Engineering Wonders of the World
Volume III

GREAT BRITISH DAMS AND AQUEDUCTS. 179 tunnels at a cost to make them for cir- The modern engineer enjoys the immense advantages conferred by the employment of iron and steel pipes able to withstand very high pressures, and the ability Modern , , . J Aqueducts. t0 dnve 10“8 sufficiently low it worth while to substitute cuitous surface sections. He lays out his aqueduct on the shortest possible line between its ends consistent with economical construc- tion ; and it should be pointed out that shortness increases the steepness of the gradient, that steepness promotes velocity of flow, and that the faster water moves the smaller and cheaper is the pipe or channel which will convey a given quantity in a given time. According to the physical features of the country passed through, the most suitable of three methods of construction is selected. Where a hill is encountered and a detour is inadvisable, a tunnel is driven through it on the hydraulic gradient, and, where necessary, lined with cement or brick to prevent erosion of the rock and obstruction of the channel. On sections where the surface of the ground follows the hydraulic gradient closely, cut- and-cover becomes practicable. This form of construction consists of dig- ging a trench, building on the bottom an inverted arch (some- times a flat floor is used), raising the side walls upward from this, and covering over the channel thus formed with an arched roof, on which some of the material excavated is placed to restore the natural level of the surface. At intervals manholes are fixed to give access to the conduit. Through undulating country and across valleys pipes are used. An unbroken length of pipe with its ends on the hydraulic gradient and intermediate parts below the gradient, is known as an inverted syphon, or, more shortly, as a syphon. To prevent the pipes being subjected to an Three Methods of Construction. excessive “ head ” of water, open “ balancing reservoirs ” are, where necessary, and where physical conditions permit, built on the hydraulic gradient. Balancing Into each of these water is ^eservo*rs« discharged from the lower end of the syphon immediately above, to be passed into the upper end of the syphon immediately below. The reservoirs also serve for local supply service, and assist in the regulation of the flow through the aqueduct. The hydraulic gradient of both tunnel and syphon sections is in many cases made steeper than the general gradient, as these two classes of construction are more costly than the cut- and-cover or conduit, and because, as has already been pointed out, steepness allows reduction in the size of the channel. Tunnels and conduits are made full size in the first instance—that is, are given a cross section of sufficient area to pass the full supply for which the aqueduct is designed. In syphon sections the flow is distributed among a number of separate pipe lines, which are laid successively as the need for an increased supply arises. From these preliminary remarks we proceed to a description of some of the most notable British aqueducts.* The first chronologically is that which leads water to Glasgow from a series of lochs Glasgow —Katrine, Drunkie, and Ven- Mueducts. nachar. Across the mouths of the first and last of these lochs were built masonry dams ; the level of the second was raised by means of earthen embankments. From the lochs the water passes through an aqueduct 25f miles long, to the Mugdock reservoir, where it is strained for delivery to the city. Of its length, 13 miles consist of tunnels, driven mostly through sound hard rock ; 9 miles of cut-and-cover ; and 3| miles of syphon, made up of two lines of 48-inch pipes—one only * Lack of space prevents a description of the Dublin and Edinburgh aqueducts.