Why flash floods are so powerful

Unlike regular floods, flash floods can quickly sweep you off your feet and move cars.

Transcript

00:00 (upbeat music)

00:02 - Certainly when one gets up to inches per hour,

00:09 you're looking at the potential for flash flooding.

00:13 - During storms, keeping track of the rainfall intensity

00:16 and the volume of water per unit time

00:19 indicates if flash flooding is possible or likely.

00:23 Land elevation, soil conditions, and ground cover

00:26 also play a huge role.

00:28 - It would be very surprising to people in many places

00:33 what the magnitudes of the floods are

00:35 that are actually possible on the landscape.

00:38 - Flash floods are extremely powerful.

00:41 Victor Baker, professor of hydrology

00:43 and atmospheric sciences, says water flowing in a stream

00:47 typically runs an average of a few feet per second.

00:50 A flash flood flow could be 10 times that.

00:53 - If the water velocity is 10 times what you usually see

00:58 the energy is a hundred times what you usually see.

01:03 That is because energy goes up

01:05 with the square of the velocity.

01:08 - Fast moving water can be deadly.

01:10 It can also sweep away cars, roll boulders,

01:12 tear out trees, and destroy bridges and buildings.

01:15 - Think about what you normally see in a river

01:19 and what it does flowing along by your feet

01:22 and think of a flow more than a hundred times more energetic

01:27 and that is an example of why those two things,

01:31 why you have the problem from these floods.

01:34 - Flood waters also pick up massive amounts of debris

01:37 as they move, which can restrict the flow of water

01:39 and increase flooding.

01:41 Rapidly rising water can reach heights of 30 feet or more.

01:44 - So the water may back up behind that

01:47 and you may actually get a depth of water

01:50 five feet, seven feet deep,

01:53 just coming in all together, all at once.

01:55 For AccuWeather, I'm Emmy Victor.