Welsh Bridges: 8. Garreg-Ddu Bridge

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.

I still don't have an explanation for why some spans at one end are much shorter than the others. Comment if you have any idea.

Coflein has some great photos of the bridge under construction, as well as one at low water level.

Further information:

• Google maps
• Wikipedia
• British Listed Buildings
• Coflein
• Powys Digital History Project
• Elan Valley

Welsh Bridges: 7. Craig Goch Dam

 

Craig Goch Dam is the uppermost of a series of dams built in the Elan Valley from 1893 to 1904. Most of these dams are simple spillways, and this is the only one with an exposed spillway and a viaduct across the crest. Together, this Grade II* Listed dam and bridge are both spectacular and very beautiful.

The other Elan Valley dams are also well worth visiting, but in the absence of bridges I won't be featuring them here. Someone else will have to start their own blog. "Weir Weirdos", I suggest.

The Elan Valley dams and reservoirs were constructed under the direction of engineer James Mansergh on behalf of the Corporation of Birmingham, a city some 75 miles away. Water carried from the Elan reservoirs apparently takes about a day and a half to reach Birmingham, all by gravity flow.

The dam is some 156m long, 36m tall, and 32m thick at its base, a masonry giant surmounted by 13 segmental arch spans. Note that I've taken the 156m figure from an information board nearby - the plaque picture above gives a shorter length for the "length of weir".

It's a beautiful and very well-put-together structure. The viaduct piers merge smoothly into the curved profile of the dam spillway, and the contrast between the spillway's smooth stone facing and the viaduct's rough stone facing is effective (although I think this is the effect of many years of water flow, not an original feature).

The detailing of the viaduct is admirable in pretty much every respect, and doubly so on the lovely octagonal valve tower.

The viaduct is masonry faced on its elevations, but with a narrow band of brickwork forming the arch barrels in between. There are drain holes at every span in the roadway, and surface water drains out through the piers directly onto the dam spillway.

The view downstream from the dam is spectacular, and the road journey from here westwards through the beautiful hills to Cymystwyth is also recommended (I had come that way after visiting Llanilar Suspension Bridge).

The Coflein website has aerial photos, as well as some photographs of the dam under construction.

Further information:

• Google maps
• Wikipedia
• British Listed Buildings
• Coflein
• Powys Digital History Project
• Elan Valley