The Dangers of Flash Floods

When folks anticipate impacts of incoming strong to severe thunderstorms, they typically think of high winds, large hail, and tornadoes as the main concerns for damage to life and property. Because these facets of storms are treacherous, the dangers brought on by flooding can sometimes be pushed into the background. Initial impacts from intense thunderstorms generally don't include flooding. Although, once the storm has settled in, the dangers associated with it can quickly turn to flash flooding. In certain situations, storms may become nearly stationary over an area, which can lead to major flooding. Other times, situations arise where storms will line up in an orientation that results in prolonged storm activity over a specific area. This phenomenon is known as 'training,' as the storms line up just like the cars of a train. Either way, the result is flash flooding, and it's important to know what to do when a flash flood hits your area.

According to the 30 year average, no other natural disaster has taken more lives each year than flooding. This is true not only in the U.S., but also across the globe. No area of the U.S. is immune to flash flooding either. Even predominantly dry areas in the western U.S. can find themselves the target for flash floods. Mountainous terrain can be particularly vulnerable to the impacts of flash floods. This is because the topography can quickly funnel water into valley areas. Even desert areas deal with the impacts of flash flooding from time to time.

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During a typical rain shower, some of the water is soaked up by the land, while the rest of it runs off into lakes, rivers, or streams. As the intensity of the rain increases, a larger portion of the rainwater becomes runoff, as less is able to be absorbed by the land. The excess runoff results in fast-swelling rivers and streams, along with pooling water in low lying areas. Streams that were just a trickle hours before can become raging torrents in a matter of minutes. In addition, city sewer systems can quickly become overwhelmed and back up, resulting in street and overland flooding. It is the sum of these factors that creates a flash flood.

The Power of Moving Water

During a flash flood, a typical encounter for a person on foot or in a vehicle is the presence of moving water on a sidewalk or roadway. Unfortunately, it is easy to underestimate the power of moving water, as the force is often much greater than one would think. This underestimation is the cause of most flood related deaths. It takes only 6 inches of moving water to knock a person off their feet and carry them away. One foot of water has been known to put a vehicle out of control, only to be swept up by flood waters. All too often, drivers underestimate the power of moving water over a roadway. Because of this, more than half of the flood-related fatalities in the U.S. each year occur while people are in their cars. It is for this reason that you should never drive your car into a flooded roadway. Even if the water looks fairly shallow, the pavement below it could be washed out in some sections, and the true depth and force can never be estimated until one has gone too far. When you find yourself facing water over a roadway, turn around and find another route to get home safely. Don't become the next statistic.

Image Courtesy: Marvin Nauman

Flash Floods in the Desert?

It can seem counter-intuitive, but some of the most dangerous flash floods occur in the desert areas of the western U.S. As you would imagine, the amount of runoff increases as the earth becomes increasingly saturated. The land can only soak up a certain amount at a time, and eventually becomes drenched to the point where it can no longer take in any more water. In the deserts and semi-arid regions of the western United States, the opposite happens. In regions like Arizona and Nevada, the land routinely becomes so dry and brittle that it becomes nearly impermeable to water. This translates into rapid runoff that collects in dry river basins, called Arroyos. The result is a sudden raging river in a spot that seemed safe only seconds before. Unfortunately, hikers sometimes inadvertently use these dry channels as their guide for navigating the terrain. Hikers can be lulled into thinking their route is safe due to the sunny and dry conditions where they are. Meanwhile rains may be collecting on distant mountain and ridge tops. Eventually the water collects down the sides of the higher terrain and brings a raging torrent, full of debris from above, down to the lower elevations. The video below is a classic example of how an arroyo can go from a seemingly harmless sandy-bottomed gulch, to a raging river, in a matter of seconds.

 Flash flood that occurred in July of 2014, near Carson Valley, Nevada.

How to Stay Safe

The key to staying safe during any weather situation is to be prepared. Have a plan of where to go if a flash flood develops. If you live near a river or creek, you are at a higher risk for flash flooding. Know the quickest route to higher ground in the event of a flash flood. While evacuating, remember that it's never a good idea to drive through flood waters. For those camping, find an area that is away from rivers and streams, as these are the first places to flood. Find a campsite on higher ground so you don't have to worry about getting caught off-guard during the night. In order to stay informed of potential flooding, stay updated with the latest forecasts and keep a NOAA Weather Radio nearby. The National Weather Service issues Flash Flood Watches when conditions are ripe for flooding to occur in a specific area during a certain time. A Flash Flood Warning is issued when flooding is already occurring. If you are under a watch, be prepared for the heightened risk; if under a warning, move to a safe place (higher ground).  

Keep in mind that that no area is unsusceptible when it comes to the risk of flooding. Even arid regions of the southwest can be subjected to major floods, especially during the late summer monsoon season (typically active during July and August). Certain weather patterns can focus flooding risks anywhere over the U.S. Make sure to be familiar with the area you live and work in, so you can act quickly and stay safe in the event of a flash flood.

Image Courtesy: FEMA (Areas shaded in red have seen 4 or more disaster declarations related to flooding since 1965)