On December 29, 2001, at about 1500 eastern standard time, a Glaser-Dirks Flugzeugbau DG-200,glider, N5457T, registered to a private owner, operating as a 14 CFR Part 91 personal flight collided with trees while maneuvering in the vicinity of Benton, Tennessee, following the loss of thermal lifting. Visual meteorological conditions prevailed and no flight plan was filed. The glider sustained substantial damage. The private pilot reported minor injuries. The flight originated from Chilhowee Glider Port, Benton, Tennessee, at 1300. The pilot stated he was flying north in a ridge lift. He turned back over a plateau area where he had previously encountered good thermals. He looked at his airspeed indicator before turning and it indicated 90 knots. He continued his turn toward the front of the ridge toward the valley and encountered a severe sink rate about 180-degrees into the turn. The vertical speed indicator went from a climb to a 1,300 foot per minute descent. According to the pilot, the glider was about 400 feet above ground level which under normal circumstances should have been enough altitude to clear the ridgeline. As he continued the turn, the sink rate increased between 1,500 feet per minute to 3,000 feet per minute. The glider descended into trees short of the ridgeline. The pilot further stated he did not experience any mechanical failure or malfunction of the airframe or flight control system.