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Latitude: 51.7683 / 51°46'6"N
Longitude: -3.78 / 3°46'47"W
OS Eastings: 277270
OS Northings: 209240
OS Grid: SN772092
Mapcode National: GBR Y5.ZT6J
Mapcode Global: VH5G2.FTJ8
Plus Code: 9C3RQ69C+82
Entry Name: Canal Aqueduct over Afon Twrch, including weir
Listing Date: 4 December 2001
Last Amended: 20 December 2002
Grade: II*
Source: Cadw
Source ID: 25953
Building Class: Transport
ID on this website: 300025953
Location: Situated some 50m N of Twrch Bridge between Ystradgynlais community and Ystalyfera community, reached by footpath along river from Gurnos Road.
County: Powys
Town: Swansea
Community: Ystradgynlais
Community: Ystradgynlais
Locality: Ynyscedwyn
Built-Up Area: Ystradgynlais
Traditional County: Glamorgan
Canal aqueduct carrying former Swansea Canal over the Afon Twrch, 1794-8 by Thomas Sheasby, engineer, the largest aqueduct on the canal. This was the first aqueduct to use hydraulic lime for waterproofing. The aqueduct stands on a 4m high stone-faced weir which diverted water into a canal feeder. The 16-mile Swansea Canal was opened in 1798 to Hen Neuadd, Abercraf, declined after the railway opened in 1852 and closed in 1931. The course of the canal around Ystradgynlais is substantially lost under the bypass. The aqueduct was restored 1993-4.
Canal aqueduct of 21 m span, 9 m broad, squared rubble stone with 3 broad low arches, each of 7m span, 2.7 m height, the arch soffits and voussoirs rendered over. String course under deep walling to canal sides and later C19 rock-faced stone coping. Downstream side has 2 small triangular cutwaters carried up as triangular piers with 2 offsets. Upstream side has been reinforced in C19 with 2 big stepped rounded stone piers covering earlier cutwaters. This side has a concrete spillway in centre, and parapet top courses in red brick. Under E abutment is circular tunnel for the tailrace of a former mill, with stone voussoirs to entries.
Canal channel 3.35 m wide and 1.25 m deep, has tow-path on S side, canal bed concreted in restoration, canal sides rubble stone, probably rebuilt in 1993-4. There is a cast-iron mounting for a small emptying sluice on the N side and the broad concrete overflow to centre has slots for sluice gates each side.
Included as the principal surviving engineered structure of the Swansea Canal.
Scheduled Ancient Monument GM 396(NEP).
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