The pilot of the glider reported that, while on a sightseeing flight with two passengers, about 10 minutes into the flight, the glider encountered an area with a loss of lift. He turned toward an area where lift was expected; however, the glider continued to descend. The pilot stated that there was not sufficient altitude to reach the airport, and he elected to land on a dirt road. During the landing flare, about 4-5 ft above ground level (agl), the glider abruptly yawed to the left due to a collision with an object and dropped onto the dirt road in a flat attitude; the glider bounced once and then came to rest upright on the dirt road. The glider sustained substantial damage to the fuselage and both wings. The pilot reported that there were no preaccident mechanical malfunctions or failures with the glider that would have precluded normal operation.