A few times a week I surf on a "low head dam" It has a hydraulic controlled wing that adjusts the height and slope of the drop from tailwater to the run out. This way a perfect and surfable hydraulic jump is created which behaves much like an ocean wave. However sometimes the wave collapses and turns into a "keeper". We have ropes on hand to throw to anyone trapped and the level can be quickly adjusted by the controller to reform the wave and remove the keeper so it's fairly safe. It's still something to have a lot of respect for.
A video of me surfing this wave.
https://www.instagram.com/reel/CPgfVHPlNJx/?utm_medium=copy_...