I'm really unsure about when this attraction will be opening. I say this because they will first have to remove and demolish the entire flume and any buildings they aren't keeping. Next remove a large number of trees (I assume) to prepare the site. Then they will need to remove water from the 'lake' if they plan to build here. This all has to be done before construction of the coaster can even be considered. In regards to teasing the ride 'so early' it's not really that crazy to think Merlin want to give an explanation to why one of their most loved attractions has been closed, and why that entire area is fenced off.
So I'm not going to say it MUST be 2017 or 2018 because right now we dont know the scale of the project, which when plans come out we will and then we can make a more accurate assumption to when this attraction shall be operational.