As M Spinhirne eluded to 3" rock is not going to technically "filter" silt out of runoff. You would use it to create a rock check dam. These are what you see on construction sites. When properly installed they stair step down a ditch so that the pool created by one touches the check dam upstream of it. They create slow moving pools of water that allow the silt to fall out of suspension. They don't have any "filtering" properties. They are really velocity control structures.

Dave in el dorado's design is very effective at slowing the water, and may have slight filtering properties due to his addition of smaller and smaller mediea (rocks). He's really created a slow fore-bay that he can clean out before the silt impacts his larger pond.

Erich