According to a written statement from the pilot, during the landing to runway 20 (3,500 feet by 50 feet, turf) the nosewheel impacted a "significant hump" on the approach end of the runway. The nose gear collapsed, the propeller struck the ground, and the airplane nosed over coming to rest inverted. The left wing was crushed aft and broken and the vertical stabilizer was bent. The pilot did not report any mechanical anomalies with the airplane. In a follow-up conversation, the pilot reported that he was previously aware of the rough runway environment.