After performing police patrol activities, the aircrew landed, refueled the helicopter, then took off to practice autorotative landings. Two autorotative landings were made on RWY 22, then 2 more were made on an adjacent grass area. After the 2nd landing on the grass, the pilot started to take off again. He stated the helicopter was traveling level along the ground with very little weight on the skids at a speed of 5 to 10 mph. He then saw a drainage ditch, surrounded by hi grass, about 10 ft ahead. He applied power & increased the collective to clear the ditch. Reportedly, the sounds of the engine increased; however, the helicopter started to roll & yaw to the right. Also, the power was reportedly reduced to correct the yaw, but the helicopter's right skid contacted the turf, the aircraft rolled to the right & the main rotors contacted the ground. An exam of the engine & airframe were made, but no pre-crash, mechanical failure or malfunction was found.