What are Floods: Causes and Effects of Flooding

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Flood is an overflow of water from water bodies that partially or completely submerges land that is usually dry. Floods are events produced by natural phenomena such as rain, hurricanes, or melting ice and snow. It can also be produced by human activity.

Types of Floods

There are various types of floods according to the official disaster prevention agencies around the world.

Flash Floods

These are events in which water accumulates and saturates the ground in a very short period of time. They are generally floods caused by sudden onset but intense rainfall. That is, rains that are generated suddenly and also last a long time.

Flash floods are considered one of the most dangerous categories, since by their nature they make prevention and eviction tasks difficult for affected populations.

One of the most outstanding characteristics of flash floods is that they tend to be generated on steep slopes and rapidly flooding low-lying areas. This facilitates the rapid descent of water and flooding of land, with the structural and human damage that this implies.

Slow-Onset Floods or Groundwater Floods

This type of flood is generated by persistent rains that can be intense or moderate, which causes a gradual saturation of the land. In these cases, rainfall can last for at least two to four days.

The persistent nature of rainfall can cause material damage to poor structures, as well as damage crops. However, in this case it is possible to activate damage prevention and containment protocols because the time frame is longer.

River Floods

Overflowing of rivers causes this type of floods. Rivers are overflowing either due to problems associated with the quality of the land on their banks or due to a considerable increase in water flow. A sharp increase in water level can be caused by heavy or prolonged rains, or by melting snow in areas near the river.

Storm Flooding

It is any saturation of the land caused by rains. In some countries, floods of pluvial origin are a seasonal phenomenon. In India for example, rains caused by monsoon winds generate large floods during the summer months.

Coastal Flooding

Rising sea level leads to this kind of floods. During such events, large amounts of water enter the mainland, due to the effect of heavy rains, tidal waves, tsunamis, storms or typhoons. In this sense, coastal flooding can occur in one of three ways:

  • Direct: the water directly enters the mainland in the absence of natural or artificial barriers.
  • Barrier failure: water breaks a natural or artificial barrier as a result of strong waves.
  • Over barrier: the size of the tide exceeds a barrier, either natural or artificial, and floods the land.

Urban Floods (drain and sewer flooding)

This is a type of floods that affects cities and urban communities. Urban floods can have a natural origin such as heavy rainfall or overflowing of rivers. However, it can also be a result of human activity. For example, inadequate drainage systems, breakdown of embankments, population density, soil erosion, and increased garbage production are some of the factors that cause urban flooding.

Causes of Floods

Floods can have several causes. These are the events or activities that can produce floods:

Natural Causes

They have to do with natural events that occur without human intervention, such as:

  • Rains: floods are caused when rains saturate the land and water is not drained fast enough.
  • Thaw: water produced by thawing ice and snow reaches the rivers and increases their water volume, causing them to overflow.
  • Storms, hurricanes, typhoons, tsunamis: generate waves of great magnitude that knock down natural or artificial barriers and facilitate the entry of water inland at high speed.

Human Activity

They are factors related to human activities that have a direct impact on bodies of water and land quality. Some of those activities are:

  • Hydraulic failures: dam or dike bursts, which release a large amount of water in a very short time cause overflowing of water bodies and rapid soil saturation. 
  • Water pollution: solid waste and sewage can obstruct the flow of water in water bodies, causing their elevation and subsequent overflow.
  • Erosion of the land: urban works, excess cultivation or contamination decrease the quality of the land, increasing its permeability, which makes it susceptible to rapid saturation.

Consequences of Floods

Floods have an impact on multiple levels, and end up affecting not only the environment but also local and national economies.

Environmental Consequences

Floods can generate slight, moderate or severe modifications in the affected topography. These changes in the terrain can be part of natural dynamics (such as monsoon rains) or human activity, and can affect local ecosystems.

If a crop is washed away by a flood, the ecosystem of which it was a part is also altered. Pollinating agents (such as bees) and animals that benefited from crops to feed themselves, can move to other areas and negatively alter that new place.

Economic Consequences

Floods can cause considerable structural damage, affecting homes, commercial premises, and crops. All this has a negative impact on the local economy, which in turn deteriorates the living conditions of the affected people.

In 2017, monsoon rains flooded the tea crops of Darjeeling, India. The loss of the harvest had an impact on the Indian economy and increased the value of Indian tea worldwide, hurting end consumers.

Social Consequences

In the most severe cases, floods can cause human loss or serious injuries. In addition, this type of event can induce forced population displacements, following the loss of housing or a source of income.

It is also possible that the authorities order the suspension of educational, work or recreational activities, which directly impacts the quality of life.

Health Consequences

Floods can generate foci of diseases such as dengue, infectious, dermatological diseases, digestive disorders, etc. In the most disadvantaged areas, these types of situations worsen the already precarious conditions of the local health systems, by collapsing their care capacity.

Is it Possible to Predict Floods?

There are numerous early warning systems to anticipate the arrival of floods, especially when faced with processes of climate change or seasonal periods of storms.

In general, these systems are based on mathematical formulations and calculations. These models allow specialists to predict flood margins of rivers and lakes. In some cases, it is possible to predict a flood days in advance, which can translate into lives saved. At the international level, many organizations are dedicated to fighting the effects of floods.

Most Vulnerable Regions

Usually the coastal plains and low elevation regions are more susceptible to flooding. Much of India, which is laced with rivers, is considered to be a high risk flooding area. Also, the states of Florida, Louisiana and New York in the United States, as well as Canton in China and Ho Chi Minh City in Vietnam are floodplains.

Other regions that chronically suffer from floods are the north of Argentina, part of Uruguay and the Caribbean islands. As well as Asia Minor (Asian Turkey) and the Mediterranean area due to rainfall events occurring in the autumn.

How to Prevent Floods?

Since ancient times people built defenses against water flows such as breakwaters, dikes, embankments or intricate drainage systems to protect cities from flooding. Not much more can be done about it other than keeping the population trained for the dangerous situations that a flood can bring.

On the other hand, global warming makes floods worse, since the  melting of the poles raises the level of sea water. Thus, preventing global warming through ecological measures, can also be considered as a flood prevention measure.

5 Most Devastating Floods in History

Frequent floods are considered the deadliest natural disasters in history, causing some between 145,000 deaths to nearly 4 million. The main cause of these floods are storms and heavy rains that last for days or even weeks. These are the most devastating floods in the history of mankind:

The Great Mississippi River Flood of 1927

It is remembered as the most destructive flood in the history of the United States, having moved 65,000 cubic meters of water every second. During the summer of 1926, heavy rains were witnessed in the central part of the Mississippi River. By Christmas Day the water level of the Cumberland River was above 17 meters, a record that remains to this day.

The river exceeded its levels in 145 zones, flooding more than 70,000 square kilometers with a depth of 10 meters. It caused $400 million in damage and claimed the lives of 246 people. Although it has not been one of the deadliest floods, the destruction still remains a record for the United States.

The overflow of the Amazon in 1953

The reason for this flood was the 1953 overflowing of the Amazon river which caused the flow of 370,000 cubic meters of water per second. The Amazon river is immense, it is responsible for 20% of the planet’s freshwater discharge into the ocean basins. The destruction caused by its overflow is enormous due to the large amount of infrastructure and dams that are built around the river.

The Volcanic flood of Alaska (Aniakchak flood)

But sometimes floods are caused by other factors, for example volcanoes. This is the case of the Aniakchak flood, which 10,000 years ago was caused by the eruption of a volcano. Following the eruption a crater known as a caldera, was created. This crater was filled with rainwater over the years. When the water reached the limit, it eroded the caldera and all the water flowed out with the speed of 1 million cubic meters of water per second.

The Altai Flood

At the end of the last ice age, rupture of ice dams formed by glaciers caused major flooding in the Altai republic of today’s Russia. It is estimated that it had a water flow of 10 million cubic meters per second.

These events usually take decades to complete. It starts with the movement of glaciers that block rivers, filling a huge lake and then the dam is breaking. It is believed that these events are the result of climate change. However, it is also estimated that these floods helped to slow down the process. Large quantities of water that were displaced during the flood, distorted ocean current and thus affected climate patterns.

The Ice Age Flood

Some 20,000 years ago, in what is now known as Washington and Oregon states of the US, one of the largest known floods took place. It is believed to have been caused by a huge glacier that fell into the Clark Fork River. This glacier blocked the flow of water and created a large glacial lake, called Glacial Lake Missoula. But when this massive ice dam finally failed, it flooded the entire Willamette Valley. Including much of Oregon and Washington states, at a speed of 17 million cubic meters of water per second. The enormous force of the water destroyed everything in its path. Today large rocks are found on high hills, raised there by the water. Scientists are using these rocks to study ancient floods.

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