What Causes Flooding


Flooding occurs when an area that is typically dry is inundated by flowing water. Flooding can be extremely dangerous and can actually occur in all 50 states, no matter how dry an area is normally. Most people consider rainfall to be the reason for flooding, and while that is a major factor, it is a little more complicated than that.

One cause of flooding that is not caused by rainfall is the failure of a dam or levee. This can lead to a rapidly rising water level. In the United States, there are approximately 76,000 dams with 80% of them being earth fill construction. This is an important factor because this style of dam is more easily compromised than concrete dams. Levees, which are embankments on the side of rivers, could also lead to flooding. For example, if water levels rise too much, water can overflow the sides of the levee.

Another cause of flooding, that’s not directly caused by rain, is snow or ice melting. For instance you might see this if there is an unusual amount of snow over the winter, and then air temperatures rise quickly. This will cause abnormal melting, which in turn, leads to a high amount of water runoff. A similar situation can occur with ice. The melting of ice can create unexpected runoff for an area. Furthermore, the breakup of ice could be problematic as ice pieces may temporarily block the river. When the ice block eventually melts, a gush of water is released.

Tsunamis are an additional factor that could contribute to flooding. Tsunamis are caused by underwater earthquakes or landslides, and can lead to damaging storm surges. A storm surge occurs when the force of the tsunami mixes with a high tide to create an extraordinary high tide. This surge can be 15 feet or more higher than the normal level, putting roads and buildings in significant danger of flooding.

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As you can imagine, severe storms such as tropical cyclones and hurricanes are another cause of flooding. Flooding is actually one of the most dangerous parts of these storms. If a tropical cyclone is moving slowly, it can produce 20-40 inches of rain over an area. Plus, just like tsunamis, these storms bring dangerous storm surges.

Certain factors can increase the likelihood of a flood. For instance, soil on a steep terrain may weaken during heavy rainfall and lead to debris falling. The falling debris can create a blockage, that will eventually break, creating a surge of water. Additionally, if the soil is rocky or clay based, it won’t absorb as much water as other types. It also difficult for water to be absorbed in cities. Because of the construction of roads, buildings and parking lots, water is redirected through storm drains and gutters. However, during torrential rain fall, drainage spots can be overwhelmed, and the few natural landscapes won’t be able to absorb much water.

Flooding can typically be divided into two categories- flash flooding or river flooding. A flash flood forms rapidly and occurs within hours of the rainfall. While it doesn’t last very long, a flash flood is quite dangerous because there isn’t much time to prepare for it. Furthermore, flash floods may not occur in the area that has the rainfall, but rather where the water runoff from the rain occurs. As you can imagine, this can lead to confusion and makes it challenging to convince people to evacuate. The rapidly rising levels plus the fast current of a flash flood create a treacherous situation, and may lead to deaths because of drowning.

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Flash floods are typically caused by multiple thunderstorms moving consecutively across an area. When this occurs, it is know as training thunderstorms since it’s like the passing freight cars of a train. This consistent rainfall over a concentrated area can overwhelm bodies of water and drainage, leading to flash floods. An example of this occurred in Johnstown, Pennsylvania in 1977. A continuous run of thunderstorms formed in Northwest PA and moved southeast, creating torrential rain in excess of 15 inches. The flood ended up causing hundreds of millions of dollars in damages and caused 77 deaths.

Thunderstorms aren’t the only cause of flash flooding however, flash floods can also occur after heavy rainfall. This is a common occurrence on the West Coast of the United States, where orographic precipitation is common. Orographic precipitation occurs when moist air isforced over mountains. This type of precipitation is often heavy, and thus, creates a risk for flash flooding.

As opposed to flash floods, river floods take place over days or months versus hours. River floods occur in large basins and are normally caused by excessive rain from storms or the combination of rainfall and snow melt. For example, if there are multiple heavy rain storms within a few weeks, tributaries will rise, and eventually raise the level of the river. River floods usually occur across a large area and can cause costly damage to crops and buildings. Since river floods occur over time, they do not pose as much risk as flash floods. Unfortunately, there is the potential for flash floods to develop during river flooding.

As you can see, precipitation from rain or thunderstorms is a major cause of flooding but it’s not the only cause. Floods may also occur due other reasons such as storm surges, snow melting, or a dam failure. This factors could lead to a sudden flood, known as a flash flood, or a river flood, which would occur gradually. No matter the cause or the classification, if you find yourself in an area that may have flooding, proceed with caution.

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Doswell, CA. “Flooding.” Georgia Institute of Technology, Georgia Institute of Technology, 2003, curry.eas.gatech.edu/Courses/6140/ency/Chapter8/Ency_Atmos/Flooding.pdf.
“Floods The Awesome Power.” NWS Flood Safety , National Oceanic and Atmospheric Administration , Mar. 2005, www.floodsafety.noaa.gov/resources/FloodsTheAwesomePower_NSC.pdf.
picture 1- Sadatsugu Tomizawa and released via Jiji Press on March 21, 2011https://www.jpl.nasa.gov/news/news.php?feature=6829
picture 2- By National Park Service, Mount Rainier National Park [Public domain or Public domain], via Wikimedia Commons https://commons.wikimedia.org/wiki/File%3ANisqually_River_2006_flood_raging.jpg
picture 3- http://calag.ucanr.edu/Archive/?article=ca.v061n02p53