The time of concentration in minutes is assumed by judgment for the first area. For all subsequent areas it is the sum of the time of concentration for the area or areas tributary to the inlet next above and the time of flow in the sewer from the inlet next above to the inlet in question. For example, in line 2 the time 8.1 minutes is the sum of 7.0 minutes time of concentration to the inlet at the corner of State and North Carolina St., and the time of flow of 1.1 minute in the sewer on State St. from North Carolina St. to South Carolina St. Where two sewers are converging as at the corner of Varennes Road and Tennessee St. the longest time is taken. For example, the time of concentration down Varennes Road to Tennessee St. is shown in line 25 as 11.4 + 0.6 = 12.0 minutes. The time to the same point down Tennessee St. is shown in line 21 as 16.2 + 0.3 = 16.5 minutes. This time is therefore used in line 26.
R, the rate of rainfall in inches per hour is determined by Talbot’s formula.
Q, is in cubic feet per second and is the product of the 8th and 10th columns. Since the 8th column is the sum of the products of the 5th and the 6th columns for the lines representing tributary areas, then the 11th column is the product of A, I, and R.
S, is the slope on which it is assumed that the sewer will be laid. It is usually assumed as parallel to the ground surface unless the velocity for this slope becomes less than 2 feet per second. In such a case the slope is taken as one which will cause this velocity.
V, the velocity in feet per second, is computed from diagrams for the solution of Kutter’s formula. The length in feet is scaled from the map as the distance between inlets or groups of inlets, and the time is the length in feet divided by the velocity in feet per minute.
Having computed the quantity of flow to be carried in the sewer, the design is completed by drawing the profile and computing the diameters and slopes by the same method as used in the design of separate sewers.
CHAPTER VI
APPURTENANCES
55. General.—The appurtenances to a sewerage system are those devices which, in addition to the pipes and conduits, are essential to or are of assistance in the operation of the system. Under this heading are included such structures and devices as: manholes, lampholes, flush-tanks, catch-basins, street inlets, regulators, siphons, junctions, outlets, grease traps, foundations and underdrains.
56. Manholes.—A manhole is an opening constructed in a sewer, of sufficient size to permit a man to gain access to the sewer. Manholes are the most common appurtenances to sewerage systems and are used to permit inspection and the removal of obstructions from the pipes. The details of the Baltimore standard manholes are shown in Fig. 27 and a manhole on a large sewer in Omaha is shown in Fig. 28. The features of these designs which should be noted are the size of the opening and working space, and the strength of the structure. Manhole openings are seldom made less than 20 inches in diameter and openings 24 inches in diameter are preferable. A man can pass through any opening that he can get his hips through provided he can bend his knees and twist his shoulders immediately on passing the hole. For this reason the manhole should widen out rapidly immediately below the opening, as shown in Fig. 27 and 38.
The walls of the manhole may be built either of brick or of concrete. Brick is more commonly used, as the forms necessary for concrete make the work more expensive unless they can be used a number of times. The walls of the manhole should be at least 8 inches thick. Greater thicknesses are used in treacherous soils and for deep manholes, or to exclude moisture. A rough expression for the thickness of the walls of a brick manhole more than 12 feet deep in ordinary firm material is t = d
2 + 2, in which t is the thickness in inches and d is the depth in feet. The thickness of brick walls may be changed every 5 to 10 feet or so. Concrete walls may be built thinner than brick walls.