west of Edgworth, Lancashire, UK
date 1832 - 1838
era Victorian |
category Dam/Reservoir |
ICE reference number HEW 545
Entwistle Dam was the first in Britain to exceed 100ft (30.48m) in height. It was also the first to have an outlet tunnel driven through the rock side of the valley, as distinct from a cut-and-cover under-bank culvert or pipeline.
The dam was designed, and the construction directed, by Thomas Ashworth, a local land surveyor, though the specification was vetted by Jesse Hartley, the famous Liverpool Docks engineer. The finishes and remedial works were by Joseph Jackson, an engineer and surveyor of Bolton.
The dam was built for, and by, a group of local mill owners who formed the Commissioners of the Turton and Entwistle Reservoir, the reservoir name shown on maps. They obtained an enabling Act of Parliament in 1832. The idea was to regulate the supply of water in Bradshaw Brook for water power, principally for the finishing of textiles.
Written records suggest that it was built entirely of puddle clay with no distinct core. Earth dams usually have a waterproof cutoff under their earthworks — a protective layer designed to stop seepage under the dam. It's doubtful whether there was any significantly deep cutoff constructed at Entwistle. A siphon draw-off pipe was incorporated in addition to the novel rock-cut outlet tunnel.
The dam has a maximum height of 108 ft (33m) above the original stream bed, a crest length of 110m and side slopes of 1:4, steepening to 1:2.5.
The present-day overflow channel and 'baronial' valve tower were added later by the Bolton Corporation Water Works who took over the reservoir for public water supply in 1864.
Specification vetting: Jesse Hartley
Resident engineer: Thomas Ashworth
Finishes and remedial works: Joseph Jackson (Bolton)
"Reservoirs from Rivington to Rossendale" by N. Hoyle
North West Water Authority, Warrington, 1987