Thirlmere Aqueduct

The Thirlmere Aqueduct is a 95.9-mile-long (154.3-kilometre-long) pioneering section of water supply system in England, built by the Manchester Corporation Water Works between 1890 and 1925. Often incorrectly thought of as one of the longest tunnels in the world, the aqueduct's tunnel section is not continuous.

The aqueduct was built to carry approximately 55000000 impgal per day of water from Thirlmere Reservoir to Manchester. The construction of the reservoir and aqueduct was authorised by the Manchester Waterworks Act of Parliament. The first phase was completed in 1897 and, for the pipeline sections, subsequent phases were completed in 1925. The first water to arrive in Manchester from the Lake District was marked with an official ceremony on 13 October 1894.

The route of the reservoir passes through Lancashire and then enters Manchester through Salford. It has been mapped on OpenStreetMap here: https://www.openstreetmap.org/relation/7937446

History
In 1874 John Frederick Bateman advised Manchester Corporation that the increasing demand for water, then averaging 18000000 impgal per day, would soon exhaust the available supply from Longdendale. His first recommendation was to source water from Ullswater, but it was eventually decided to seek powers to acquire Thirlmere and build a dam there. In the face of local opposition the project received Royal Assent in 1879. Under this act Manchester was granted priority of right to 25 impgal per person per day.

Tunnel under Dunmail Raise Pass
This tunnelled section under Dunmail Raise was dug by two teams mining towards each other. The two tunnel sections joined within 20 cm of centre.

Thirlmere Dam
The dam at Thirlmere 54.5615°N, -3.068°W rises 64 ft above the old stream bed, and the reservoir when full has a surface area of 814 acre, and a holding capacity of 8235000000 impgal above the level to which water may be drawn (540 O.D.) The total dry-weather yield of Thirlmere Reservoir is reckoned at about 40500000 impgal per day, out of which compensation water in respect of the area now draining into the Lake 10120 acre, amounting to  4658000 impgal per day average, is sent down the St. John's Beck. Manchester Corporation has acquired the drainage area of 10800 acre (in addition to other lands).

Aqueduct technical data
The aqueduct is 95.9 miles long from Thirlmere reservoir to Heaton Park Reservoir 53.5421°N, -2.2614°W, Prestwich. Its most common form of construction is cut-and-cover, which consists of a "D" section concrete covered channel, approximately 7.1 ft wide and between 7.1 ft and 7.9 ft high. There are 37 mi of cut and cover, made up of concrete horseshoe-shaped sections 12 in thick. Typically, the conduit has 3 ft of cover and traverses the contours of hillsides.

It is the longest gravity-fed aqueduct in the country, with no pumps along its route. The water flows at a speed of 4 mph and takes just over a day to reach the city. The level of the aqueduct drops by approximately 20 inches per mile (30 cm/km) of its length.

Construction history
Sections of the route of the aqueduct have over time been modified for the construction of modern motorways. During the construction of the M6 and M61 connection a short section was diverted. A short section of the aqueduct near Worsley, Greater Manchester, was also re-routed in the late 1960s during the construction of the M62/M63/M602 motorway interchange.