Garreg-Ddu Bridge is a Grade II* Listed viaduct built circa 1900 as part of the Elan Valley reservoir project.
When I first saw this bridge, I was puzzled by several aspects of its construction. It spans a wide reservoir, which seems odd, as it would surely have been cheaper to build a bridge at a narrower point. It is curved in plan, rather than running straight across the water. The width of the spans is irregular, with a series of much shorter spans at one end.
The stonework is similar to the Craig Goch Dam bridge, although there are fewer ornate details, and at least one odd difference: there are significant projecting stones visible in the piers which did not feature at Craig Goch.
The answer to the puzzle is that the bridge is supported on a hidden dam. The crest of the dam is submerged well below the surface of the reservoir, and would only normally be visible following a prolonged drought. The intention is to ensure that some water is available from the reservoir even when levels have dropped dramatically: further down this reservoir, the bed level is too low.
Garreg-ddu Bridge hence has much taller support piers than Craig Goch, which explains the need for projecting stonework to support arch centring during construction: at Craig Coch, the centring would have been supported directly off the body of the dam.
Coflein has some great photos of the bridge under construction, as well as one at low water level.