Human or Alien Work?

Supposedly made by man in 1955 inside the waters of the "Naves riverside" located in the Serra da Estrela (Continental Portugal biggest moutain), this is a ring type structure with 4,6 feet tall meters , being the ring 48 meters with the purpose of receive the water and carry it by a 1519m tunnel to the "long lagoon" (a dam begin to be built in 1912 showed below) , another structure made by man(?) also located in the same Serra da Estrela.