The Water-works of Manchester, Eng.
About ten years ago the corporation of Manchester, England, realizing the need of obtaining a larger supply of water, to meet the steadily growing demands of the city, set upon foot preliminary arrangements for the construction of works for the utilization of the pure water of Thirlmere, a lake situated nearly 100 miles distant, in Cumberland. This work will, it is expected, be completed within a little over a year, and none too soon, for the consumption of water in the city already uearly equals the supply.
An interesting summary of the history of the Manchester water supply system is given by an Kuglish contemporary.
In the earlier decades of the century the city depended for its water on two comparatively small reservoirs at Gorton, almut 245 feet almve datum level, and but four miles from the centre of the city, and wten there was a deficiency, as frequently happened, it was made up from the Stockport canal, the waters being much purer than they are to-day in any of the canals. In 1846 the corporation bought out the company which had constructed the earlier works, being amongst the first to take the water supply over. Liverpool and Glasgow followed at short intervals. The services of Mr. Bateman were requisitioned by the corporation with the view of having the supply augmented, and the result was that he laid out and superintended the construction of the works by which Manchester is at present supplied. The drainage area, extending to about 19.300 acres, is in Longdendale, on the River Kthcrow, one of the large tributaries of the River Mersey, and on the river a series of reseivoirs, at various heights, were constructed. The largest is Torside, eighteen miles from Manchester, having an area of 160 acres, a depth of eighty four feet and a capacity of 1,474,000,000 gallons. Woodhead is next in size, having a capacity of 1,181,000,000 gallons. The total area of the reservoirs is 854^ acres, and the capacity is 5,985.000.000 gallons. The quantity obtainable for the supply of the city is about 25 000,000 gallons per day, exclusive of compensation water to the river to the extent of about 13,500,000 gallons gallons per day. The works have been carried out step by step during the past forty-five years, and when first designed the provision made for the future appeared to be ample. Forty years ago the average consumption was about 8,000,000 gallons per day ; it is now 23,000,000 gallons per day, and great care and attention are given to prevent waste. Manchester has been growing steadily, and the suburbs, which now cover a large area of country, have been extending, liy 1870 the population had increased eight per cent in the decade, and the water consumption had gone up nearly fifty per cent. It is worth mentioning here, too, that the annual revenue of the corporation for water supply in 1855, when the extended works were put in operation, was only about £50,000, but by 1870 it had increased to ,£131,000, the increase in ten years being ninety-two per cent. These facts, and particularly the steady growth in the consumption, awakened the water committee to the necessity of considering the future. Not only was the population increasing with a corresponding increase in the consumption, but the water supplied was being put to varied uses. Calico printers for instance, finding that the streams around were polluted, utilize it in their operations.
The consumption for domestic and trade purposes within the area of supply of the corporation ultimately approached so rapidly towards the safe quantity of water to be relied upon that in view of the time which must necessarily be occupied in obtaining fresh sources of supply and in the construction of the works, the corporation about the year 1877 took the whole question into consideration, and determined on the advice of Mr. Bateman to ask the sanction of Parliament to appropriate Thirlmere and the drainage area adjoining to it as the source from which the city might obtain further supplies of water when the existing works in Longdendale were insufficient.
In choosing Thirlmere the corporation adopted a commendable policy. To have taken a less copious source of supply would have only postponed for a season further operations, whereas in going to the Cumberland lake named the necessities of Manchester for many years to come can be met. A substantial augumentation was required, for now the average consumption is about 23,000,000 gallons per day ; some days it is as great as 28,500,000 gallons. In twenty years it may almost be said that the consumption and water revenue have doubled, although the population of the supply district has only increased tweuty per cent. Should the growth in the next twenty years be as great, then Manchester may require to add another line of piping in the valleys along the line of Thirlmere aqueduct, as for the present the aqueduct is only to pass about one-fifth the quantity available, and only one of five pipes has been laid across these valleys.
The scheme for adopting Thirlmere as a source of supply was presented to Parliament with great completeness of detail, and after considerable opposition the act of Parliament was obtained in the year 1879. Shortly afterwards arrangements were made with the landowners. When the corporation decided to proceed with the work, it appointed G. H. Hill as the sole engineer. The works were commenced in January, 1886. They consist, generally speaking, of the adaptation of Thirlmere as a reservoir, including the construction of dams at the outlet of the lake, discharging arrangements and other works; of an aqueduct 95 miles 1364 yards long, including long lengths of tunnel, cut and cover, and piping ; of wells and valves for regulating the flow of water; of numerous bridges to carry the aqueduct over streams and valleys, those over the rivers Lune, Kibble and Irwell being of an important character ; and finally of a reservoir at Prestwich, four and a half miles north of Manchester, from which the water will be passed to the city.
Thirlmere is situated in Cumberland about five miles south of Keswick. The nearest railway station is Threlkeld. The lake in its natural state is two and three-quarter miles in length, and as the plan shows, varies considerably in width. At one point it is narrowed by promontories on both sides ; but ultimately artifice will have made it more uniform, as the scheme includes the raising of the water level, and these promontories wili be covered. The coach road is on the east side of the lake, but as the raising of the lake will submerge a length of this road, a diversion has been constructed. There is no road at present, along which carriages can pass, on the west side of the lake, but the corporation is constructing oneThe watershed of this lake is situated on the lower Silurian formation, consisting of volcanic ash and breccia. The highest summit is that of Ilelvellyn, which is 3118 feet above the sea ; the level of the lake is 533 feet above the sea. The rainfall varies from about 137 inches on the west of the valley to about 90 inches, or 100 inches in the valley in wet years ; and in dry years from 80 inches to 60 inches in the same places. On two days recently the fall is stated to have been 3.53 inches and 3.43 inches. In the district of Seathwaite the rainfall one day last year was over 5 inches, and at Grasmere it was4.i5 inches, both these places being near the drainage area. The rain which falls on the mountains surrounding the valley, and included in the drainage area appropriated by the corporation, does not all flow into the lake. The natural drainage area to the lake is 7400 acres, but a supplemental area of 3600 acres is included in the scheme, nearly the whole of which forms part of the main valley, and is only separated from the lake by a low ridge. The hills are so steep and the rock so near or actually on the surface that the evaporation and absorption is exceedingly small, and the water is delivered into the lake in a state of very great purity. The same circumstances occasion enormous floods. The present area of the lake is 328 acres, but the act provides that the water in it may not at any time be reduced below its present level. It is therefore intended to obtain the necessary storage by raising the level fifty-one feet. The area of the lake will be increased to 793 acres, and an available capacity of 8,130,686,193 gallons will be created. This capacity is equal to a rainfall of about thirty-two and three-quarter inches over the drainage area, and to this extent is available to supplement the rainfall of dry years. After giving the statutory compensation to the river which flows down St. John’s Vale and through Keswick, the supply available for the city has been assumed at 50,000,000 gallons per day. This supply will be in addition to the 25,000,000 gallons per day from present sources.
The lake lies approximately north and south, the natural flow being towards the north. The valley at the north has for boundaries two large hills, the one named Raven’s Crag, which the ravens deserted after operations were commenced, and the other Great How. Between these two there is a small hill of exceedingly hard and close rock, the summit of which is about the new top water level of the lake, and this divides the valley into two. The natural outlet of the lake, St. John’s Beck, is by the eastern half of the valley. The western half is about fifteen feet higher. A dam is in course of construction to confine the waters of the lake at the northern end. This dam, viewed in plan, shows a reverse curve for the half of its total length. This has not been adopted to give greater resistance to the water ; but by adopting this line the dam has been made to follow the ridge of the rock across the centre of the valley, and thus a large amount of work has been saved. Through the rock in the small hill a tunnel has been driven to discharge the water of the lake, and a pipe is laid under the floo: to carry off a regulation quantity of water to St. John’s Beck. This tunnel was constructed before the work of erecting the dam was commenced, and was used as a bye-wash. West of the embankment an overflow swallow has been formed at the top water level with a tunnel through the rock to a waste watercourse by which the water flows to St. John’s Beck, entering it at the gauge basin. Our illustrations give comprehensive views of the works.