My company needs me to accomplish a similar effect, but I can't do it anyway.Or I can get a little hint.please help me.
Thank you very much.

sorry for my english.

Dam spillways are a very specific design. Your simulation is naive even at a glancing view.

Your reference footage shows water pouring over the lip of the first stage of the spillway with a mostly laminar flow then hitting a second stage in the spillway that diffuses the energy (energy dissipation part of the spillway) of the water aerating the fluid. Classic short fall spillway design. We have a similar design in a small town near by me and it's amazing to watch this time of the year. The Lakefield spillway uses actual granite rocks strategically placed (we have a lot of that here) as the energy diffuser.

Without reference of the dam without water, you can't tell what technique the dam is using in the second ramp but if you just put three boxes or wedges in that second section plateau where the water should be hitting will certainly cause the water to collide and aerate nicely.

Redo your collider after doing 10 minutes of research on dam spillways and include a energy diffusion stage and MUCH LESS initial velocity and make sure it is laminar flow over the top. In other words, get rid of that turbulent velocity you are injecting in to your fluid! Let the diffuser collision geometry do that for you.

And use Metres as your scale.

Your reference image is definitely telling you what you need to do.