The pilot landed the experimental nose wheel-equipped airplane 500 feet beyond the landing threshold of the grass runway, having performed a full-flap, power-off approach. The runway was traversed by two asphalt taxiways, and during the ground roll, he observed a depression in the turf just before the first taxiway intersection. He then applied back pressure to the elevator control; however, the nose wheel dug into the depression, and the airplane nosed over. The fuselage and wings were substantially damaged. The runway was normally 3,500 foot long, but at the time of the accident, the second half was closed. This was reflected in a NOTAM (notice to airmen), and the pilot was unaware of the closure. According to the airport manager, the depression was a drainage ditch, and was located to the left of runway, adjacent to the parallel asphalt runway. The pilot reported no preimpact mechanical malfunction or failure with the airplane that would have precluded normal operation.