[griffzhowl]

If it's based on a real flood, they presumably follow a power law distribution, where you have relatively frequent "normal" floods, and progressively larger floods are rarer and rarer, till you get an occasional gargantuan one.

My guess though is it's probably a plot device where the storyteller takes a known phenomenon and just exaggerates it to magical-mythical proportions, which may contribute to the story being repeated as it strikes the balance between the relatability of the real phenomenon and the attention-grabbing otherworldliness of its exaggerated version.