The pilot reported that he was taxiing from the transient parking towards the runway for takeoff. It was dusk at the time and he was proceeding at a walking pace (2-3 miles per hour) to the taxiway when he encountered a large ditch at the end of the tarmac. The airplane rolled into the ditch bending the prop and the empennage. The pilot commented that there were no signs, reflectors, warnings, or other indications that the ditch was there. The airport manager reported that the taxi route from the transient area is clearly marked with in ground reflectors which lead to a lit taxiway and then the runway. The manager stated that it appears that from the transient parking area where the pilot was parked he proceeded directly to the taxiway, crossing a set of double solid lines that demarcate the taxiway edge, and crossed into a helicopter operating/parking area. From there he crossed a marked service road, past a helicopter landing pad, and past a lit windsock and into the drainage ditch. The airport manager further reported that as the area is clearly marked and only intended for helicopter traffic there is no extra signage warning of the ditch.