Roof drainage: water velocity

[paduk]

Hi there,
I am designing a roof drainage water system.
The roof features a double curvature: south east 20 degrees, north west 5 degrees.
The gutter will collect the water coming from the roof and will drain it in a end point downpipe.
My concern is that the water velocity might be too high (the protection is a 200 years period) and hence jump the gutter.
Do you have any formula or tips to calculate this velocity?
and which is the velocity value that should make me concerned?
many thanks

 

[bltseattle]

You could use Manning's equation to figure out the velocity in the gutter, then assume that at the end of the run, the jump height will be v^2/2g; add that to the normal flow depth, that should be about the max height where it hits the end of the gutter and drops down. But it never hurts to have a splash pad just in case the downspout clogs.

 

[PEDARRIN2]

You could have more than one downspout and slope the gutter on both sides to it. That way you do not accumulate too much water and will not have to worry about overflow or jumping.