Powys Digital History Project

The Elan Valley dams
The unfinished dam

The location of
the Elan Valley dams
and reservoirs
is shown on the
sketch map.   
Dol-y-Mynach dam
The original 1890’s scheme for the sequence of dams and reservoirs in the Elan and Claerwen valleys included provision for three dams on the River Claerwen, which were intended to be constructed later when additional water supplies were needed for Birmingham. Part of the plans for these, drawn up by James Mansergh in 1892, are reproduced below.

Dams originally
planned for the
Claerwen Valley,

 The original proposals for the sequence of dams, shown in a cross-section of the Claerwen Valley.
Caban Coch dam in the Elan Valley is at bottom right.
The massive
masonry base of the
Dol-y-Mynach dam,
Claerwen Valley
The Caban Coch dam was to create a reservoir with a top water level which would be above the level of the foundations of the dam at Dol-y-Mynach, the lowest of the planned three on the River Claerwen. (The above plan shows this dam as the second one from the right).
It was thus necessary to build the base of the Dol-y-Mynach dam at the same time as the other dams in the adjacent valley of the River Elan.
The section drawings
on this page are from
"The Future Water
Supply of Birmingham"
by Thomas Barclay,
published in 1898.
Powys County Archives.

The unfinished dam, which was intended to rise to a height of 101 feet, and to be 938 feet long, gives an interesting insight into the construction methods used for the early dams. When exposed above the waters of Caban Coch reservoir it reveals huge blocks of stone weighing anything up to ten tons set in concrete which form the solid core of the huge structure, between the outer faces of dressed masonry. The unfinished dam is submerged when the reservoir is full.

There is a low tower just upstream of Dol-y-Mynach dam which marks the submerged entrance of the Dol-y-Mynach Tunnel. This feeds water from the Claerwen directly into the Careg-Ddu reservoir close to the Foel Tower, thus adding these supplies to those of the Elan above the submerged dam of Careg-Ddu.

The two other dams originally planned for locations higher up the Claerwen Valley were not even started, and were replaced at a much later date by the huge Claerwen dam, completed in 1952.

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