# The Dynamic Response of an Arch Dam During a Record Low-Intensity Earthquake
## 1 Introduction  
Monitoring the behavior of structures whose operation involves significant risks is essential to detect anomalies and intervene promptly to prevent any structural accidents. Periodic evaluation of the dynamic behavior of structures is an important tool because it allows for the detection of corresponding structural modifications over that period based on changes in dynamic behavior. These changes can pertain solely to the evolution of the mechanical characteristics of the material (favorable or unfavorable), variability of actions, or significant structural changes, particularly related to cracking. In order to implement this methodology, it is useful to determine the dynamic behavior of the structures for reference states and, from that point on, monitor their evolution. The dynamic behavior of concrete dams, given the potential risks involved, has been the subject of extensive research [1, 2]. Over the past few years, techniques for conducting forced vibration tests on concrete dams and other structures have been developed at the LNEC to characterize their dynamic behavior. These testing methodologies are part of a continuous evolution process, with considerable improvements implemented over time, particularly in the control of dynamic actions applied to the dam, the reliability of the records obtained concerning structural behavior, and their processing to identify the structure's dynamic parameters. The combination of experimental and numerical methods is an indispensable tool for supporting the safety assessment of structures.  
In recently constructed dams, systems that allow for monitoring the dam's dynamic behavior over time have been implemented. These systems can be divided into two types: (i) seismic monitoring systems (SMSs) that allow for recording of the action imposedon the structure and the corresponding structural response during seismic events, and (ii) continuous dynamic monitoring systems (CDMSs) that, based on dynamic response records due to ambient actions (noise, traffic, wind, plant operation, etc.), can determine dynamic parameters and verify possible behavior changes over time. Records of the dam's response obtained during seismic events have been an important source of information [3; 4].  
To support the structural safety assessment of a dam, numerical models representing the dam-reservoir-foundation system have been developed and calibrated with experimental results. Models that incorporate the discretization of the reservoir to simulate the hydrodynamic interaction water effect are typically preferred to simplified approaches, such as [PERSON]'s added mass method, which overestimates the water effect, requiring a correction coefficient [5; 6]. The representation of the foundation and the dynamic boundary conditions have also been examined by researchers [7]. Model calibration with experimental data for case studies has been presented [8; 9]. Within the context of structural health monitoring, the issue of damage detection by means of dynamic measurements has also been addressed for concrete dams [10; 11]. Various benchmarking workshops have allowed for comparison of the performance of different numerical approaches and the discussion of modelling uncertainties [12]. Extensive research studies have investigated the effects of seismic action on geotechnical structures, including dam foundations [13] and slopes [14; 15].  
In the present paper, the response of a concrete arch dam during a low-intensity earthquake that recently occurred in the region is analyzed by means of a numerical model of the dam-reservoir-foundation system. The Baixo Sabor dam is briefly described in the following section. Then, the numerical model, which had been previously calibrated by means of experimental data from forced vibration tests and ambient vibration monitoring, is presented. The response of the dam recorded during the recent seismic event is then compared with the model simulations under this dynamic input. Lastly, the methodology employed and the results obtained are discussed.