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.
There are three factors to any project:
Time - Cost - Quality
Contractors can optimize any two but not all three...
-
Deliver quick time and low cost but sacrifice quality.
Deliver quick time and high quality but sacrifice cost.
Deliver low cost and high quality but sacrifice time.
So with all of the upper dams full, a huge snow pack that could take all summer to melt off, the time it takes to draw new plans, plan check and engineering, and the possibility of another heavy winter next year, there may be a huge logistics problem here. Plus the fact of unforeseen problems discovered after demolition and environmentalists breathing down the contractors necks about polluting the feather river.