Powys Digital History Project

The submerged dam 2
Maintaining the flow

viaduct and
Foel Tower with
normal water
levels, 2000
(far right)

Careg-ddu viaduct
The small submerged dam at Careg-Ddu holds water back on the upstream side of the River Elan so that sufficient water can always be taken at the Foel Tower, which is the starting point of the delivery of water to Birmingham.Recent view of viaduct & tower
It is more usual for water to be drawn off from the base of the lowest dam in the series, but the height of Birmingham above sea level required a take off point at a higher level so that water could be delivered along the 73 mile aqueduct by natural gravity alone.
This required a starting point at least 169 feet (52m) above the holding reservoir at Frankley, Birmingham, and the outflow at the base of the lowest dam, Caban Coch, is only 120 feet (30m) higher. This determined the need for the submerged dam and the location alongside it of the valve tower at Foel. The relative heights of Caban Coch dam and the submerged dam can be seen clearly in the 1892 cross-section of the Elan Valley dams featured on another page.

submerged dam
revealed by
low water, 1947

County Archives

Low water showing dam,1947The photograph on the left, taken during very low water levels in November 1947, shows the small dam at the base of the pillars carrying the road viaduct which leads to the Claerwen valley.
The Foel Tower is visible beyond the viaduct. The submerged dam had played its intended role of maintaining the flow to Birmingham ten years earlier, in September 1937, after a similar period of exceptional drought.
The location of
the Elan Valley dams
and reservoirs
is shown on the
sketch map.   

Low water,1995The soundness of the design work on the Elan Valley waterworks scheme of over a hundred years ago has paid off several times since. The most recent occasion on which the submerged dam has been doing its job was in the summer of 1995, when the water levels in the Elan Valley were again to drop alarmingly. The photograph on the right shows the top of the submerged dam exposed above the water on this occasion. This view is looking towards the new Baptist church referred to on the previous page.

This is the second of two pages on the Careg-Ddu dam.

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