# The Dynamic Response of an Arch Dam During a Record Low-Intensity Earthquake
## 2 Case Study--The Baixo Sabor Dam  
The Baixo Sabor dam is part of a hydroelectric project that was designed and built by EDP in the north of Portugal. The dam is on the Sabor river near its confluence with the Douro river [16] (Figure 1). This project is currently under concession to Movherad-Hidroeletricas do Norte, S.A., with Engie-Hidroeletricas do Douro, Lda, being the company responsible for operation and maintenance. There are two dams in the project: a concrete arch dam, which is the main dam, located upstream, with its hydraulic circuit and underground powerhouse (Figure 2); downstream, there is a concrete gravity dam, intended to allow pumping into the reservoir.  
Figure 1: Location of the Baixo Sabor dam (adapted from [17]).  
The Baixo Sabor arch dam is a double-curvature concrete vault. The river valley has a V configuration at this location, with the right bank having a higher inclination than the left bank (Figure 3). The dam is 123 m high, and the crest, at an elevation of 236 m, is 505 m long and the arch is 6 m thick. The concrete vault is composed of 32 blocks, defined by vertical contraction joints. The spillway is placed at the center of the crest, and its 4 openings, 16 m wide, have radial gates. The reservoir's capacity is 1095 km\({}^{3}\), for the water level at an elevation of 234 m, corresponding to a flooded area of 2819 ha.  
The dam was built in a granitic rock mass that intruded the phyllite-greywacke metassediments of the Douro-Beiras Group. From a petrographic point of view, this rock corresponds to a medium-to-coarse-grained, biotitic-muscovitic, porphyroid granite [17].  
The monitoring system installed at the Baixo Sabor dam provides information on the loads, material properties, and structural response. The collected data include concrete and air temperatures, reservoir water level, seepage, leakage, displacements of the dam, foundation, and contraction joints, concrete strains and stresses, and water pressures [19]. The measurement of the reservoir water level over time is performed using a high precision pressure meter and a level scale. An automated weather station allows for the determination of the air temperature and humidity.  
In order to evaluate the dynamic behavior of the arch dam, and its time evolution, it was decided that a continuous dynamic monitoring system be implemented, compris  
Figure 3: View of the Baixo Sabor arch dam and cross-section through the central cantilever.  
Figure 2: General layout of the Baixo Sabor arch dam (adapted from [18]).  
ing 20 uniaxial accelerometers with a radial position in three upper galleries (Figure 4), synchronized via GPS antennas [20].  
In addition, a seismic monitoring system (SMS) was also installed, intended to record the dynamic response of the arch and gravity dams when subject to seismic action. It is an active system, in permanent operation, which records the vibration of the structures and nearby areas whenever an earthquake takes place. The assessment of the structural response implies a comprehensive characterization of the seismic action reaching the dam; thus, the SMS also provides information on the propagation of the seismic waves in the rock mass along various directions.  
In order to achieve these goals, the SMS of the Baixo Sabor scheme includes these devices:  
- Six remote stations, placed in the rock mass around the reservoir, at the locations shown in Figure 5.  
- Six stations located in the galleries of the arch dam (Figure 6).  
Figure 4: Measuring points of the continuous dynamic monitoring systems (CDMSs) in the Baixo Sabor arch dam.  
Figure 5: General layout of the seismic monitoring system (SMS) of the Baixo Sabor hydroelectric scheme, with the location of the remote seismic stations.  
Figure 6: Location of seismometers of the SMS in the Baixo Sabor arch dam.