Major flash flood in Zion Jun 29th. Locked

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mfinley
mfinley Senior Member • Posts: 5,419
Re: Major flash flood in Zion Jun 29th.

bflood wrote:

The amount of rain in this case is unremarkable - about 1 inch total in about 1 hour.

But the reality is that most of the rain, about 0.6 inches, fell in about 15 minutes.

The flooding is more the consequence of geology than meteorology. Most of the country has soil that has been rained on hard for millenia - all the loose sand/gravel washed away a very long time ago. That leaves a soil surface that copes pretty well with an inch of rain in an hour.

A desert, OTOH, hasn't been rained on much, and only a fraction of the rain it gets is hard rain. The surface soil is sand and gravel, and is easily washed away by a high rainfall rate like 0.6 inches in 15 minutes. Even the best efforts of road building departments using their best compacting techniques can't build a road in the southwest that can't be washed out fairly easily. And that's what happened in Zion. Springdale's experience demonstrates another characteristic - the fine grain sand/gravel doesn't allow much absorption, so most of the rain will be runoff - like down the streets of Springdale and into the town hall. Whoever thought that building the town hall at an elevation below the main street probably now understands the error in that idea.

We just got 3 inches of rain in 30 minutes in some isolated areas of Denver, caused flooding and havok, sh*t happens, nature does what she wants, man builds roads, bridges, and structures in places that nature puts a bullseye on. Sometimes they get by unscathed, sometimes they get jacked up. This is why we have surveying designating flood planes and stuff called 100 year, 500 year, and 1000 year floods. Zion was founded in 1919, so historically all the records for the place could only have data for a 100-year storm/flood, and mankind and engineers don't know exactly what a 100-year flood is exactly in an area until it happens. Wait till the 500 year one comes along.

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