The Types of Flood Events

Rainwater isn't the only cause of flooding

Floods (weather events where water temporarily covers land it normally doesn't cover) can happen anywhere, but features like geography can actually increase your risk for specific types of flooding. Here are the main types of floods to look out for (each is named for the weather condition or geography that cause them):

Inland Floods

Trees In River After Flood
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Inland flooding is the technical name for ordinary flooding that occurs in inland areas, hundreds of miles from the coast. Flash flooding, river flooding, and pretty much every type of flooding except coastal can be categorized as an inland flood. 

Common causes of inland flooding include:

  • Persistent rainfall (if it rains quicker than the can , water levels will rise);
  • Runoff (if the ground becomes saturated or rain runs down mountains and steep hills ); 
  • Slow-moving tropical cyclones;
  • Rapid snowmelt (the melting of snowpack -- layers of deep snow that accumulate over winter in the northern tier states and mountainous regions of the U.S.);
  • Ice jams (chunks of ice that build up in rivers and lakes, creating a dam. After the ice breaks apart, it releases a sudden surge of water downstream).

Flash Floods

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Flash floods are caused by heavy rain or the sudden release of water over a short period of time. The name "flash" refers to their fast occurrence (typically within minutes to hours after the heavy rain event) and also to their raging torrents of water that move with great speed. 

While the majority of flash floods are triggered by torrential rain falling within a short amount of time (like during intense thunderstorms), they can also occur even if no rain has fallen. The sudden release of water from levee and dam breaks or by a debris or ice jam can all lead to flash flooding. 

Because of their sudden onset, flash floods tend to be thought of as more dangerous than ordinary floods.

River Floods

Germany, Hesse, Eltville, Flooding of River Rhine Island Koenigskling Aue, aerial photo
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River flooding occurs when water levels in rivers, lakes, and streams rise and overflow onto the surrounding banks, shores, and neighboring land. 

The water level rise could be due to excessive rain from tropical cyclones, snowmelt, or ice jams. 

One tool in predicting river floods is the monitoring of flood stage. All major rivers in the U.S. have a flood stage -- water level at which that particular body of water begins to threaten the travel, property, and lives of those nearby. The NOAA National Weather Service and River Forecast Centers recognize 4 flood stage levels:

  • At Action stage (yellow), water levels are near the top of river banks.
  • At Minor flood stage (orange), minor flooding of nearby roadways occurs.
  • At Moderate flood stage (red), expect flooding of nearby buildings and the closure of roadways. 
  • At Major flood stage (purple), extensive and often life-threatening flooding is expected, including the complete inundation of low-lying areas.

Coastal Floods

Insurance Claim: Flooding from a hurricane
Jodi Jacobson / Getty Images

Coastal flooding is the inundation of land areas along the coast by seawater.  

Common causes of coastal flooding include:

  • High tide;
  • Tsunamis (large ocean waves generated by underwater earthquakes that move inland);  
  • Storm surge (an ocean swell that "piles up" due to a tropical cyclone's winds and low pressure which push water out ahead of the storm, then comes ashore).

Coastal flooding will only worsen as our planet warms. For one, warming oceans lead to a rise in sea level (as oceans warm, they expand, plus melt icebergs and glaciers). Higher "normal" sea height means it will take less to trigger floods and they will happen more often. According to a recent study by Climate Central, the number of days U.S. cities have experienced coastal flooding have already more than doubled since the 1980s!

Urban Floods

manhole cover bubbles over
Sherwin McGehee / Getty Images

Urban flooding occurs when there is a lack of drainage in an urban (city) area. 

What happens is that water that would otherwise soak into the soil cannot travel through paved surfaces, and so it is redirected into city sewage and storm drain systems. When the amount of water flowing into these drainage systems overwhelms them, flooding results.  

 

Resources & Links:

Severe Weather 101: Flood Types. The National Severe Storms Laboratory (NSSL) 

National Weather Service (NWS) Flood Related Hazards

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Means, Tiffany. "The Types of Flood Events." ThoughtCo, Jul. 5, 2016, thoughtco.com/the-types-of-flood-events-4059251. Means, Tiffany. (2016, July 5). The Types of Flood Events. Retrieved from https://www.thoughtco.com/the-types-of-flood-events-4059251 Means, Tiffany. "The Types of Flood Events." ThoughtCo. https://www.thoughtco.com/the-types-of-flood-events-4059251 (accessed January 24, 2018).