SEWER CONSTRUCTION. The underground channels provided by the local authority for removing liquid wastes from buildings are termed sewers, and are usually situated in the public thorough fares. The smaller sizes up to 24in. diam eter may be formed of glazed stoneware, but over this size it is more convenient to use brick or concrete. Pipes of artificial stone are also being used. The two com monest forms of cross-section for large sewers are circular and "eggshaped," the latter of the relative dimensions shown in fig. 1. The idea of the "eggshape" is to concentrate low flows and so retain the scouring action of the water. It is, however, much weaker against external pressure than the circular shape. When bricks are used they are made wedge shaped for the smaller sizes so as to lessen the thickness of the joints, and consist usually of at least two rings. The mortar should be made of one part of Portland cement to one part of clean sand. As the invert of eggshaped sewers may be of a sharp curve the "invert block" shown in fig. 2 is often provided. Such sewers are rarely made more than 7 feet in height. With very large sewers, such as those used for intercepting purposes or for out falls, reinforced concrete is corn ing into use. For these, semi elliptical, "horse-shoe," semi-cir cular and other forms of cross section are common.
Rainfall Run-off.—Where "combined" sewers are used the channel must be large enough to take the rainwater received as well as house wastes, even if the former liquid is soon afterwards overflowed to a stream. It is now customary to calculate the rain fall run-off from such a formula as :— Q=--6o.5 Apv where Q is run-off in cubic feet per minute, A is the area drained in acres, p is the coefficient of impermeability, dependent on the nature of the ground surface, and v is the rate of rainfall in inches per hour, and is obtained from a chart plotted from auto matic records if possible, and will be of higher value the shorter the storm. It is customary to consider that all combined dis
charge, diluted more than six times the normal dry weather flow, may be overflowed to a stream, but there is no rule, as the sewage may vary so much in quality. The duration of the storm is as sumed equal to the time of travel in the sewer from the highest point in the area to the point of discharge from the area A; p is frequently assumed as the ratio between the unbuilt-up and total areas. The flow of actual sewage is usually taken as equal to the water supply, but the rate at which it is received by the sewer will vary, being often double the mean at maximum periods. It is customary to calculate the maximum discharging capacity of sewers to be when they are running three-quarters or seven eighths full and hence never under pressure, and the formula for the mean velocity is of the type :— v=c Vmi. feet per second, where m is the "hydraulic mean depth" and i is the "hydraulic gradient," c is a constant dependent on the material of the sewer lining and on its condition. (See HYDRAULICS.) Storm-Overflows.—Two types of these are shown in figs. 3 and 4. In fig. 3 the water is intercepted above a predetermined height and turned into a parallel storm-water sewer. In fig. 4 the main sewer has a weir placed on one or both sides and the water above weir level falls into the storm-water sewer alongside. The latter type has the disadvantage of requiring a great length of sill, and even then a large amount of water escapes into the down stream portion of the main sewer.