The pilot was returning from a three hour cross county flight in an area with which she was very familiar. Although too low to complete a final glide to the airfield, the pilot continued on expecting to pick up a thermal to give her the height necessary to complete the flight. The thermal source selected, a quarry, was known to be reliable. On this occasion the expected thermal failed to materialise and, at a very low height, the pilot was forced to select an area to make an outlanding. The pilot selected a field, initially not realising it was too short and covered in long grass. When she realised the field was covered in long grass she decided to improve aileron performance by setting the flaps to full negative from the previous setting of full positive. This resulted in a significant loss of lift, culminating in a heavy landing. The long wings dipped into the grass resulting in a violent ground loop. Significant Factors The following factors were considered relevant to the development of the accident: 1. The pilot descended to a very low altitude, anticipating a thermal over a known geographic feature. 2. When the thermal did not materialise, the pilot was then limited in choice of suitable areas for an outlanding. 3. The area selected for outlanding was short and covered in long grass. 4. The pilot incorrectly manipulated the flaps. 5. The glider landed heavily and ground looped.