For figuring rates of runoff to determine storm-sewer requirements, the so-called rational method may be used. It employs the formula

The runoff coefficient C indicates the degree of imperviousness of the land. It ranges from 0.6 to 0.9 for built-up areas and paved surfaces and from 0.30 to 0.50 for unpaved surfaces, depending on the surface slope. In storm-sewer design, however, it is necessary to know not only rate of runoff and total runoff, but also at
what point in time after the start of a storm the rate of runoff reaches its peak. It is this peak runoff for which pipe must be sized and sloped. (The conduit designed to handle the peak runoff is for conveyance of runoff volume only and should not be considered for storage.)

Determination of Sewer Size

Sanitary sewers or lines carrying exclusively industrial wastes from a building to disposal must be sized and sloped according to best hydraulic design. The problem is generally one of flow in a circular pipe. (C. V. Davis and K. E. Sorenson, Handbook of Applied Hydraulics, McGraw-Hill Publishing Company, New York.)
Gravity flow is to be desired, but pumping is sometimes required.
Pipe should be straight and of constant slope between access holes, and access holes should be used at each necessary change in direction, slope, elevation, or size of pipe. Access holes should be no farther apart than 200 ft for pipes 24 in and smaller, and 500 ft for pipes 30 in and larger.
The sewer from a building must be sized to carry out all the water carried in by supply mains or other means. Exceptions to this are the obvious cases where losses might be appreciable, such as an industrial building where considerable water is consumed in a process or evaporated to the atmosphere. But, in general, water out about equals water in, plus all the liquid and water-borne solid wastes produced in the building.
Another factor to consider in sizing a sewer is infiltration. Sewers, unlike water mains, often flow at less pressure than that exerted by groundwater around them.
Thus, they are more likely to take in groundwater than to leak out wastewater. An infiltration rate of 2000 to 200,000 gal /(day  mi) might be expected. It depends on diameter of pipe (which fixes length between joints), type of soil, groundwater pressure, and workmanship.
In an effort to keep infiltration down, sewer-construction contracts specify a maximum infiltration rate. Weir tests in a completed sewer can be used to check the contractors success in meeting the specification; but unless the sewer is large enough for workers to traverse, prevention of excessive infiltration is easier than correction. In addition to groundwater infiltration through sewer-pipe joints, the entry of surface runoff through access hole covers and thus into sewers is often a factor. Observers have gaged as much as 150 gal /min leaking into a covered access hole.

Size and slope of a sanitary sewer also must satisfy a requirement that velocity under full flow be kept to at least 2.5 ft / s to keep solids moving and preventing clogging. In general, no drain pipe should be less than 6 in in diameter; an 8-in minimum is safer.

Sewer-Pipe Materials

Vitrified clay, concrete, ductile iron, polyvinyl chloride (PVC), acrylonitrile butylene  styrene (ABS) composite pipe, or steel may be used for pipe to carry wastewater and industrial wastes. PVC or ABS are used for the smaller diameters. Steel or reinforced concrete are generally used for larger sizes (24 in).
Choice of pipe material depends on required strength to resist load or internal pressure; corrosion resistance, which is especially vital for pipe carrying certain industrial wastes; erosion resistance in sewers carrying coarse solids; roughness factor where flat slopes are desirable; and cost in place. Sewer piping installed on the discharge side of pumps should have a pressure rating well in excess of the pressure that will be experienced.
Reinforced concrete pipe must be made well enough or protected to withstand effects of damaging sewer gas (hydrogen sulfide) or industrial wastes. Ductile-iron sewer pipe is good under heavy loads, exposed as on bridges, in inverted siphons, or in lines under pressure. Steel is used chiefly for its strength or flexibility. Corrugated steel pipe with protective coatings is made especially for sewer use; its long lengths and light weight and ease of handling and jointing. Plastic pipes are used because of corrosion resistance, light weight, and low installation cost.

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