Results from a field study of the flow-induced vibrations of a full-scale, long-span, shell-type gate undergoing simultaneous over- and underflow are presented. Data include experimentally determined mode shapes and frequencies for both horizontal and vertical bending modes, the maximum center-span vibration amplitudes in the horizontal and vertical direction as functions of underflow gate openings and overflow depths, and the trajectories of gate motion. Predictions of gate vibration, based on theories previously developed from model-scale data, were in good agreement with the experimentally observed behavior. Theoretical considerations, coupled with earlier model-scale experiments, permit the classification of the observed self-sustained vibrations resulting from the coupled-mode press-shut behavior of the gate. The basic mechanism is as follows: when the gate moves toward the downstream side, it also moves downward; the flow rate under the gate decreases causing the deceleration of the upstream approach flow; accordingly, the hydrodynamic pressure in the upstream channel increases and the increased hydrodynamic pressure pushes the gate in the downstream direction, amplifying the original downstream motion of the gate. Analogous, but reversed, behavior occurs as the gate moves upward and upstream simultaneously, amplifying the original upstream motion.