Many hydroelectric dams, such as the Hoover Dam, also serve to contain floods.
The weather event known as a flash flood can be especially dangerous due to its sudden formation and potential for widespread destruction. Television forecasters routinely broadcast flash flood warnings along with severe storm and tornado warnings. Cities located along rivers or under dams are especially vulnerable if the amount of water generated during a storm exceeds protective barriers.
Flash flooding can occur during a severe storm.
There are a number of factors, some natural and some man-made, that can determine whether a storm system will produce flash flooding or just a significant amount of rain. One factor is the nature of the storm cell itself. A fast line of thunderstorms, for example, can move through an area so fast that rain is absorbed into the ground or sewer systems and doesn’t return. On the other hand, a slow-moving independent storm system can dump a large amount of rain on a small area and create the potential for flooding to develop quickly. A short, intense burst may be preferable to a long, slow rain event.
Flash flooding can create dangerous road conditions.
Assuming there is a slow-moving storm system, dumping a lot of rain in the area, the next factor in a flash flood is ground absorption. If the ground is frozen or covered in ice, rainwater will not absorb naturally and will continue to seek lower areas. The same goes for urban areas with significant amounts of concrete and asphalt. When rain is prevented from reaching the ground, it will flow into sewer systems and natural ditches. If these drainage systems or gutters become overloaded, they can cause flooding.
A fast-moving storm line may be preferable to a slow-moving storm system.
Rivers and streams also have a natural saturation level, and if enough rain falls in a short period of time, the water will overflow into an area known as a floodplain. If a levee or dam were to break during a severe storm, a destructive flash flood would be unleashed on anything and anyone living below it. A flood of this magnitude is often called a 100- or 500-year flood, primarily because destruction at this level should occur only once every 100 or 500 years. This does not mean that a second flood of equal power will not occur the following year.
In flash floods, heavy rain or other waters quickly submerge low-lying areas.
In short, a flash flood occurs whenever the amount of rainfall during a storm exceeds natural and man-made drainage and absorption systems and continues to increase in a low-lying area. This type of flooding can happen at any time, so meteorologists often advise people to stay off the streets for several hours after a slow, intense storm. Runoff from upstream storm systems may not reach an area for a few hours, meaning there is still a chance of flooding in some places even after the rains have stopped.