The bidding process is for sure a very problematic one for every project. But in this case, I assume that the bidding wasn't a fixed package, but rather a price per certain action, e.g. for a ton of RCC poured etc. As the bidding happened before the plans were finalized, that would be the only honest way to project the costs. And as we know, the work executed was way more extensive then assumed during the bidding process. There was much more RCC used - they couldn't know where exactly the bedrock was before they dug it out, and more of the upper spillway replaced with a final solution. So some of the costs which would have occurred next year have been part of this years budget. Considering those changes, it is no surprise, that the costs were overrun, actually the overrun is reasonably modest compared to other projects where there was time for a detailed planning.