Let me show you the coolest spillway design. The spillway in Lake Berryessa behind Monticello Dam in Napa Valley, California.
Spillways manage water levels and prevent overtopping of dams.
This particular design is a Bell-Mouth Spillway. They are also called Morning Glory Spillways (named after the flower whose shape is similar), and this is, in fact, the official name of the one in the video.
This is not the only one around and there are a few major dams that use this design.
The reason for a Bell-Mouth design, in this case, was the constraints of the dam location. There was no space in the valley to have a bypass-type spillway on the sides. Also, the tunnel used to divert the river during the dam construction was adapted to become this spillway outlet - saving a lot in construction and schedule.
They don't always look this spectacular, and they are usually not required. For example, it became active for the first time in 10 years in February 2017. I also previously posted about the Hoover Dam Spillway, which has only been used twice in a hundred years.
When the dam reaches capacity, the spillway swallows water at a rate of 1400 m3/s (that is, an Olympic-size swimming pool every two seconds), emptying through an enormous concrete pipe.