An interesting story, all in all. Allows for good after the fact thinking.
How would one stop a hydroturbine, if its electric load disappeared? I would suppose that it would overspeed in seconds, so anything to stop a turbine, would need to be done quickly and automatically.
Open some sort of a sluice in the line that feeds turbine with water, to divert and dump the water elsewhere, would seem like the only possible solution.
The amount of available options would depend on how quickly the water would overspeed the turbine. Such a calculation should be possible based on available data, such as water column height, turbine mass, and turbine radius.
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