The general lack of attention paid to waterhammer primarily due to the belief that velocities less than around 10 to 15 fps are perfectly safe.  Flow velocity is only half the problem.  Accelaration or decelleration is the other half.  A mass moving at 5 fps decellerated over 30 seconds may not result in much force.  A mass moving at 5 fps, decelarated in 1 second can result in relatively quite a lot of force.The gates are large vanes that are controlled like venetian blinds. As they are servo controlled to maintain the generator's power input, they are opened and shut slightly.
It's my understanding and experience that they were typically all controlled by a single huge yoke assembly.   However lately they have been migrating to individual servos for each vane, all individually fed by communications lines.  I think they'd just gone to this and were initially setting up the new control Bellows Seal Valves system.
As for the other generators they're HUGE units that have extremely large rotors moving at very large tangential speeds only a few millimeters from their stator assemblies.  When the water from the failed unit(s) poured into that space hydraulic forces immediately disassembled them.

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