Ancient Monuments

History on the Ground

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Nant-y-Gro Dam

A Scheduled Monument in Rhayader (Rhaeadr Gwy), Powys

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Latitude: 52.2589 / 52°15'31"N

Longitude: -3.5809 / 3°34'51"W

OS Eastings: 292191

OS Northings: 263479

OS Grid: SN921634

Mapcode National: GBR 9F.ZWHR

Mapcode Global: VH5CV.WH86

Entry Name: Nant-y-Gro Dam

Scheduled Date: 25 June 2008

Source: Cadw

Source ID: 1247

Cadw Legacy ID: BR387

Schedule Class: Water Supply and Drainage

Category: Dam

Period: Post Medieval/Modern

County: Powys

Community: Rhayader (Rhaeadr Gwy)

Traditional County: Brecknockshire


The monument comprises the remains of a small gravity dam, built during the construction of the Elan valley dams and reservoirs between 1893 and 1904. It is located on a moderate NW-facing slope across the Nant-y-Gro valley. The concrete and masonry dam is aligned NE-SW and measured 58m in length, 3.1m in width at its base and 11m in height. The dam impounded c. one million gallons of water used in the workmen's village below. Following its abandonment, it was used on 24th July 1942 as the location for Barnes Wallis' early and highly secretive tests for the development of a means to attack the German dams in the Ruhr valley. The successful detonation of 280lbs of explosive against the masonry wall of the dam removed a central section of dam wall measuring 19m in width and 8m in height. The NE and SW ends of the dam stand to full height. The debris field of broken dam wall extends for 20-30m downstream.

The monument is of national importance for its potential to enhance our knowledge of the secretive scientific endeavour undertaken during World War II and the strategic consequences that might follow. Following the successful proof of concept at Nant-y-Gro, Wallis went on to develop Upkeep, the bouncing bomb used in Operation Chastise by No. 617 Squadron to destroy two dams and damage four others on the night of 16th - 17th May 1943. The raid was not as effective as intended, but did have an adverse effect on German food production and morale. Strategically, the raid boosted British morale and, by keeping German resources away from the Soviet Union, helped to persuade Stalin and Roosevelt that Britain could be an effective ally. The monument retains significant archaeological potential, with a strong probability of the presence of both structural evidence and deposits.

The scheduled area comprises the remains described and an area around them within which related evidence may be expected to survive. It is trapezoidal in shape and measures 63m ENE-WSW by 40m transversely.

Source: Cadw

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