The pilot was taxiing the airplane after landing on a gravel and clay bar the second time that day. He said the ground was soft and muddy, so he kept the airplane moving so it would not become stuck. The wheels bogged down during the taxi, the airplane nosed down, and the wind carried the airplane over onto its back. The vertical stabilizer and the right wing outboard of the lift strut sustained substantial damage. The pilot said the visibility was 5 miles in rain, and the winds had increased from the first trip, and were 25 knots, with gusts to 40 knots.