What causes flash floods to start so abruptly?

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Flash foods seem to appear as a sudden wall of water and debris several feet deep and across a whole currently dry channel at once.

What’s the mechanism for this to happen? I would expect even a sudden heavy rainstorm to form a small stream that builds up over time.

In: Physics

2 Answers

Anonymous 0 Comments

So, there are varying factors that can cause one, but to simplify, usually it’s cause the soil/land simply cannot absorb the amount of rain that is coming down.

Commonly, if someplace is normally dry (like California), the soil actually becomes hard and resistant surface. Rain doesn’t get absorbed as well and the water has to go somewhere. So it just runs off downhill… thus creating a flash food.

Think of a layer of burned crusties on your pan after leaving it in the sink overnight without soaking. It takes a while for the crusties to soften. So that’s what happens with the dirt and rain. It takes a while for the water to penetrate and so the water just runs off as a result.

Anonymous 0 Comments

In arizona where flash floods are fairly common, its mostly because the ground is packed, dry, rocky and doesn’t have much vegetation so it does a very poor job of absorbing water when there is a significant rain. The water just rushes along the ground toward low points and rapidly accumulates in dried creek beds and ditches (or anywhere low), and very quickly forms a ‘flash flood’.