Actual Velocity in Sewer vs Design Velocity by Manning's Equation

I have a question about the actual velocity in the sewer, especially stormwater pipe. In many cases, people calculate the design velocity using Manning's equation and I've seen a lot of people calculate the velocity over 3.0 m/s or even more than 12 m/s according to the pipe slope. They have calculated the velocity based on the assumption that water flows full in the pipe, but I think water does not flow full in the pipe in most cases. Design guideline states the design velocity shall not exceed 3 m/sec. The acceleration of gravity in the vacuum condition is just 9.8 m/s2, so it's not possible for the design velocity to exceed 9.8 m/s. I've heard the terminal velocity of rain drop is about 2 m/s. Is there anyone who has data or experience with actual and design velocity in the sewer under gravity?