Engineering Wonders of the World
Volume III

GREAT BRITISH DAMS AND AQUEDUCTS. 183 number of 12-inch steel pipes furnished with flexible joints, and having valves at one point in the circumference. An 800-foot length of this piping was fitted together on sliding ways in a trench on the Lancashire side of the river. Both, ends were plugged to exclude water. When all was ready, steam winches on the Cheshire side, hauling on steel cables attached to the near end, drew the pipe off the ways and across the river. Within an hour of the start the plugs had been withdrawn, con- nections had been made with the pipe line at both ends, and water was flowing through the pipes. Then the Lancashire end was plugged to allow the water to issue at high pressure through the valves—the pipes had been so arranged that this should be at the lowest side—and scour a trench for the pipes in a bank of sand and silt at mid-stream. This ingenious method of trenching proved very successful. The area of Lake Vyrnwy is 1,121 acres. Tunnels now connect the reservoir with, the Marchnant and Cowny Rivers, forming gather- ing grounds of 27,000 acres extent. In 1892, almost exactly eleven years after the laying of the memorial stone on which is recorded the commencement of the works, the undertaking was declared open by the Duke of Connaught. Prior to the opening of the Thirlmere Aqueduct in 1894, Manchester depended en- tirely for its water on the supply—25,000,000 gallons a day—drawn from the river Etherow, at Longdendale, 18 miles east of the city. As early as 1875 it became evident that measures must be taken for tapping some The Thirlmere- Manchester Scheme. other source, in order to pre- vent the demand overtaking the supply. The Corporation decided to obtain water from Thirlmere, one of the Cumberland lakes, into which drains an area subject to a very high annual rainfall. The surface of the watershed being free from peat, the water that flows off is well suited for human use. An Act of I Parliament was obtained in 1879, authorizing the construction of a dam across the northern end of the lake to create a reservoir that should supply Manchester with a maximum of 50,000,000 gallons a day for 160 days without replenishment by rain, and the con- struction of an aqueduct able to pass this amount of water. The dam, which was begun in 1890, is 857 feet long at the top, and has a greatest height above the foundation of 104 feet 6 inches. At present it increases the depth of the lake by a maximum of 35 feet, but if raised to its full projected height, will add another 15 feet, and produce a storage capacity of 8,135,000,000 gallons. A small hill divides the dam into two portions. Through this hill was driven a tunnel for the discharge of surplus and com- pensation water. No water passes over the dam itself. It may be added that the area of the lake has been increased from 330 to 690 acres by the creation of the dam, and that, as a consequence of the rise of water level, an entirely new coach road has had to be built along the west bank of the lake, in addi- tion to a road along the crest of the dam to connect the two sides of the valley. The aqueduct is made up of 13 miles 1,517 yards of tunnel, 37 miles 120 yards of cut- and-cover—all for 50,000,000 gallons a day— and 45 miles of syphons. For the two syphons nearest the ^Iie ^hir^mere lake three lines of 48-inch Aqueduct, pipes are specified, and for the other syphons five lines of 40-incli piping, except in the part of the aqueduct south of Little Hulton, where the gradient is steeper, and 36-inch pipes are able to deal with the flow. Aqueduct pipes are generally of cast iron. Where exceptionally high pressures have to be borne—as at the lowest point of a deep syphon