The pilot reported that while on downwind he elected to land on the grass runway parallel to the asphalt runway. As he crossed the threshold for the landing area, he realized he was landing onto a grass field next to the grass runway. With the glider too low to transition to the grass runway, the pilot continued to land onto the field. During the landing sequence, the glider impacted two culverts and sustained substantial damage to the fiberglass fuselage. The pilot reported no preimpact mechanical failures or malfunctions that would preclude normal operation of the glider.