Floods vs. Flash Floods

flooded road
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Floods and flash floods happen whenever water overflows onto normally dry land. But while the outcome is the same, and the weather events that cause them (slow-moving low pressure systems, hurricanes, and monsoons) may be the same, all floods are not created equal.

The main differences between floods and flash floods are the time it takes their flood conditions to develop, how long they last, and how wide sweeping their impact.

Floods: Slow-rising, but Long-Lasting

Like the Great Flood that came after heavy rain poured down on the earth and Noah's ark for forty days and forty nights, the world's flood events are often longer duration flooding. And as Noah's flood continued for one-hundred and fifty days, likewise today's flood events begin and end gradually and are considered long-term events that typically last days or weeks.

Related: Are floods considered weather or climate events?

Besides impacting transportation, floods often bring health hazards, like mold, and disease brought on by standing water.

When weather conditions lead waters to rapidly rise, flash flooding occurs.

Flash Floods Develop within Minutes to Hours

As the name suggests, flash floods are rapid flooding events. How rapid? According to the NOAA National Weather Service, flash flood situations develop within six hours (or less) of the start of the causative event.

While the majority of flash floods are triggered by heavy rain falling within a short amount of time (like during intense thunderstorms), non-rain-related events can also trigger them such as:

  • a levee or dam failure,
  • sudden snow melt or thawing of glaciers, or
  • a sudden release of water by a debris flow or ice jam.

    Because of their sudden onset, flash floods tend to be thought of as more dangerous than regular floods. Adding to this flash floods are also associated with raging torrents of fast-moving water against which there is little protection (even from a vehicle) from being swept away.

    Flash flood waters often subside as swiftly as they swell. Once the torrential downpours or ends, flash flood conditions do too.

    Another difference between flooding and flash flooding is where each commonly occurs. Flooding can involve widespread flooding of waterways or the accumulation of rainwater on saturated ground and roadways. In contrast, flash flooding more often involves localized flooding of small rivers, streams, creeks, and storm sewers.

    Is is possible to be under a flood alert AND a flash flood alert?

    It may seem redundant to have both an active flood watch or warning and a flash flood watch or warning too, but if this happens you should take both seriously. It means that your area is at risk for both gradual and immediate flooding. An example weather situation where this could happen is if your area had seen prolonged rainfall in days prior and then had a hurricane approach. Your flood risk would be elevated from the longer duration flooding, but also from the heavy tropical moisture associated with the hurricane.

    More: Advisory, Watch, or Warning: Do You Know the Difference?

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    Means, Tiffany. "Floods vs. Flash Floods." ThoughtCo, Jul. 7, 2017, thoughtco.com/floods-vs-flash-floods-3443861. Means, Tiffany. (2017, July 7). Floods vs. Flash Floods. Retrieved from https://www.thoughtco.com/floods-vs-flash-floods-3443861 Means, Tiffany. "Floods vs. Flash Floods." ThoughtCo. https://www.thoughtco.com/floods-vs-flash-floods-3443861 (accessed November 22, 2017).