I was very interested to read the article about the Manapouri second tailrace tunnel on p2 of your June 2002 issue of IWP&DC. I worked for the South Island hydro region of the Electricity Corporation of New Zealand (ECNZ) through the 1980s before it became Meridian Energy, and I remember that the costs and benefits of the second tailrace tunnel were already being studied in the late 80s. I think the idea goes back much further, and it is pleasing to see that it has at last come to fruition.
I keep seeing reports asserting that the Manapouri turbines could not reach their designed output because of flow restrictions in the original single tailrace tunnel. However, I remember as a boy living near Manapouri in the 1960s there was enormous controversy (for New Zealand) over plans to raise Lake Manapouri as part of the power station scheme. In the end the lake was not raised by anything like the amount intended and I wonder if the original turbines were designed for the raised lake level, and that is the real reason the turbines did not perform as intended. Perhaps another reader will answer this question, or perhaps everyone has simply forgotten.
In closing, thank you for the improvements you have made to IWP&DC since you became Editor, it is now much more readable and no less interesting.