Use of Aqueducts in the Manchester Waterworks, England
The works by which the city of Manchester and its suburbs are now supplied with water, and which have been in course of construction from the year 1848 to the presents time (1874), are perhaps, in some respects, the most stupendous works of the kind which have ever been constructed, in which difficulties of no ordinary character have been successfully overcome. These remarks, however, relate especially to the impounding reservoirs, which are seven in number, with embankments varying from 70 to 100 feet above the level of the valley in which they are constructed, and cannot, therefore, be properly alluded to here. As the conveyance of the water to the city is, however, by aqueduct, a few words explanatory of the general scheme will be necessary.
The water is collected from the river Etherow and its tributaries, which, rising on the westerly slope of the Pennine chain of hills, flow into the river Mersey, and so into the Irish Sea. The drainage ground from which the water is collected lies nearly midway between Manchester and Sheffield. Its area is about 19,300 acres. It rises in parts to an elevation of about 1800 feet above the level of the sea, and about 1200 or 1300 feet above the deep and romantic valley of Longdendale, in which the main collecting reservoirs are situated. The district consists of the shales and sandstones which constitute the lower portion of the Coal Measure formation-the upper millstone grit forming the cap of the steep escarpments on each side of the valley, while the lower millstone, grit, which may be said to separate the Coal Measure shale from the limestone shale, is found in the bottom. The waters produced by this geological formation are among the purest in the world. The spring water is especially brilliant, highly aerated, containing little or no foreign matter, and extremely soft. It is at all times very abundant, the district yielding much more spring water than the usual quantity, in proportion to the area from which the springs issue. The quantity of water flowing from the drainage ground would, if wholly stored, afford a gross supply of about 40,000,000 gallons per day, of which 13,000,000 gallons per day have to be delivered as compensation to the mills on the stream, leaving about 27,000,000 gallons per day as the supply available for the city and its suburbs.
The water of heavy rains and wet season is collected in large impounding reservoirs, the gross capacity of which is 4,233,000,000 gallons. In some of these reservoirs the turbid and coloured water is impounded, where it is allowed to settle and purify, and is subsequently given as compensation to the stream; and in others, the pure water, when more than sufficient for the wants of the city, is collected. Here it is stored till required, and then given in addition to the spring water, when that is in itself deficient in quantity. The spring water is separately collected, and conveyed to the city by aqueducts specially constructed for the purpose. In heavy rains, which swell the streams, and especially in autumn, the water is discoloured, but by a simple and ingenious contrivance, every stream is made to separate its coloured water from its pure water-the coloured water being passed to reservoirs set apart for the storage of such water, and the pure water being sent at once to Manchester, or passed to reservoirs in which it is stored for future use.
The aqueducts by which this water is conveyed from the springs and from the reservoirs, consists for the most part of tunnel and covered conduit, 6 feet in diameter, with a fall of 5 feet in a mile, with cast-iron siphon pipes of large dimensions across one valley, which has to be passed before the highest service reservoir is reached. From this reservoir to the city, about 8 miles distant, cast-iron pipes are laid along or under the public roads, to convey the water to various other reservoirs at lower elevations, from which the city and its suburbs are conveniently supplied.
In the main valleys in which the spring water is collected or the flood and turbid waters conveyed by separate channels, the aqueducts are chiefly open, and are, to a great extent, formed of concrete 6 inches thick on the sides and bottom, faced with dry stone pitching 9 inches in thickness. They are cheap, easily constructed, and perfectly successful.
The area and capacities of all the reservoirs are as follows: