Floods occur when a normally drier land area is temporarily submerged in water overflowing from rivers, dams, runoff (water that accumulates and flows after heavy rainstorms or snowmelts), or tides.
Floods can be caused by several factors: heavy rainfall over a short period, moderate rainfall over a long period, melting snow and hurricane storm surge (a dome of water that builds up as a hurricane moves over water).
Floods are the deadliest form of natural disasters, killing more every year than tornados, lightning, earthquakes, and forest fires combined.
Flood Description Criteria
Scientists describe floods according to three criteria: the maximum height of the water above normal levels during the flood, the time period required for the flood waters to rise and fall, and the size and frequency with which similar floods are likely to occur.
Height of the floodwater: In describing the maximum height of a flood, scientists refer to the crest of a river (its maximum height during a flood) or the height of the floodwaters over the floodplain, a low-lying area near a water source that is normally dry, but is subject to overflow by a river, lake, or water from a man-made water barrier.
When describing floods according to the time period required for the water to rise and fall, scientists often refer to flash floods. Flash floods are floods that occur in only a few hours or even minutes, usually due to heavy rainfall or a dam break. Flash floods can also occur when ice or debris to obstruct the flow of river water, causing water to back up upstream, breaks loose and a wall of water rushes downstream and can cause flash floods.
Speed of the flood: Flash floods are dangerous because they are usually accompanied by fast moving water. Fast moving water usually occurs in drainage ditches, canyons, and in rivers and creeks. Flash floods can produce fast-moving walls of water up to 20 feet (6 meters) high. Two feet of fast moving water can wash away cars, and flash floodwaters only 6 inches (15 centimeters) deep have knocked down people.
Size and frequency of the flood: Scientists also categorize floods according to the size of the flood and the likelihood of another similar flood in the same place within a one-year time-frame. For example, a 100-year flood means that the chance of a similar flood occurring within the same year is 1 in 100. A 5-year flood has a 5 in 100, or 20% chance of a similar flood happening in the same place within a year.
The classifications of 5-year flood, 10-year flood, 25-year flood, 50-year flood, 100-year flood, or 500-year flood, actually refer more to size of the flood than to predictions when a similar one will happen again in the future. The water volume of the flood increases along the scale as the frequency decreases. A 5-year flood is almost always a mild occurrence, whereas a 500-year flood tends to have very high and violent water flow, and covers a wide area of the floodplain.
Measures To Prevent Flooding
- Introduce better flood warning systems:
Giving people more time to take action during flooding, potentially saving lives. Advance warning and pre-planning can significantly reduce the impact from flooding.
- Modify homes and businesses to help them withstand floods:
The focus should be on flood resilience rather than defence schemes. Concreting floors, waterproofing homes and businesses and moving electric sockets higher up the walls to increase resilience.
- Construct buildings above flood levels:
Constructing buildings, say one metre, from the ground to prevent flood damage.
- Protect wetlands and introduce plant trees strategically:
The creation of more wetlands – which can act as sponges, soaking up moisture – and wooded areas can slow down waters when rivers overflow. These areas are often destroyed to make room for agriculture and development. Halting deforestation and wetland drainage, reforesting upstream areas and restoring damaged wetlands could significantly reduce the impact of climate change on flooding.
- Improve soil conditions:
Inappropriate soil management, machinery and animal hooves can cause soil to become compacted so that instead of absorbing moisture, holding it and slowly letting it go, water runs off it immediately. Well drained soil can absorb huge quantities of rainwater, preventing it from running into rivers.
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