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I noticed this Tweet about the Oroville Dam concerns this week:

enter image description here

Seems most media is not emphasizing/noting the distinction. Is there a difference in the consequences of a spillway failure? Seems intuitively that the release of water might be slower due to what may well be a smaller gap.

But is that true? And would unmitigated spillway failures generally add concern that additional stress/erosion can result in full dam failure shortly thereafter?

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    $\begingroup$ This seems like a classic example of "how not to create a failsafe device". The actual procedure seems to have been (1) build the device (the emergency spillway), (2) ignore it for almost 50 years because it's never been needed, (3) discover that when you do need it, it's now broken. $\endgroup$
    – alephzero
    Feb 14 '17 at 4:57
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Spillways are designed to control the water flow (and excess waterflow) past a dam.

Spillway failure could mean a variety of things, but the primary difference is that the spillway is most always located at the upper level of the dam. Water that escapes this area may be damaging, but unlike dam failure, not all the water in the reservoir is going to be released.

2017 Oroville Dam Crisis Wiki

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  • $\begingroup$ That's the theoretical answer. "... in practice..." $\endgroup$ Feb 14 '17 at 14:39
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If a dam collapses catastrophically, the entire volume of the lake is released in one 'pulse.' This means the downstream area will be subjected not only to a wave of great height but also of great peak power (energy per unit time). That's a BadThing(TM) . You could look up info on what happened eons ago when Ice-Age ice dams gave way in the Northwest United States. Canyons were carved out in hours.

If a spillway gives out, then, depending on just how it fails, there'll be a gradually increasing flow rate as the new escape channel is widened. To a limited extent this means the same total water volume is released, but over a longer time period -- again unless at some point the spillway hillside blows out catastrophically. So the peak wave height and the peak power will be smaller, albeit applied for a longer period of time.

Either way, you will not be happy if you're downstream.

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