FLOODS | Causes of Floods

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Definition and Causes of Floods

A flood occurs when water covers an area that is not normally covered with water.

Most flooding is the result of a rise in the level of water in a river - after heavy rain in a catchment area.

The river will overflow its banks and water will cover the flood plain.

Physical Causes

  • Extended periods of heavy rain including cyclones

  • Steep mountain slopes that lead to high water run-off

  • Saturated soil that prevents water infiltration

  • Volcanic eruptions can melt ice, ash can dam rivers

  • Earthquakes can cause tsunamis or divert rivers

  • Flat land where rivers can naturally overflow

  • Human Causes

  • Urban areas have rapid water run-off, causes rapid rise in river levels

  • Altering the shape of river channels may cause rivers to flow faster

  • Building dams that change the natural flow of rivers – threat of dams breaking or overflowing

  • Deforestation and vegetation removal increases run off

  • Destruction of wetlands reduces soil’s capacity to retain water

  • Poverty and overpopulation may force people to live in flood risk areas

Distinguishing between Physical and Human causes of flooding

Physical

1. Gradient- Steeper slopes allow for a high runoff speed. Flat land where rivers can naturally overflow.

2. Vegetation- If the area has little vegetation, for example, due to overgrazing, there is very little interception and direct runoff occurs.

3. Rock/ soil type- Water will not infiltrate impermeable rock such as granite. Clay soil will also result in direct runoff. Surface runoff increases the risk of a flood. In the same way, the surface run-off of rainwater will be high on impermeable rocks like granite

4. Types of rain - Convectional thunderstorms will cause excess rain, which will saturate the first layers of soil and then run off.

5.River basin- A round drainage basin allows for tributaries to converge simultaneously, increasing the chance of flooding.

6. Tropical cyclones- Tropical cyclones and hurricanes cause excessive rain. In 2000, tropical cyclone Eline produced 544 mm of rain in one day.

7. Tsunami - An earthquake from tectonic plate movements on the sea floor causes coastal flooding, such as in Japan in 2011 (10m high wave)

8. Climatic phenomena- Increased seasonal rainfall as a result of climatic phenomena such as La Nina and summer monsoons

9. Tectonic shifts - Cause dams and lakes to break.

10. Volcanic Eruptions- Can melt ice and snow. The ash can dam rivers.

Human

1. Vegetation may be cleared for building or by overgrazing. With no vegetation to intercept rainfall, there is more direct runoff and less infiltration into the soil.

2. Urban development leads to the ground being covered by materials such as brick, tar, and cement, resulting in more direct runoff.

3. The floodplain shows the extent to which the river floods. Houses that are built on flood plains are more likely to be flooded.

4. Dams can reach their capacity, burst their banks, or d the dam wall. Areas downstream are flooded.

5. If a river's course is re-directed by making a meander when the river naturally flows straight when the river flows strongly it is likely to follow its old course, which is more direct.

6. Changing river channels, walls, and beds. Concrete river channels allow for vast amounts of fast-flowing water. This could be dangerous in urban areas as the river might reach capacity and flood. The water is fast-flowing and the land surface does not allow for infiltration. There is also decreased friction in the channel as the channel surface is concrete which increases the river's velocity.

7. Changing river channels, walls, and beds. Concrete river channels allow for vast amounts of fast-flowing water. This could be dangerous in urban areas as the river might reach capacity and flood. The water is fast-flowing and the land surface does not allow for infiltration. There is also decreased friction in the channel as the channel surface is concrete which increases the river's velocity.

8. Building too close to the coastline leaves many dwellings susceptible to coastal flooding.

Main Types of Floods

1. Flash floods are fast-moving waters that sweep everything in their path. They are caused by heavy rainfall or rapid snow thaw. Floods usually cover a relatively small area and occur with little to no notice, generally less than six hours. The rapid water torrents can move large objects such as cars, rocks, and trees.

2. Coastal floods are caused by strong winds or storms that move towards a coast during high tide. When powerful waves breach the coast’s dune or dike, the area is usually flooded. Coastal areas with fewer defenses and lower elevations are the most affected. The best time to repair the breach is during low tide.

3. River floods are characterized by gradual riverbank overflows caused by extensive rainfall over an extended period of time. A  river flood occurs when a river has more water in it than the river's channel can hold. The water then flows over the banks of the river, onto the adjacent land areas. The areas covered by river floods depend on the size of the river and the amount of rainfall. River floods rarely result in loss of lives but can cause immense economic damage.

4. Urban floods occur when the drainage system in a city or town fails to absorb the water from heavy rain. The lack of natural drainage in an urban area can also contribute to flooding. Water flows out into the street, making driving very dangerous. Although water levels can be just a few inches deep, urban floods can cause significant structural damage.

5. Pluvial floods form in flat areas where the terrain can’t absorb the rainwater, causing puddles and ponds to appear. Pluvial flooding is similar to urban flooding, but it occurs mostly in rural areas. The agricultural activities and properties in areas where pluvial floods have occurred can be seriously affected