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Floods

Floods are some of the most dramatic and damaging natural disasters humans have experienced for thousands of years. Water is essential to life, but too much can have severe consequences on our communities and the environment. We’re going to take a closer look at precisely what floods are, how they happen, and what impacts they have.

Floods definition

What is the definition of a flood? Floods are when water from a body of water like a river or lake overflows and fills typically dry areas. Flooding is distinct from standard variations in water levels, like the tide rolling in and out or rivers filling up from spring snowmelt. When the water rises to levels far beyond these regular changes, it becomes a flood.

Flood: A flood is a natural disaster where water rises beyond its average level and flows into areas that are usually dry.

Causes of floods

Not every flood happens for the same reason. Let’s see what some of the main causes of floods are.

Rainfall

Periods of excess rainfall can overfill rivers, ponds, and lakes, causing them to overflow and flood. The rain does not even have to be particularly intense, but if it rains constantly over a long period of time it might be too much for bodies of water to handle. Most rain showers don’t lead to floods as a lot of the water gets absorbed into the ground or bodies of water rise but not enough to overflow.

Floods rainfall in city StudySmarterFig. 1 - intense rainfall can result in floods

Geology and vegetation

The type of land and plants of an area also heavily impact the risk of flooding. Certain rocks and soils absorb water more easily than others, and plants can help absorb water too. Typically, a more barren landscape is more susceptible to flooding with little water able to absorb into the ground. A heavy rainstorm in a desert is rare but when it does happen, flooding can be intense! Areas downhill are at higher risk of flooding too, with gravity forcing water into areas like valleys.

Man-made

Features of the human environment can also cause flooding. We mentioned earlier how certain types of rocks and soils can absorb water more easily than others, and man-made surfaces like concrete are very bad at absorbing water. Cities require complex sewer systems to allow water to move away from our roads otherwise water from rain storms would have nowhere to go. Dams are structures meant to hold water back in a river and slowly release it but sometimes they can fail, causing catastrophic flooding.

Types of floods

Next, let’s look at some different types of floods.

River floods

Rivers are channels of water that make their way through the landscape eventually ending in an ocean or other body of water. Rivers tend to have natural changes in the amount of water flowing through them, but sometimes periods of intense rainfall can cause them to overflow and spread out into surrounding areas. The chance of flooding is made worse if the land around rivers has surfaces that are poorly drained or sloped towards the river, causing more water to flow straight into the river.

Coastal floods

When water from the sea rises and flows onto the land, that is known as a coastal flood. During strong storms like hurricanes, wind and pressure push water far into land in what is known as a storm surge. Sea level rise is a consequence of global warming, and also results in flooding in low-lying areas, with the risk increasing as polar ice continues to melt. Another type of coastal flood is a tsunami. Tsunamis are dramatic events where a massive wave is caused by earthquakes in the ocean. The sheer force of a tsunami can reach far inland.

Floods Waves crashing StudySmarterFig. 2 - Surging water from the ocean and tsunamis can overwhelm coastal areas and settlements

Sewer floods

Sewers are critical infrastructure to cities to ensure waste is properly disposed of and water does not pool in streets or basements and cellars, but sometimes it’s not enough. Intense rain or debris clogging sewer systems can result in backups, with sewage spilling back out onto roads and in homes. No matter if it’s a city of ten thousand or ten million, sewers need to be managed and maintained to prevent flooding.

Flash floods

A sudden rush of water in low-lying areas as a result of rain is known as a flash flood. As the name implies, flash floods happen suddenly and with little warning. Very dry areas are particularly prone to flash flooding when rain storms hit because the dry ground is not able to absorb the water well. Flash floods can also be man-made, caused by dams bursting as we discussed earlier. The sudden nature of flash floods makes them particularly dangerous to humans who can find themselves in the midst of rushing water without warning.

Floods dry riverbed StudySmarterFig. 3 - Dried riverbeds like this are particularly prone to flash flooding

Effects of floods

Now that we’ve talked about the causes and types of floods, let’s take a look at some of the effects of floods.

Human health

Floods have caused some of the most deadly disasters in human history. Not only being direct causes of death, but flood waters can also bring diseases like cholera, especially sewer floods. The subsequent destruction of infrastructure also hampers the supply of clean water. Minor floods can cause homes to build up mould and mildew, leading to respiratory illness in some people. The mental impact of flooding is another big risk. Seeing your home destroyed or losing the lives of loved ones can take a major toll on an individual’s and community’s mental wellbeing.

The deadliest flooding event in modern history was the 1931 China floods, with estimates of up to 4 million lives lost directly and indirectly from the flood.

Infrastructure

The power of floods is enough to topple homes and buildings in some cases, and the electricity supply can get cut as well. When flood waters remain and block roads or other methods of transportation, rescuing people and bringing supplies becomes much more challenging. The use of boats and helicopters is often necessary to help relieve people stuck in a flood-stricken area. The damage can last much past flood waters receding; homes flooded with even a centimetre of water might need costly repairs. Water and moisture damage building foundations and structural components, meaning they need inspection and repairs to make sure they are safe to be in.

Environment

Of course, floods can also result in the death of any plants or wildlife stuck in their path, as well as contaminating habitats. The local water supply can also become contaminated with pathogens and algae ruining groundwater quality. It’s not all bad with flooding, however, as sediment from the floods can help replenish nutrients in the soil improving the growth of plants. While flooding can contaminate groundwater, it’s also important to help refill groundwater supplies. Finally, wetland environments that become too dry also benefit from flooding to refill them. As you can see, flooding has lots of ups and downs regarding its impacts on the environment.

Floods - Key takeaways

  • Flooding is when water overflows and inundates areas that are normally dry.
  • Floods are commonly caused by rain and are made worse by poorly absorbent surfaces.
  • The main types of floods are river floods, coastal floods, sewer floods and flash floods.
  • Flash floods are one of the most dangerous kinds of floods, with water rushing suddenly and rapidly.
  • Floods are very damaging to infrastructure, humans, and the environment and are exasperated by the effects of climate change.

Frequently Asked Questions about Floods

Floods happen when there is too much water for a body of water or water system to handle, and it overflows into areas that are typically dry. Water bodies like rivers and oceans have a normal fluctuation in how much water they contain, but when it goes far above normal it is considered a flood. 

One of the main causes of floods is excess rainfall, either because it is very intense or occurs over a long period of time. The type of ground can also cause floods, with less absorbent surfaces leading to a higher risk of flooding. Man-made causes include dam failures and sewers overflowing or becoming clogged.

The types of floods are:


  • River floods

  • Coastal floods

  • Sewer floods

  • Flash floods

Because flash floods occur suddenly, they are very dangerous. Even moderate rainfall can cause flash floods, especially in low-lying areas. Because they happen quickly and before it appears there is much rain, many people do not anticipate them leaving little time to get to higher ground.

With climate change, floods are increasing in frequency and are predicted to become more frequent unless something is done to help mitigate greenhouse gas emissions. Rapid polar ice melt resulting in sea level rise and more severe storms are attributed to climate change and result in flooding becoming more common.

Final Floods Quiz

Question

Which of the following are a type of flood?

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Answer

Coastal

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Question

Why are flash floods particularly dangerous?

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Answer

Because they occur suddenly and with little warning.

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Question

True or false: Excess rainfall can cause flooding.

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Answer

True

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Question

How does the material the ground is composed of impact flooding?

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Answer

If the ground is made of materials that absorb water well, flooding can be mitigated. If the surface does not absorb water well, like concrete, it increases risks of flooding.

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Question

Why are sewer floods particularly dangerous to human health?

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Answer

Sewers contain human waste and other harmful substances, and if it floods an area sewage can result in the spread of harmful diseases.

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Question

What is a storm surge?

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Answer

A storm surge is a rush of water from the ocean in coastal areas resulting from severe storms like hurricanes.

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Question

What most commonly causes tsunamis?

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Answer

Earthquakes on the ocean floor.

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Question

Would a flash flood be more of a risk in a low part of a dry lake, or high part of a dense forest? Why?

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Answer

Low part of a dry lake, because water pools at the lowest point and dry ground without vegetation does not absorb water well.

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Question

True or false: Floods can cause mould and mildew in homes, but it's of little concern to human health.

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Answer

False, mould and mildew buildup can lead to respiratory diseases.

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Question

How is climate change making flooding more common?

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Answer

Increased rainfall, more intense storms, and polar ice melt as a result of climate change are making flooding more common.

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Question

In what way can flooding be good for the environment?

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Answer

Flooding can allow the spreading of nutrient-rich sediments which improve the health of plants in a habitat. It's also useful for replenishing groundwater supplies and wetlands.

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Question

Is tide rolling in an out an example of flooding?

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Answer

No, tide is a natural variation in the water levels of an ocean.

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Question

Which of the following are causes of flooding?

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Answer

Irrigation

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Question

What is a man-made example of a flash flood?

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Answer

Dam breaks

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Question

Why is it particularly challenging to rescue people and bring critical supplies during a flood?

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Answer

Flooding does not allow easy transportation access, so boats and helicopters need to be employed to reach people in need. A loss of power may also make it harder to reach emergency services.

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