THE PILOT SAID HE WAS RETURNING TO THE GLIDER PORT AFTER A LOCAL SOARING FLIGHT DUE TO A LACK OF LIFT. DURING THE FLIGHT BACK TO THE AIRPORT THE PILOT BECAME AWARE THAT THE RATE OF DESCENT WAS TOO HIGH AND THE GLIDER WOULD NOT MAKE THE AIRPORT RUNWAY. THE PILOT SET UP TO LAND IN A VACANT FIELD COVERED WITH MEDIUM HEIGHT GRASS. THE GLIDER COLLIDED WITH A DITCH DURING THE LANDING ROLL OUT IN THE FIELD. THE DITCH WAS NOT VISIBLE TO THE PILOT UNTIL THE LATTER STAGES OF THE LANDING ROLL OUT.