system
Given the following post, respond as if you're commenting on Reddit:
human
How do flash floods happen?
gpt
If you have ever had a potted plant that you forgot, or left unattended over vacation, and came back to it bone dry, then added water, you probably noticed the water didn't soak into the soil right away. Maybe you even had the water flow over the edge of the pot?

Its the same idea on a larger scale. Long periods of drying out will cause the soil to be hydrophobic until thoroughly wetted. If you have massive rains, the water doesn't soak into the ground fast enough and instead just runs down hill. A sufficiently hard rain will get you a flood.