While participating in a soaring competition, the pilot of the glider realized that a layer of high cirrus clouds had moved into the area and had stopped the heating of the ground by the sun. The thermal activity began to get progressively weaker and the pilot began to look for a place to make an off airport landing. He observed a mowed hay field and did not see any obstructions and chose it for his landing site. After turning on final approach with the wing flaps and spoilers extended, the glider struck two electrical transmission lines that were running diagonally across the glide path, about 15 feet above ground level. During the encounter with the transmission lines, the transmission lines cut into the wing root and landing gear door, broke the main pivot point for the landing gear, broke the canopy, cracked the seat pan, and cracked the aft fuselage approximately halfway back between the trailing edge of the wing and the vertical stabilizer.