Mulm Suspension Speeds

For perspective, I am trying to design a combination tropical water
garden indoor/outdoor pond/aquarium.

That sounds pretty daunting by itself, but please don't let that
derail the discussion -- my present question, (I think, hope) is
pretty focused:

That is, "is there any general rule-of-thumb for flow velocities that
will keep aquarium tank mulm in suspension?"

If you will kindly refer to the link below, I have made a sketch that
will hopefully save a lot of words. I am referring specifically to
the "U" shaped pipe that extends downward out of the "screened bottom
drain".

I realize that there are many factors involved, (including pipe
diameter and vertical height of the standpipe), but my question is:
"Is there any general rule-of-thumb for flow velocities (i.e., in the
U pipe) that will keep tank mulm sufficiently suspended such that it
will be expelled from the top of the U standpipe, and not simply
settle IN the pipe?" (Please ignore the complications of leaves,
sticks, plant matter, string algae, etc. -- I am only talking about
fish feces and similarly sized particles.)

Thank you very much for any thoughts,
David

http://www.nmia.com/~dmjeppe/TWGASingleTank.jpg