What is a Flood?

Believe it or not, flooding is the deadliest type of severe weather. There’s probably a lot about floods and flooding you don’t know.

In this guide, we’ll go over everything there is to know about flooding; answering common questions like “What causes flooding?” and “Where does flooding occur?”

Use the buttons below to jump to the flood facts you care about the most, or read the entire Flooding 101 guide to become an expert!


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What is a Flood? | Types of Floods

Flooding gif with flowing floodwaters

When discussing floods, it’s important to understand what they are. Let’s start with a flooding definition.

A flood is an overflow of water that submerges land that is usually dry. Floods are an area of study in the discipline of hydrology. They are the most common and widespread natural severe weather event.

Floods can look very different because flooding covers anything from a few inches of water to several feet. They can also come on quickly or build gradually. To better answer the question of “What is a flood?” we’ll answer what each specific type of flooding event is.

According to our friends at the National Severe Storms Laboratory, there are five types of floods. They include:

A dark blue "1" in a red circle River Flood

A dark blue "2" in a red circle Coastal Flood

Storm Surge

A dark blue "4" in a red circle Inland Flooding

a dark blue "5" in a red circle Flash Flood

As you can infer from the list above, flooding can happen anywhere, including both coastal and inland locations. Let’s take a detailed look at the different types of floods.

What is a River Flood?

The first type of flooding we will explore is river flooding.

What is River Flooding?

A river flood occurs when water levels rise over the top of river banks. This flooding can happen in all river and stream channels. This includes everything from small streams to the world’s largest rivers.

Causes of River Flooding

River flooding typically happens for four reasons. They are:

Excessive rain from tropical storm systems making landfall

 Persistent thunderstorms over the same area for extended periods

Combined rainfall and snowmelt

Ice jam

More on Riverine Floods

River floods can happen suddenly or slowly. Sudden river flooding events occur more often on smaller rivers, rivers with steep valleys, rivers that flow for much of their length over impermeable terrain, and normally dry channels.

On the other hand, low-rising river floods typically occur in large rivers with large catchment areas. In case you didn’t know this already, a catchment area is any area of land where precipitation collects and runs off into a common outlet.

What is a Coastal Flood?

houses flood

The next type of flood you should know about is a coastal flood. 

What is Coastal Flooding?

A coastal flood is the inundation of normally dry land areas along the coast with seawater.

Causes of Coastal Flooding

Coastal flooding is typically a result of a combination of sea tidal surges, high winds, and barometric pressure.

These conditions typically come from storms at sea like:

Tropical cyclones

Tsunami

Higher-than-average tides

What is Storm Surge?

Another type of flooding that is often associated with coastal flooding is storm surge.

What is Storm Surge?

Storm surge is an abnormal rise in water level in coastal areas over and above the regular astronomical tide.

Causes of Storm Surge

Storm surge is always a result meteorological storms that cause higher than normal tides on the coast. There are three parts of a storm that create this surge. They are:

Wind

 Waves

Low atmospheric pressure

Learn about storm surge from one of our expert meteorologists, Andrew Rosenthal in the below snippet from our weather safety warmup webinar on hurricanes:

 

More on Storm Surge

Storm surge is an extremely dangerous form of flooding. It can flood large coastal areas at the same time. It can also cause flooding very quickly.

Extreme flooding occurs when storm surge happens at the same time as high tide. This can cause storm tides to reach over 2o feet!

Our meteorologists always stress that storm surge is the most dangerous aspect of any tropical system. It poses the most threat to both life and property. In the past, we’ve seen truly devastating storm surge impacts. For example, storm surge took the lives (directly and indirectly) of over 1,500 people during Hurricane Katrina.

What is Inland Flooding?

The next type of flood on our list is an inland flood. Some organizations refer to inland flooding as urban flooding. A flash flood is also a type of inland flood.

What is an Inland Flood?

An inland flood is flooding that occurs inland or not in a coastal area. Therefore, coastal flooding and storm surge are not inland floods.

Causes of Inland Flooding

Rainfall is almost always to blame for inland floods. Rain causes inland flooding in two ways. It can happen with steady rainfall over several days or it can happen because of a short and intense period of rainfall.

Snowmelt also causes inland floods, although rainfall is a more common cause.

Another way inland flooding happens is when water ways get blocked by debris, ice, or dams.

More on Inland Floods

Inland floods are often worse in urban areas because there isn’t anywhere for the water to go. The following urban features can create urban flooding or make inland floods worse:

Paved roads and streets

Low-capacity drainage equipment

Dense buildings

Low amounts of green space

What is a Flash Flood?

The most well-known and deadly type of flood is a flash flood.

What is a Flash Flood?

A flash flood is flooding that begins within 6 hours, and often within 3 hours, of heavy rainfall (or other cause).

What Causes Flash Floods?

Flash floods can happen for several reasons.

Most flash floods happen after extremely intense rainfall from severe thunderstorms over a short period of time (normally 6 hours or less). There are two key elements to determine is flash flooding is likely:

A dark blue "1" in a red circle Rainfall rate

A dark blue "2" in a red circle Rainfall duration

Flash floods also happen when damns break, when levees fail, or when an ice jam releases a large amount of water.

Dangers of Flash Floods

Flash flooding is the #1 severe weather killer in the United States.

Flash floods are extremely powerful. They have enough force to roll boulders, tear trees from the ground, destroy buildings and bridges, and scour out new channels. This type of flood is characterized by raging torrents that rip through river beds, urban streets, or canyons, wiping out everything in their paths. With heights reaching 30 feet, flash floods can completely cover communities.

Another reason why flash flooding is so dangerous is that it can happen with little to no warning. This is especially true when dams or levees break.

The National Weather Service recommends knowing your area’s flood risk before a flash flooding event happens. They also recommend having a family or business disaster plan ready in the case of a flash flood.

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Flood Causes & Effects

house with rain flooding iconNo matter what type of flood you’re dealing with, they are generally caused by the same key factors and there are always negative effects.

In this section, we’ll cover the basic causes and effects of flooding to help you better understand this dangerous meteorological and hydrological phenomenon. If you read through the above section on types of floods, you might just want to skip down to flooding effects.

What Causes Flooding?

As we mentioned above, there are plenty of different causes of flooding. While different flood types typically have different causes, most floods are caused by one of the following activities.

A dark blue "1" in a red circle Heavy rainfall is the simplest cause of flooding. When there is too much rain or it happens too fast, there just isn’t a place for it to go. This can result in floods like flash flooding.

A dark blue "2" in a red circle Overflowing rivers are another cause of floods. You don’t necessarily need heavy rains though to experience river flooding. As we mentioned before, river flooding can happen when there is debris in the river or dams that block the flow of the water.

Speaking of dams, broken dams are another cause of flooding. Older infrastructure can fail when heavy rains come and water levels rise. When dams break, they unleash torrents of water on unsuspecting households. This is part of what happened when Hurricane Katrina hit New Orleans in 2005.

A dark blue "4" in a red circle Storm surge and tsunamis also cause flooding. Storm surges from hurricanes and other tropical systems can cause sea levels to rise and cover normally dry coastal areas in several feet of water. Tsunamis on the other hand are giant waves caused by earthquakes or underwater volcanic eruptions. As these waves move inland, they build height and can push a lot of water inland in coastal areas.

a dark blue "5" in a red circle Channels with steep banks are also to blame for flooding. Flooding often occurs when there is fast runoff into lakes, rivers, and other basins. This is often the case with rivers and other channels that feature steep sides.

a dark blue "6" in a red circle A lack of vegetation can cause flooding. Vegetation can help slow runoff and prevent flooding. When there is a lack of vegetation, there is little to stop water from running off and overflowing river banks and streams.

a dark blue "7" in a red circle Melting snow and ice is another common reason for flooding. When a large amount of snow and/or ice melts quickly, it often doesn’t have somewhere to go except low-lying areas.

These aren’t all the reasons that flooding can happen, but they are some of the most common.

Flooding Effects

No matter what causes of a flood, it can have devastating effects on your community. There are actually many dangerous flooding effects. Besides physical danger, floods also cause economic and social problems. In this section, we’ll cover the most common flood effects.

Loss Of Lives

The gravest effect of flooding is death. In fact, flooding is the number one severe weather killer. Floods have claimed thousands of lives throughout history. But how does flooding kill?

Floods kill by carrying people away in fast-moving water or drowning them. It only takes six inches of water to wash a person away. Floods can also kill people by destroying buildings and creating unsafe environments. One often-overlooked deadly effect of flooding comes from waterborne illnesses.

From 2010 to 2018, the National Weather Service recorded hundreds of flooding deaths across the United States. Texas saw most of those deaths, with the 8-year total sitting at 212 fatalities.

2010 to 2018 u.s. flood fatalities from the National Weather Service and NOAA

Property Damage

Since it only takes two feet of flood water to wash a car away, flooding can also cause great loss of property. Surely you’ve seen images of cars floating away in flood waters. This is why it is so important to avoid flooded areas when driving. You don’t want to be in your car when it gets washed away in the flood!

Flooding also causes property damage to buildings by blowing out windows, sweeping away doors, corroding walls and foundations, and sending debris into infrastructure at a fast pace. Not to mention the furniture and items inside a home or business that are damaged when flood waters make it inside. 

Economic Impacts

The economic impact of flooding can be devastating to a community. This comes from damage and disruption to things like communication towers, power plants, roads, and bridges. This brings business activities in an area to a standstill. Oftentimes, major flooding results in dislocation and dysfunction of normal life long after flood waters recede.

Flooding hinders economic growth and development because of the high cost of relief and recovery associated with floods.

In frequently flooded areas, there is less likely to be any investment in infrastructure and other developed activities.

Psychosocial Flooding Effects

Flooding can also create lasting trauma for victims. The loss of loved ones or homes can take a steep emotional toll, especially on children. Displacement from one’s home and loss of livelihood can cause continuing stress and produce lasting psychological impacts.

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Flood Safety Tips

While floods have many causes, there are proven flood safety and flood prevention tips to help keep families, businesses, and communities safe.

Here are some steps you can take before, during, and after flooding to keep yourself and those you care about safe.

What Do I Do Before a Flood? 

The first thing you should do before a flood is assemble an emergency kit. Your flood emergency kit should include food, water, and medicine. You should pack a supply of these items that will last each person three days. You should also put the following in your kit:

A red check mark with a transparent background Phone Charger

A red check mark with a transparent background Batteries

A red check mark with a transparent background Blankets

A red check mark with a transparent background Flashlights

A red check mark with a transparent background First Aid Kit

A red check mark with a transparent background Rubber Boots and Gloves

A red check mark with a transparent background NOAA Weather Radio

You should also have a cell phone on hand. This way, you can communicate with your friends, family, and coworkers.

The next thing you can do to prepare for a flood is research your home or organization’s location to assess flood risk. Are you located in a flood plain? What’s the fastest way to get to higher ground? Knowing the answers to these questions can help you in the case of floods.

One last flood tip we recommend is to sign up for flooding notifications. The National Weather Service issues Flood Advisories, Flood Watches, and Flood Warnings to help people prepare for flooding dangers.

Familiarize yourself with your local emergency management agency and how they communicate with your community. Do they have a mobile app you can download? Should you follow them on social media? Will an announcement about flooding air on the TV or local radio stations?

Finally, for hyperlocal weather alerts that include NWS flooding notifications, current precipitation information, and rainfall totals in your neighborhood, you should look into getting a subscription to professional-grade weather tracking software.

Flood Watches And Warnings

The first level of flood watches and warnings is a flood advisory. A Flood Advisory means you should be mindful of possible flooding because a weather event is forecast to occur that could result in flooding. The National Weather Service issues these advisories when flooding isn’t expected to be bad enough to issue a warning, but could still pose minor threats.

A Flood Watch means you should prepare for flooding because conditions are favorable for a specific hazardous weather event to occur.

A Flood Warning means a hazardous weather event with flooding is imminent or already happening.

A Flash Flood Warning means a flash flood is imminent or occurring. If you are in a flood prone area move immediately to high ground.

You should always pay attention to these notifications and understand what the different levels mean.

For businesses, access to real-time flood watches and warnings through a hyperlocal weather map like Sferic Maps can help keep employees safe and protect outdoor equipment from being damaged during a flooding event.

What Do I Do During a Flood? 

A red question mark over a grey circle background

If a flood event is happening, there are steps you can take to stay safe. The first and most important thing to remember is to steer clear of flood waters. This is true even if you are in a car.

Remember: Turn around, don’t drown! Flood waters can rise or gain intensity in the blink of an eye. Stay away from flood water at all costs.

Another important tip that will help you survive a flood is to listen to evacuation orders from authorities. We also stress that you listen to evacuation orders the first time you hear them. Waiting even just a few minutes can be the difference between life and death. And staying behind can put you and others at risk if you need to be rescued from rising waters.

One last useful flood safety tip to pay attention to during a flood is to stay away from rooms where water covers electrical outlets. This is pretty self-explanatory; you don’t want to be electrocuted walking through your flooded home or business.

What Do I Do After a Flood?

Once the flood waters recede, you might think you’re safe. However, there are still plenty of flooding dangers that can threaten your health and property. There are also things you should in case a flood damages your property.

One of the most important safety precautions after a flood is making sure your water is safe to drink. Groundwater may have been contaminated during the flood so don’t assume you can just turn on the faucet and the water is safe.  Local authorities will let you know if water is safe to drink or if you should boil it before using.

After a flood, standing water is also a danger. While it might look tame compared to a raging flood, standing water left behind is a breeding ground for bacteria and can carry toxins or chemicals. If you have to touch standing water for cleanup or any other reason, protect yourself with rubber gloves, boots, and other protective gear.

Floodwaters can also hide sharp or dangerous objects, so it’s best to avoid going into the water if you can.

One last flood safety tip to keep in mind for after waters recede is to wait for the all clear from local officials to return to buildings or areas compromised by the flood. While your home or office might look safe to you, there could be major structural or electrical dangers. Always wait for the all clear.

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Common Flood Locations & Worst Flooding Events

While flooding can happen anywhere in the world, there are certain places that are more prone to flooding. These locations often see the worst flooding events in history.

In this section, we’ll cover where floods occur most often in the U.S. and then take a look at some of the worst floods in history.

Where Do Floods Occur in the U.S.?

As we’ve mentioned above, floods can happen anywhere in the U.S., including coastal and inland locations. However, there are some places that see flooding more often than others.

FEMA compiled flooding data and produced a map of the U.S. showing where the most flooding occurred in the U.S. between 1996 and 2013.

As you can see from FEMA’s map, the Northeast, desert Southwest, Midwest and Gulf Coast see some of the most flood activity in the U.S.

In the Northeast, snowmelt and urban flooding are to blame for the high frequency of flooding events. Tropical storms also contribute to flooding along the coast in the Northeast.

In the desert Southwest, a lot of flooding events happen during monsoon season. Since the ground is dry, it’s tough for the ground to soak up water that falls at a high rate.

In the middle of the country, the Mississippi River contributes to river flooding throughout its length. Some of the flooding in the Midwest also comes from snowmelt. It’s a common occurrence for Mississippi River flooding to last for months. In early 2019, the Mississippi River flooded for over three months, which is longer than the famous flood of 1927.

Along the Gulf Coast, flooding events are commonly caused by hurricanes and tropical systems. Hurricane Harvey is an example of a hurricane that caused a lot of flooding along the Gulf, especially in Houston, Texas.

What Were the Worst Floods in History?

Unfortunately, some of the worst flooding events in history didn’t happen so long ago. Over the past few years, extreme flooding events have been common throughout the United States. Here’s a list of the worst flooding events in U.S. history:

Mississippi River Flood, 1927

The most destructive river flood in U.S. history was the Mississippi River flood of 1927 that killed 500 and left another 600,000 homeless throughout Illinois, Missouri, Kentucky, Tennessee, Arkansas, Mississippi and Louisiana. In Vicksburg, Mississippi the river was over 80 miles wide. This is quite extreme when compared to a normal day when the widest part of the river is just over 11 miles.


Ohio River Flood, 1937

The Ohio River Flood of 1937 cost over $20 million in damage and was so widespread it destroyed homes located 30 miles away from the riverbank.


Mississippi River, 1993

Another terrible flooding event happened on the Mississippi in 1993 when waters remained at flood stages for 81 consecutive days. This flood killed over 50 people, destroyed 50,000 homes, and covered 75 towns in flood waters. Damages totaled more than $15 billion.


Hurricane Katrina, 2005

Possibly one of the most infamous flooding events of all time, Hurricane Katrina killed over 1,800 people when storm surge came ashore and rain amounts broke levees throughout Louisiana, Mississippi, and Florida. Hurricane Katrina cost over $160 billion.


Big Thompson Canyon, Colorado, 1976

The next flood on our list happened in 1976 when flash flooding from snowmelt on the Rocky Mountains combined with a high rain rate to cause extreme damage in Big Thompson Canyon, leaving 144 dead.


Rapid City, South Dakota, 1972

The 1970s were a bad decade for flooding. The Rapid City flood of 1972 killed 238 people and caused over $165 million in damages when flash flooding happened at night while most people were asleep.


Galveston, Texas, 1900

It’s hard to fathom this, but when a Category 4 hurricane made landfall in Galveston, it came with storm surge that killed over 8,000 people. The “Great Galveston Storm” made landfall at night and, even though residents could see the storm brewing offshore, they had no idea how bad it would be. This was a full 65 years before the National Hurricane Center was established.


Johnstown, Pennsylvania, 1889

This flooding tragedy was the result of a burst dam. Over 2,200 people perished when the South Fork Dam failed and unleashed a 30-foot high wall of 20 million tons of water. The water came crashing down on the town with the force of Niagara Falls.


Central Valley, California, 1861-1862

This flooding event is known as the storm that caused California to go bankrupt. A tremendous amount of rain lasting 43 days caused this flood and over $725 billion in damages. It even forced the state to move the capitol from Sacramento to San Francisco for six months while the city dried out!


Hurricane Camille, 1969

Some readers might remember this last flooding event on our list. Hurricane Camille was a natural disaster that caused damage from the Gulf of Mexico to the Mid-Atlantic. It hit the hardest in Virginia, where it overflowed the James River and caused widespread flooding.

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Additional Resources

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