This was a rather weak point for us. We did not grasp early enough the simple fact that it was essentially an engineering project. For planning permission, we went first to an architect. The result was a very attractive design, but It would have been better to start with the engineering and what that would require, then go to the architect. Advice from professional engineers is needed, which for us came initially from consultants, and later from two recently retired engineers who stepped in and guided us through.
There is a need to understand the whole technical side, not just how to build something in a river. Hydropower involves water flow, generation, transmission, data on the river flow and level, and a lot of analysis. As I have a physics background I was reasonably comfortable with all this, but a group that does not have access to these skills is very much in the hands of the consultant, and there is a worrying number of projects that have under-performed.
Analysis can produce a fairly reliable estimate of what income to expect, but we don't know if we have a viable project without a figure for the cost. Hydropower is not off the shelf, each project needs a custom design. A quantity surveyor needs the design to work on, contractors don't like to give estimates without some understanding of the detail, and they have to spend time on it to work out a cost. They are cautious about giving a figure at all, in case the customer ignores the ifs and buts and assumes that will be the final figure. We had a few estimates at different stages and found the cost increasing.
A special skill that engineers bring is an understanding of risk. Anything that could affect the project in an unpredictable way, needing extra work or causing a delay, is risk. That will add to the cost, so money has to be spent to reduce the unknowns. A good sound design is essential, needing as much knowledge of the site as possible. A river is quite a hostile environment, and sooner or later it will find out any weaknesses. All aspects of the construction and operation must be thought through, remembering that if any changes are made after the quote, however small, the contractor will increase the cost.This applies to the contract as well as the design.
Attention to detail is key!