dredger mounted on a barge, or on a riverbank, scoops sediment from the river channel (lowering the riverbed) and dumps it onto their banks to raise the height. This double approach is sustainable and has minimal impact on the environment
  3. Concrete walls and embankments provide a more-effective form of defence, but cost more to build.
4. Diversion Spillways (by-pass channels)
  1. Diverts excess water away from built-up areas. During high flow periods, sluice gates are opened to allow the excess water to flow along the new channel.
  2. Very expensive to build and had positive impact on the environment by creating new wetlands.
  3. Popular for recreational activities, like walking and fishing.
Soft Engineering Options
1. Planting Trees
  1. Afforestation programmes, often in upland areas, increase interception and slow down water transfer
  2. Trees use water so reduces the amount flowing into river channels.
2. Establishing Wetlands
  1. Important habitats and breeding grounds as well as being efficient in storing water
  2. Areas that have been drained and developed are being returned to their natural state, as wetlands.
3. Riverbank Construction
  1. Stabilising riverbanks to prevent their erosion and collapse - and the subsequent silting-up of the river channel.
  2. Planting vegetation which bind together the loose sediments forming the banks.
4. Land Use, Management and Floodplain Zoning
  1. Areas close to rivers that are prone to flooding can be kept clear of expensive and vulnerable developments like housing. Instead used for pasture and recreational use.
  2. This type of land use reduces the amount of water flowing into rivers (and its speed) but, should a flood event occur, the damage will be minimal. This approach of land use is called floodplain zoning.
5. River Restoration
  1. This involves returning a river that has been artificially altered to its natural state.
  2. River restoration projects often involve restoring features like meanders to rivers that have been artificially straightened.
Flood Forecasts and Warnings
1. There are three levels of warning that can be issued, using the Internet and emergency services like the police
  1. Flood Watch: Flooding of low-lying land and roads is expected. People are urged to be prepared and to watch river levels
  2. Flood Warning: There is a threat to homes and businesses. People are urged to move items of value to higher floors and to turn off services such as electricity and water. Evacuation is a possibility
  3. Severe Flood Warning: Extreme danger to life and property is expected. People urged to seek high place in their homes for safety, or to evacuate the property

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Rivers, Floods and Management: Flood Management

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