While taxiing the airplane to park, the pilot selected an open spot located on the side of the parking apron. The pilot noticed a drainage ditch covered with a metal grate but did not perceive that only a portion of the ditch was covered by the grate. As the pilot maneuvered the airplane to park, the nose wheel entered the uncovered ditch and the propeller contacted the ground. The pilot shut down the airplane and exited normally. Damage was sustained to the airplane's firewall. The pilot was not injured. No markings were present to make the pilot aware of the hazard.