According to the pilot, he hover-taxied the helicopter downwind in an 18-knot wind that gusted to 26 knots, before he landed and purchased fuel. For the subsequent takeoff, he again hover-taxied downwind to his requested runway. He said the winds were exacerbated either by gusts or a "wind tunnel" effect from two rows of hangers. As the helicopter hovered downwind, the pilot was unable to arrest his descent, forward travel, or a loss of rotor rpm with the adjustment of throttle, collective, and cyclic controls. The helicopter continued over and descended into a drainage "lagoon" before it struck the opposite bank and nosed over. The pilot reported that there were no mechanical deficiencies with the helicopter. According to the FAA Rotorcraft Flying Handbook, among the conditions conducive to settling with power are downwind approaches at low forward airspeed.