Garreg-Ddu Viaduct, in the Elan Valley, is actually a submerged dam controlling the water supply to Foel Tower which in turn supplies water to Frankley reservoir near Birmingham via a 73 mile long gravity fed aqueduct.

Garreg-Ddu Viaduct, in the Elan Valley, is actually a submerged dam controlling the water supply to Foel Tower which in turn supplies water to Frankley reservoir near Birmingham via a 73 mile long gravity fed aqueduct.