How long does it take to tear out, re-compact soil, form, tie steel and pour new concrete on a rugged hillside. ( maybe 3000 feet worth.?)
It really varies based upon how much has to be dug out and mucked out below grade after you demo the lower potion of the spillway. Additionally it can be done a lot faster if you want to pay for overtime work — big money can be used to accelerate it.
I would say, four months for starters after the approved plans and access.