Matt, lots of people use gravity methods in their bottom drain / filter systems with no problems, and end up with a clearer pond and less bottom cleaning to do. The plumbing between the drain and filter is a minimum of 3" in diameter. So, I think that the water will flow through the filter as fast as the pump takes water from the filter, as long as the filter is below ponds water line.
In a way, you are right about the filter media becoming saturated with debris. I think this is only a big concern in fall, because of leaf drop. In the begining of fall, I usually go out with a net everyday and scoop anything on top of the pond. In late fall, I cover my pond with this heavy duty plastic screen. It's holes are only 1/8". So, the pond is able to breath and get sunlight, with very little debris reaching it. Maybe I can create some sort of settling chamber before the filter. This way all the heavy stuff will sink there befor it gets to the filter media.
Anyway, I am trying to post a new diagram below. In the new design, the plumbing connects to the filter from the side, just above the filter media instead of below it. This way the water and debris will fall in to the media, instead of being pulled through. I also created a shelf to hold the media above the pump (in red). As I consult with other ponders, I will keep you posted on changes in design.
I WILL FIND SOMETHING THAT WORKS.
http://watergardening.infopop.net/3...a&ul=3482950476