Dynamic monitoring of a concrete arch dam during the first filling of the reservoir

•Continuous dynamic monitoring of a concrete arch dam.•Characterization of modal parameters and vibration levels.•Automated modal analysis using the covariance driven stochastic subspace identification method (SSI-cov).•Mitigation of the reservoir water level effect on natural frequencies.•Comparison with results from a forced vibration test and a numerical model. The Baixo Sabor dam is a concrete double-curvature arch dam, 123 m high, located in the north-east of Portugal, which is being monitored by a dynamic monitoring system that comprises 20 uniaxial accelerometers. The paper starts with the description of the structure and of the installed monitoring equipment, as well as the monitoring software used to process the data that is continuously retrieved from site through an internet connection. Then, the results obtained during the system’s first six months of operation are presented, which include the characterization of acceleration levels for major events, such as the opening of spillway gates after intense rain, and the evolution of the dam modal properties (natural frequencies, modal damping ratios and mode shapes) during the reservoir first filling. In particular, the continuous characterization of the dam modal properties during important variations of the water level is a very unique experimental result (up to the authors knowledge, this is the first journal paper describing the continuous tracking of the modal parameters of a dam during its first filling), which is particularly interesting for the calibration of numerical models that take into account water-structure interaction. Additionally, it should be noted that the accurate tracking of the modal properties, in quite demanding conditions motivated by the very low vibration levels, associated to the massive nature of the structure, and by the presence of harmonics induced by the turbines of the hydroelectric power plant, was only possible thanks to the use of a carefully designed installation and advanced processing methodologies. At the end of the paper, it is presented a comparison between the results estimated with ambient excitation and the ones obtained with a forced vibration test and also predicted by a numerical model that includes the water effect with the added mass technique.