Rivers, Floods and Management: River Flooding

River Flooding
1. Thursday 19th November 2009 was the wettest day ever in the UK, it unleashed a devastating flood that inundated farmland, washed away bridges, and tore apart towns and villages in western Cumbria.
2. Flooding is where land that is usually above water becomes inundated.
3. Often occur after a period of prolonged rainfall.
4. Slow-build-up river floods are common in the UK in late winter and early spring where heavy rain coincide with the melting snow
5. Flash floods are more dramatic floods following torrential storms.
Causes of River Floods
1. Human Factors
  1. Building Construction: Building on the floodplain creates impermeable surfaces (tarmac roads etc). Sewers and drains also transfers water quickly to nearby channels causing them to fill up quickly and overtop the banks
  2. Deforestation: Trees leaves intercept the rain from hitting the floor. Much of the water that falls on trees is evaporated or stored temporarily on leaves. Trees use water for growth, so if trees are cut down then much more water is suddenly available to be transferred rapidly to river channels-increasing the risk of flooding
  3. Agriculture: With arable farming, there is a period where the soil is left bare and exposed to the elements. This can lead to increased runoff, which is made worse if the land is ploughed up and down a slope (instead of across it), because it provides a quick flow for the water downslope in groves left by the plough.
  4. Ineffective Flood Management and Warning Systems: Lack of appropriate defence schemes, or poor warning systems, can increase the risk of flooding and its impacts on people. Flooding is more likely if flood defences aren't repaired or upgraded. badly maintained culverts (underground channels) or bridges can also lead to blockages and flooding
2. Physical Factors
  1. Rock Type: Impermeable rocks encourage overland flow, which transfers water rapidly to river channels
  2. Steep Slopes: In mountain environments, steep slopes encourage a rapid transfer of water, both by overland flow and throughflow
  3. Long Profile: If a river has a steep upper course and a flat lower course, water is transferred rapidly to the lowlands, where the gentle gradient causes it to slow down. This can lead to a build up of water, which might ultimately cause flooding.
  4. Drainage Density: A high drainage density means the water reaches river channels quickly. This speeds up the transfer of water and can make flooding more likely.
  5. Snowmelt: Snow and ice form a store of water. When they eventually melt, this store is rapidly released and, if the ground is still frozen, the meltewater flows quickly into rivers and increases the risk of flooding.
  6. Intense and Prolonged Rainfall: Intense rainfall events, like summer storms in the UK, tropical storms (hurricanes) in the Caribbean, and monsoons in India, have the potential to cause significant floods
Flood Risks in the UK
1. Flood risk I the UK is managed by three separate agencies, depending on where you live: England and Wales-The Environment Agency, Scotland-The Scottish Protection Agency, Northern Ireland-The Rivers Agency
2. Each agency has produced an interactive map which plots the possible extent of river and sea flooding, and locates flood defences. This service is to raise awareness of floods in people's areas.
3. The severity, or magnitude, of a flood is generally related to its frequency-the higher the frequency, the lower the magnitude or severity.
4. High magnitude events are rare, once every few hundred years or so. Low magnitude events are much more common, possibly several times a decade.

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Rivers, Floods and Management: River Flooding

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	Created by Andrew_Ellinas about 11 years ago