Not impossible at all! The main spillway discharge enters the river channel at a 90deg. angle, easy to see how it could flow both ways with enough volume. It will never back up to lake level, but could back up enough to affect the turbines discharge.
Yup, the increased back pressure starts STALLING the turbines, and computer controlled respond by increasing the excitation voltages,....bad electrical catastrophes ensue. In the case of debris actually being inducted, the almost incalculable forces involved can break the shafts / gear trains ......the equivalent of a powerful bomb going off.
GTC
Member, Clan of the Border Rats -- “Sometimes I wonder whether the world is being run by smart people who are putting us on or by imbeciles who really mean it.”- Mark Twain
