It may be an old thread, but I'll chime in, maybe it will help someone.
My bog is built similar to and basically inspired by addy's showcase. Now, I understand that some may want to do the snorkel/centipede method, that's up to them. It seems like a good system if you're concerned with muck buildup. It can be a little more complicated to set up and quite pricey, but there are other materials that can be used to keep the cost down.
Mine is 14 feet x 5 feet and has approximately 12 inches of small river stone. I have two 2 inch PVC manifolds that span the full length if the bog. Each one has slits cut through 1/3 of the diameter of the pipe and are spaced 1-1/2 inches apart. At the end of each pipe is a 90 degree elbow with a vertical clean-out stack that ends just above the gravel. Each clean-out stack has a screw-on cap.
When my bog was first started and going through it's maturity, I had to use the clean-outs maybe once a week. But after everything settled and the bog seemed to mature, I've only had to do it maybe once every two months. If I notice the water returning to the pond has slowed down considerably, I'll do the flush.
Here's how I do it...I turn off the pump, unscrew one of the caps, stick a spare piece of 1-1/2" flex PVC down into the open 2" PVC, aim the other end toward my garden then turn the pump back on. For about 5 seconds the water comes out black, then turns clear. I shut the pump, cap the pipe and repeat the process with the second clean-out.
After that, the water returning to the pond is back to full volume.
So, I'm not backwashing the gravel, I'm just using the pump to clean out the buildup in the manifold pipes.
As far as buildup of muck within the gravel, addy stated she has had her bog in service for 10 years and never had to tear it apart to clean out any muck. I'll take her word for it and I'm looking at many years of clear water with maintenance free bog filtering.
