Installation and results from the first 6 months of operation of the dynamic monitoring system of Baixo Sabor arch dam

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1 Available online at ScienceDirect Procedia Engineering 199 (2017) X International Conference on Structural Dynamics, EURODYN 2017 Installation and results from the first 6 months of operation of the dynamic monitoring system of Baixo Sabor arch dam Sérgio Pereira a, Filipe Magalhães a, *, Jorge Gomes b, Álvaro Cunha a, José V. Lemos b a Construct-ViBest, Faculty of Engineering (FEUP), University of Porto, Rua Dr. Roberto Frias, Porto, Portugal b National Laboratory of Civil Engineering (LNEC), Av. do Brasil 101, Lisboa, Portugal Abstract The Baixo Sabor hydroelectric power plant, located in the northeast of Portugal, includes a reservoir with a storage capacity of 630 million m 3 of water created by a concrete double-curvature arch dam, 123 m high, which was equipped with a 20-channel vibration based structural health monitoring system. This paper, apart from describing the dam, presents the main characteristics of the monitoring system and the results obtained during the first 6 months of operation and the first filling of the reservoir. These results include in particular the characterization of the accelerations amplitude and the evolution of the dam modal parameters (natural frequencies, modal damping ratios and mode shapes). It is particular interesting to observe the influence of the water level on the modal parameters and the development of statistical models to mitigate this effect The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of EURODYN Keywords: arch dam; monitoring; operational modal analysis 1. Introduction The Baixo Sabor hydroelectric power plant is located in the northeast of Portugal. Its reservoir extends 60 km north along the Sabor river, a tributary of the Douro River, allowing a useful storage of 630 million m3 of water [1]. This is a strategic location, since there are four other power plants located downstream that can be supplied by Baixo Sabor * Corresponding author. Tel.: ; fax: address: author@institute.xxx The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of EURODYN /j.proeng

2 Sérgio Pereira et al. / Procedia Engineering 199 (2017) reservoir in case of need. The concrete double-curvature arch dam, embedded in a narrow valley zone, is 123 m high and its crest 505 m long. The arch is composed by 32 blocks, separated by vertical contraction joints, and includes six horizontal visit galleries. Figure 1 shows an aerial picture of the dam and the reservoir, referring to May 2016, after the monitoring had started. Figure 1 Baixo Sabor arch dam [1]. The continuous dynamic monitoring of Baixo Sabor dam is being carried out by ViBest/FEUP and LNEC, in order to identify the dam s dynamic characteristics and their evolution over time, taking into account the variation of ambient and operational conditions, as well as the possible evolution of the materials mechanical properties. Similar works have been performed in the past, as for instance in Mauvoisin dam in Switzerland [2] or Cabril dam in Portugal [3]. This paper describes the dynamic monitoring system installed in the Baixo Sabor upstream step dam and presents the results obtained during the first six months of operation of the referred monitoring system, between 01/12/2015 and 31/05/2016, which comprehend a major part of the reservoir-filling period. Results from the application of automated operational modal analysis are presented and the use of regression models for the elimination of operational effects on the estimated natural frequencies, such as the water level, is explored. 2. Description of the Monitoring System To ensure a good characterization of the dynamic behavior of the dam, 20 uniaxial accelerometers were radially installed along the three upper galleries. Figure 2 characterizes the position of the accelerometers, marked in red. In the GV1 gallery, 12 accelerometers are divided in two groups of six, disposed on each side of the spillway. Each of these groups of six is connected to a digitizer, which is linked to a field computer. In turn, the eight accelerometers on the two lower galleries are connected to a different set of two digitizers. All the equipment is connected by optic fibre and the synchronization of the data recorded by each digitizer is assured with GPS antennas. The main field computer is connected to the fiber optic network between the dam and the plant, thus allowing remote access. All the installed accelerometers are uniaxial and force balance, which were configured to measure in the range g and 0.25 g, in order to allow the accurate characterization of very low acceleration signals. The dynamic monitoring system is configured to continuously record acceleration time series with a sampling rate of 50 Hz and a duration of 30 minutes at all instrumented points, thus producing 48 groups of time series per day. Figure 2 Position of measuring points on Baixo Sabor monitoring system [4].

3 2168 Sérgio Pereira et al. / Procedia Engineering 199 (2017) Data Processing The continuously collected data is independently processed by the two institutions involved in this project (ViBest/FEUP and LNEC), this paper presenting the processing developed by ViBest/FEUP. This is accomplished with a monitoring software developed at ViBest/FEUP called DynaMo, initially designed for continuous dynamic monitoring of bridges and now adapted for the monitoring of dams [5]. As already referred, the monitoring system organizes the continuously collected acceleration time series in files with 30 minutes which are regularly downloaded through an FTP connection to the main field computer. These files are then handled by DynaMo software that was configured to perform the following tasks: - backup the original data file in a database; - pre-process of the acceleration time series, which includes trend elimination, filtering with an eighth-order low-pass Butterworth filter [6] and re-sampling with a frequency of 25 Hz; - characterize the acceleration amplitude by maxima and root mean squares values (RMS); - construct colour maps in the frequency domain, which characterize the energy distribution along the analysed frequency range over time; - identify the bridge modal parameters (natural frequencies, mode shapes and modal damping ratios), based on its response under normal operation, using state of the art output only modal identification algorithms; - apply statistical tools to minimize the water level effects on the identified modal parameters; - store all the obtained results in a database; - publish the most important results on a webpage. The most important and challenging task is the continuous automated identification of the modal parameters. In the present application, after testing alternative output only algorithms [7], it was concluded that good results could be obtained combining the Covariance Driven Stochastic Subspace Identification method (SSI-COV) with a routine based on clusters analysis to automate its application. A description of this approach and of its theoretical background can be found in reference [8]. 4. Analysis of the Results 4.1. Identified Modal Parameters The six months of data were processed following the steps described in the previous section. The first six modes of vibration were identified and natural frequencies, modal damping values and modal configurations were obtained. The three-dimensional representations of the modal configurations are presented in Figure 3, in which the dashed line represents the dam s original geometry and the modal configuration is represented in red. The first, third and fifth modes are approximately symmetric and the second, fourth, and sixth are antisymmetric. Mode 1 Mode 2 Mode 3 Mode 4 Mode 5 Mode 6 Figure 3 Modal configuration of the first six modes of Baixo Sabor arch dam. Modal estimates corresponding to the first six modes are resumed in Table 1, where minimum, maximum, mean and standard deviation frequencies and damping values are presented. It is important to notice the significant difference between minimum and maximum frequencies for each vibration mode (even after the elimination of outliers), which is reflected in the standard deviation values as well, indicating significant oscillations during the evaluation period. Additionally, the damping values present slightly higher mean values for the symmetric modes.

4 Sérgio Pereira et al. / Procedia Engineering 199 (2017) Table 1 Modal Parameters Mode ffff [Min ; Max] [Hz] ffff mmmmmmmmmmmmmmmm [Hz] ffff ssssssssssss [Hz] ξξξξ [Min ; Max] [%] ξξξξ mmmmmmmmmmmmmmmm [%] ξξξξ ssssssssssss [%] Description 1 [2,43 ; 2,75] 2,53 0,10 [1,16 ; 3,16] 1,50 0,23 Symmetric 2 [2,57 ; 2,92] 2,68 0,11 [0,85 ; 2,11] 1,42 0,15 Antisymmetric 3 [3,33 ; 3,85] 3,51 0,17 [0,55 ; 3,00] 1,67 0,25 Symmetric 4 [3,92 ; 4,50] 4,12 0,19 [0,92 ; 1,82] 1,36 0,16 Antisymmetric 5 [4,78 ; 5,34] 4,99 0,18 [0,75 ; 2,66] 1,88 0,30 Symmetric 6 [5,32 ; 5,95] 5,58 0,16 [0,50 ; 2,29] 1,44 0,26 Antisymmetric However, the evolution of the modal parameters is easier to understand when graphically depicted. Therefore, the temporal evolution of the natural frequencies of the first six modes of vibration of the structure is characterized in Figure 4 a), where each point corresponds to a 12-hour average. The small blank spaces correspond to periods of system failure or maintenance, for which no data is available. As 12-hour averages were calculated, two values are presented per day. This process leads to a visually clean figure without losing accuracy in the characterization of the modal parameters fluctuations, as it will be illustrated latter. It should be taken into account that modal parameters variations are quite slow in such very massive structures. At the same time, Figure 4 b) shows the temporal evolution of the reservoir water level. Observing these two figures (Figure 4a and b), it is possible to observe that the values of the natural vibration frequencies of the structure have decreased considerably and continuously since the beginning of the monitoring. Moreover, this decrease accompanies the increase of the water level of the reservoir, thus suggesting an inverse proportionality between these two variables. This phenomenon is clearly observed during January, when intense rains occurred, motivating the sudden rise in the reservoir water level and a sudden drop in the frequency values, which would be expected, given that a large amount of mass was added to the dam-reservoir system. Furthermore, it is interesting to observe the sensitivity of the natural frequencies to the relatively low variations of the water level occurred between February and May, thus, confirming the accuracy achieved in the natural frequencies estimates. In turn, Figure 5 presents the temporal evolution of the first three modal damping ratios, with colours corresponding to those in Figure 4. In this case, it is shown that the modal damping values of modes 2 and 3 are similarly influenced by the reservoir water level, while that influence is not significant in the case of mode 1. As observed in other monitoring applications the modal damping estimates present a much higher scatter than the observed in the natural frequencies. These experimental results obtained during the first filling of a dam reservoir are quite unique and very relevant for the tuning of numerical models that take into account the influence of the water level in the dynamic behaviour of dams. Figure 4 a) Time evolution of natural frequencies 12-hour average; b) Time evolution of reservoir water level.

5 2170 Sérgio Pereira et al. / Procedia Engineering 199 (2017) Removal of the Effect of Operational Conditions Figure 5 Time evolution of damping ratios 12-hour averages. In order to detect small variations of the natural frequencies of the structure motivated by structural alterations, which may be associated with the occurrence of damages, it is important to minimize the effects of environmental and operational factors. In this application the factor that more strongly affects the natural frequencies is the water level. In this way, statistical relationships between the vibration frequencies of the structure and the water level in the reservoir were developed, as shown in Figure 6. The obtained quadratic regressions are generally of high quality, presenting values for the determination coefficient of the order of It is also important to note that there is much more data for the highest and lowest levels of water in the reservoir during the observation period compared to those in the intermediate zone of the figures due to the intense rains that took place in January, leading to a sudden rise of the water level. Figure 6 Correlation between the reservoir water level and the first two natural frequencies [9]. Afterwards, this effect was removed from the values of the frequencies directly estimated from the acceleration time series, resulting in the evolution of frequencies in the time represented in Figure 7. This shows a great stability over these six months and thus confirms the close relationship between the level of water in the reservoir and the vibration frequencies of the structure. Figure 7 - Time evolution of natural frequencies 12-hour averages after correction of reservoir water level effect

6 Sérgio Pereira et al. / Procedia Engineering 199 (2017) The future collection of additional data will certainly permit to enrich these models with the incorporation of factors with lower influence on the natural frequencies of this type of structures, as for instance the temperature. 5. Conclusions The dynamic monitoring system installed in Baixo Sabor dam was described and the main results obtained between December 2015 and May 2016 were presented. The processing of the time series of accelerations continuously recorded during this period and the good results obtained demonstrates that the dynamic monitoring system is functioning properly and able to automatically identify the modal properties under much different operating conditions. The results show a close relation between the water level in the reservoir and the values of the vibration frequencies, thus explaining the significant variations of the latter during the six months of evaluation. However, after correcting the effect of the water level, using regression models, it appears that during the period under analysis the dynamic behaviour of the dam remains quite stable. These experimental results obtained during the first filling of a dam reservoir are quite unique and very relevant for the tuning of numerical models that take into account the influence of the water level in the dynamic behaviour of dams. The next step of this work will comprise, on the one hand, the tuning of the modal parameters identification process, including the estimation of uncertainty bounds, and, on the other hand, the minimization of environmental and operating conditions effects on modal parameters, taking into account the use of new data and new variables, such as temperature. Acknowledgements This work was financially supported by: PTDC/ECM-EST/0805/ DAM_AGE - Advanced Online Dynamic Structural Health Monitoring of Concrete Dams, funded by FEDER funds through COMPETE Programa Operacional Competitividade e Internacionalização (POCI) and by national funds through FCT - Fundação para a Ciência e a Tecnologia; FCT PhD Scholarship SFRH/BD/100587/2014 provided to the first author. The authors would like also to acknowledge all the collaboration and support provided by EDP Produção. References [1] EDP (23/01/2017). Available at: [2] G. R. Darbre and J. Proulx, "Continuos ambient-vibration monitoring of the arch dam of Mauvoisin," Earthquake Engineering and Structural Dynamics, Article vol. 31, no. 2, pp , [3] S. Oliveira and M. Espada, "Long-term dynamic monitoring of arch dams. The case of Cabril dam, Portugal," 15th World Conference on Earthquake Engineering, [4] LNEC, FEUP, and Ambisig, "Upstream step dam of Baixo Sabor hydroelectric power plant. Characterization of the dam's dynamic behaviour through continuous monitoring. Installation Report." (in Portuguese), [5] Magalhães, F., Amador, S., Cunha, A. and Caetano, E., "Dynamo - software for vibration based Structural Health Monitoring", 6th Int. Conf. on Bridge Maintenance, Safety and Management, IABMAS 2012, Vila Erba, Lake Como, Italy, [6] Mathworks, "Matlab. R2016a.," ed, [7] Magalhães, F. and Cunha, A., "Explaining Operational Modal Analysis with data from ana arch bridge", Mechanical Systems and Signal Processing, Invited Tutorial Paper, Volume 25, Issue 5, pp , [8] Magalhães, F., Cunha, A. and Caetano, E. Online automatic identification of the modal parameters of a long span arch bridge, Mechanical Systems and Signal Processing, Vol.23, Issue 2, pp , [9] Vibest/FEUP, "Dynamic Monitoring System Baixo Sabor Dam. Results of May 2016." (in Portuguese), 2016.