The behaviour of Foz Tua dam during the first filling of the reservoir and its first years of operation

Abstract

Foz Tua project, located close to Tua river’s mouth, a tributary of Douro river (right bank), North of Portugal, is the latest accomplished hydroelectric investment of EDP - Energias de Portugal. The project includes a concrete double curvature arch dam closing a narrow granitic valley, in the right bank a twin shaft powerhouse and two independent sets of headrace and tailrace tunnels. The reservoir has a capacity of 106 Mm³ and a surface area of 420.9 ha at the normal water level (NWL=170) being the crest at elevation 172 m. At the maximum flood water level (MWL=171), the reservoir capacity rises to 110 Mm³ and its surface area increases to 436 ha. Construction began in April 2011, and the experimental and commercial operation of the powerhouse started in March 2017 and April 2018, respectively. The first filling of the reservoir took place between June 2016 and June 2017. After a brief presentation of the project, the monitoring system is introduced and the evolution of the reservoir level during the first filling, including the indication of the filling stages, is described. The evolution of the displacements and of the hydraulic behaviour of the foundation since the beginning of the filling is presented, noticing the perceived agreement between observations in redundant systems, as well as the good fitness of a nonlinear finite element model. Additionally, the paper presents the safety conditions evaluation of the structure during the first years of operation, regarding the main effects of the temperature variations and the hydrostatic pressure.