Sometimes called a weir raises the level of water upstream of a dam. However it creates a hydraulic jump when super-critical and sub-critical meet very close to the dam. This results in a circular flow of water creating a spot where water currents recirculate, which creates a dangerous trap for objects and people1.
References
1.
Practical Engineering. The Most Dangerous Dams. at https://www.youtube.com/watch?v=GVDpqphHhAE (2019).