On June 23, 2001, about 1717 hours Pacific daylight time, a Schempp-Hirth Nimbus 3/24.5, N4427M, owned and operated by the pilot, experienced an in-flight loss of control during landing flare at the Siskiyou County Airport, Montague, California. The experimental glider came to rest in an inverted attitude on the airport, and it was substantially damaged. The private pilot was seriously injured. The accident occurred during a soaring competition. Visual meteorological conditions prevailed, and no flight plan was filed. The flight was performed under 14 CFR Part 91 and originated from Montague about 1250. During the accident flight the pilot was competing in an open class national soaring contest. Another participant in the soaring contest reported to the National Transportation Safety Board investigator that because of the strong surface wind condition, some of the glider pilots were landing into the wind using the east-west oriented closed taxiway rather than runway 17/35. A witness observed the accident glider approach the airport and attempt to land on the east-west taxiway while heading into the prevailing wind that was from about 270 degrees at 20 knots, with gusts to 30 knots. When the glider was about 20 feet above ground level, a wing suddenly dropped and contacted the ground. The glider spun around in a manner resembling a ground-loop. It then rolled upside down. According to the pilot, during his flight he had monitored the airport's weather condition, and he was aware of the wind speed and direction. The pilot reported that he turned onto the final approach leg with extra speed. When he had descended between 5 and 10 feet above the ground, a wind gust lifted the glider's right wing upward. It was then that the left wing contacted the ground, he lost control, cartwheeled, and nosed over.