## 1 Introduction  
The intense vibration induced by flood discharge may cause significant damage to hydraulic structures, especially to the accessory structures (e.g., guide walls [1; 2], sluice gates [3] and gate piers [4]) on the dam body. Moreover, it had been found that the vibration can be transmitted from the superstructure to the dam foundation, and then to the neighboring ground, which may seriously impact the safety of the nearby buildings as well as the physical and mental health of the building residents. Therefore, hydraulic structure damage and environmental hazards are the two major problems induced by the vibration during the process of high dam flood discharge. In the past several years, numerous studies on vibration mechanisms, health monitoring, safety assessment, and reduction measures have been carried out [5; 6; 7; 8; 9; 10; 11]. Among all the hydraulic structures that can be possibly damaged by flood discharge-induced vibration, the hydraulic gate is very vulnerable and is more likely to be destroyed than other hydraulic structures. The reasons for this are that the carrying capacity and structural strength of hydraulic gates, which are made of thin steel plates, are much lower than those of other hydraulic structures made of massive concrete (e.g., arch dam, guide wall, gate pier, spillway, water cushion pool and end sill, etc.). Moreover, the direct impact of fluctuating pressures on the hydraulic gate and the accompanying complex fluid-structure interaction mechanisms frequently lead to intense vibrations of gates, which has significant adverse effect on structural safety. To the authors' knowledge, cases of vibration-induced damage to hydraulic gates that have been reported in the public literature [3; 12; 13; 14; 15; 16; 17] are far more frequent than those of other hydraulic structures. In order to explain the mechanism of hydraulic gate vibration, extensive studies have been carried out and several theoretical models have been proposed. [PERSON] [12] reported that the vibration is due to a periodic reattachment of the jet to the flat bottom of the vertical-lift gate. [PERSON] and [PERSON] [13] presented a theory of self-exciting vertical vibrations which comprises the following elements: (1) gate raise variations induce discharge fluctuations; (2) discharge fluctuations and flow inertia cause local pressure difference variations across the gate opening; and (3) local pressure differences result in variations of the force exercised by the water on the gate. [PERSON] and [PERSON] [14; 15] believed that hydraulic gates are excited by vortices due to the dynamic interaction between the elastic gate and the unstable shear layer underneath the gate. In order to ensure the stability of Tainter gates with respect to a significant source of self-excited vibration, [PERSON] and [PERSON] [16] presented a design criterion which specified the gate bottom type, the hydraulic gate design frequency and the relative stiffness of the gate leaf and supporting arm in different operating conditions. Based on prototype observations, [PERSON] et al. [17] studied the flow-induced vibration of the radial gate in some engineering projects and evaluated the operational safety and vibration character of the gate. However, the explanations for the mechanism of hydraulic gate vibrations in different cases are quite controversial and the cause of self-excited vibration is still equivocal. Recently, the hydraulic gate vibrations with special characteristics that cannot be explained by the traditional flow-induced vibration theory were observed during testing of the prototype. Jinping hydropower station is one of many hydropower engineering projects with high dams and large reservoirs that have been built for the comprehensive utilization of water resources in the past several decades in China. One frequently occurring but still unresolved problem--the intense vibration of the hydraulic gate--was observed when floods were discharging through the orifices on the dam body in the flood season of 2014.