Learned something new today

The state required a new plat for an irrigation reservoir so we contacted an engineer who can sign off on one as larger as this one.

The dam was permitted back in the 1960's as 50 feet high and the topo is actually pretty close to the original one.

Somewhen in the past the outlet structures were changed, a headgate/outlet pipe was concreted in and a syphon was added.

The elevation for the top of dam is 3816, the bottom of the reservoir is 3767, and the top of syphon is about 3815, slightly buried in under the top of dam.
The engineer explained that the syphon will empty the reservoir fairly quickly but only to the 3783 leaving 16' of water in the reservoir.
So after the water lowers to 32' below the top of syphon it will actually begin to steam and of course stop moving the water. One atmosphere is the limit.

I've seen lots of them, but never really thought about the limit, I just kinda assumed they could empty anything.