According to the pilot, after an uneventful flight, he verified the current weather conditions about 10 miles from the airport and subsequently entered the airport traffic pattern, at which point he elected to conduct a "grass landing." The airplane touched down normally, but during the landing roll he observed an obstacle in the middle of the "runway." The pilot was unable to react before the right main landing gear impacted a knoll and the airplane ground looped. The right main landing gear collapsed, and the right wing was substantially damaged. The airplane landed on an area of grass between the runway and taxiway when it impacted the grass knoll. According to the airport manager, pilots are advised to land and takeoff on published runways only as prescribed under the airport policy page of the airport's website. This information is not available in the airport's chart supplement. The pilot indicated that he checked the chart supplement for pertinent airport information, which did not include any restrictions about landing on the grass area between the taxiway and runway. The pilot reported that there were no mechanical anomalies with the airplane that would have precluded normal operation.