flood
A flood can happen almost anywhere. In wet areas they are expected, but in dry regions they are usually the more dangerous flash floods.
The two main reasons a flood happens are when rain builds up faster or just too much for the ground to soak in, or when a dam (either natural e.g. debris like sticks, or man made) collapses.
Since soil in its dry form is hydrophopic (doesn't absorb water at first) the water just forms a wall that easily sweeps any thing in its path up. this is why floods in dry areas are usually more dangerous than those in wetter ones (that and the fact that in dry areas flood protection is not as great).
Predicting floods is as simple as predicting rain fall, which is just cloud position, type, and temperature; though this will only work if it is a flood caused by just rain and not a dam collapsing (which is usually caused by rain, but still mainly unpredictable).
If a flood can be predicted, an evacuation notice will be given for all those in the path, as well as estimated time until it strikes so posessions can be gathered.
http://www.fema.gov/hazard/flood/index.shtm
http://www.bom.gov.au/hydro/flood/flooding.shtml
http://en.wikipedia.org/wiki/flood
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