Engineering Wonders of the World
Volume III

GREAT BRITISH DAMS AND AQUEDUCTS. 191 higher ground. In this connection we may note that Lake Vyrnwy covers the site of the village of Llanwddyn, with its .church, school, three chapels, and forty cottages. The aqueduct consists of 23 j miles of cut- and-cover in nineteen sections, 12| miles of tunnel, and about 36| miles of syphon. Tun- The Aqueduct. nel.S and cut-.and-cover con- duits have a normal internal section 8 feet high, 98 inches wide at the springing of the arch, and 86 inches wide at the invert. They are lined throughout with blue brick, and are able to pass a maximum of 75,000,000 gallons a day. The syphons will ultimately have six lines of 42-incli pipes ; at present only two lines have been laid. The total fall on the 73 miles is 169 feet, or about 21 feet to the mile. The gradient of the syphons ranges between 1 in 1,760 feet and 1 in 1,570 feet. To some readers it may be a matter for surprise that so very gentle a slope should suffice for an aqueduct which has to deliver huge quantities of water through comparatively restricted channels. On leaving the lake the water passes through the Foel tunnel to the Flan filter beds, where it is strained and purified. The next 15 miles are in cut-and-cover, interrupted by four short syphons and two short tunnels. Near Dolau it enters a tunnel 4 J miles long. Emerg- ing from this, it traverses 2 miles of conduit, a short syphon, and .the 2|-mile Knighton tunnel. A mile of conduit is succeeded by the Downton syphon, 9| miles long, which at two intermediate points rises to the hydraulic gradient, and twice crosses the river Teme. The next 4 miles are mostly in cut-and-cover. Then comes the big Teme syphon, miles long, with a greatest hydraulic head of 444 feet, and a series of short conduit and tunnel sections leading to the Severn Syphon, which covers 17£ miles. At the point where they cross the river Severn, over a fine arch bridge of 150 feet span, the pipes are subjected to a hydraulic head of 540 feet, the greatest on any British aqueduct. From the Birmingham end of the syphon the water is led to the Frankley reservoirs and filter beds through 5| miles of conduit syphon and tunnel. The receiving reservoir is semicircular in plan, has an area of 25 acres, and holds 200,000,000 gallons. The Derwent Valley waterworks are of par- ticular interest, as the first great scheme for affording a supply to a combination of large towns. The cities of Leices- ter, Derby, Sheffield, and Not- Derwent tingham all wanted water from Val,ey x i. j r n Waterworks. the watershed of the Derwent. In the autumn of 1898 the first three de- posited separate plans, and applied for Parliamentary powers to carry them out. Nottingham, and the counties of Nottingham and Derby, also petitioned for a share of the water. The Parliamentary Committee ap- pointed to investigate the matter decided that all the parties concerned should unite to carry out works to obtain a supply divis- ible among the claimants in certain propor- tions ; and powers were granted for creating six reservoirs in three instalments in the valley of the Derwent. The first instalment, the Howden and Derwent reservoirs, was taken in hand in 1900. As a preliminary to building the dams, a railway seven miles long was con- structed through difficult country from Bam- ford, on the Midland Railway, to the site of the Howden dam, where a village was built to accommodate the workmen and their children. The Derwent dam has a length of 1,110 feet at the water-line, rises 114 feet above the bed of the stream, and is 169 feet thick at the widest part of the foundations. The masonry of the dam measures 360,000 cubic yards, A Huge and is computed to weigh Dam. 630,000 tons. As the rock leaked at the level of the foundations, the engineers had a trench 6 feet wide cut down into the rock and filled with masonry to form an imperme-