Sewerage and Sewage Treatment

by Harold E. Babbitt

TABLE 20
Computations for Slope and Diameter of Pipes for a Separate Sewerage System
On Street	From Street	To Street	From Manhole	To Manhole	Length Feet	El. of Surface		Total Flow, C.F.S.	Slope	Dia. of Pipe, Inches	Velocity when Full, Ft. per Second	Capacity when Full, Second-Feet	El. of Invert		Line Number
						Upper Manhole	Lower Manhole						Upper Manhole	Lower Manhole
Nebraska St.	Map margin	Alley S. Grant St.	3.5	3.4	338	105.8	102.4	1.66	0.0108	10	3.25	1.78	97.80	94.40	1
Alley S. of Grant St.	Railroad	E. of Missouri St.	8.3	8.2	328	113.5	112.0	0.046	.00575	8	2.00	0.71	105.50	103.62	2
Alley S. of Grant St.	E. of Missouri St.	E. of Kansas St.	8.2	8.1	355	112.0	107.7	0.105	.0110	8	2.78	0.98	103.61	99.70	3
Alley S. of Grant St.	E. of Kansas St.	Nebraska St.	8.1	3.4	340	107.7	102.4	0.152	.0156	8	3.27	1.18	99.69	94.40	4
Nebraska St.	Alley S. of Grant St.	Alley S. of Meridian	3.4	3.3	380	102.4	100.7	1.79	.00385	12	2.28	1.79	94.07	92.61	5
Alley S. of Meridian	Railroad	Kansas St.	7.2	7.1	400	111.8	107.0		.0120	8	2.90	1.03	103.80	99.00	6
Alley S. of Meridian	Kansas St.	Nebraska St.	7.1	3.3	400	107.0	100.7	0.123	.0157	8	3.28	1.18	98.99	92.70	7
Nebraska St.	Alley S. of Meridian	Alley S. of Smith Av.	3.3	3.2	304	100.7	99.3	1.85	.0042	12	2.36	1.85	92.37	91.09	8
Alley S. of Smith Ave.	Railroad	East of Kansas St.	6.2	6.1	305	109.3	105.3		.0131	8	3.00	1.08	101.30	97.30	9
Alley S. of Smith Ave.	East of Kansas St.	Nebraska St.	6.1	3.2	304	105.3	99.3	0.069	.0197	8	3.70	1.32	97.29	91.30	10
Nebraska St.	Alley S. of Smith Ave.	S. of Cordovez St.	3.2	3.1	300	99.3	101.1	1.92	.00213	15	2.00	2.45	90.84	90.20	11
S. of Cordovez St.	Railroad	Nebraska St.	4.1	3.1	410	100.8	101.1		.00574	8	2.00	0.71	92.80	90.62	12
S. of Cordovez St.	Map margin	Nebraska St.	5.1	3.1	380	104.6	101.1	0.054	.00854	8	2.46	0.87	96.60	93.10	13
Nebraska St.	S. of Cordovez St.	Long St.	3.1	148	172	101.1	98.7	1.99	.00213	15	2.00	2.45	90.04	89.87	14
Long St.	Map margin	Nebraska St.	149	148	380	103.8	98.7	0.030	.0134	8	3.04	1.08	95.80	90.70	15
Long St.	Nebraska St.	N. Carolina St.	148	147	492	98.7	103.8	2.02	.00213	15	2.00	2.45	89.86	88.94	16
Long St.	N. Carolina St.	Georgia St.	147	146	430	103.8	99.1	3.03	.0016	18	2.00	3.50	88.69	88.00	17
Long St.	Georgia St.	Harris St.	146	145	419	99.1	96.9	3.04	.0016	18	2.00	3.50	87.99	87.32	18
Alley S. of Janis St.	End of Janis St.	Harris St.	2.2	2.1	350	105.2	98.1		.0203	8	3.78	1.35	97.20	90.10	19
Harris St.	Alley N. of Janis St.	Long St.	2.1	145	135	98.1	96.9		.0088	8	2.53	0.89	90.09	88.90	20
Long St.	Harris St.	Kentucky St.	145	144	258	96.9	94.4		.00353	18	2.98	5.20	87.31	86.40	21
Long St.	Kentucky St.	Tennessee St.	144	143	282	94.4	93.6		.00635	18	4.00	7.00	86.39	84.60	22
Tarbell Ave.	Harris St.	Long St.	1.1	143	417	98.7	92.6		.0146	8	3.18	1.14	90.70	84.60	23
Long St.	Tennessee St.	Alley W. of Tenn. St.	143	142	185	92.6	92.3	3.13	.0016	18	2.00	3.50	83.77	83.47	24
Column No. (1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)	(11)	(12)	(13)	(14)	(15)

47. Surface Profile.—A profile of the surface of the ground along the proposed lines of the sewers should be drawn after the completion of the computations for quantity. An example of a profile is shown in Fig. 26 for the line between manholes No. 3.5 and No. 147. The vertical scale should be at least 10 times the horizontal. A horizontal scale of 1 inch to 200 feet can be used where not much detail is to be shown, but a scale of one 1 to 100 feet is more common and more satisfactory and even one inch to 10 feet has been used. The information to be given and the method of showing it are illustrated on Fig. 26. The profile should show the character of the material to be passed through and the location of underground obstacles which may be encountered. The method of obtaining this information is taken up in Chapter II. The collection of the information should be completed as far as possible previous to design, and borings and other investigations made as soon as the tentative routes for the sewers have been selected.

48. Slope and Diameter of Sewers.—After the quantity of sewage to be carried has been determined, and the profile of the ground surface has been drawn, it is possible to determine the slope and diameter of the sewer. A table such as No. 20 is made up somewhat similar to No. 19, or which may be an extension of Table 19 since the first 6 columns in both tables are the same. The elevation of the surface at the upper and lower manholes is read from the profile.

The depth of the sewer below the ground surface is first determined. Sewers should be sufficiently deep to drain cellars of ordinary depth. In residential districts cellars are seldom more than 5 feet below the ground surface. To this depth must be added the drop necessary for the grade of the house sewer. Six-inch pipe laid on a minimum grade of 1.67 per cent is a common size and slope restriction for house drains or sewers. An additional 12 inches should be allowed for the bends in the pipe and the depth of the pipe under the cellar floor. Where the elevation of the street and lots is about the same, and the street is not over 80 feet in width between property lines, a minimum depth of 8 feet to the invert of sewers, 24 inches or less in diameter is satisfactory. This is on the assumption that the axes of the house drain and the sewer intersect. For larger pipes the depth should be increased so that when the street sewer is flowing full, sewage will not back up into the cellars or for any great distance into the tributary pipes.

Fig. 26.—Typical Profile Used in the Design of a Separate Sewer System.

The grade or slope at which a sewer shall be may be fixed by: the slope of the ground surface; the minimum permissible self-cleansing velocity; a combination of diameter, velocity, and quantity; or the maximum permissible velocity of flow. Sewers are laid either parallel to the ground surface where the slope is sufficient or where possible without coming too near the surface they are laid on a flatter grade to avoid unnecessary excavation. The minimum permissible slope is fixed by the minimum permissible velocity.

The velocity of flow in a sewer should be sufficient to prevent the sedimentation of sludge and light mineral matter. Such a velocity is in the neighborhood of 1 foot per second. Since sewers seldom flow full this velocity should be available under ordinary conditions of dry weather flow. The minimum velocity when full should therefore be about 2 feet per second. Under this condition, the velocity of 1 foot per second is not reached until the sewer is less than 18 per cent full. The velocity in small sewers should be made somewhat faster than in large sewers since the velocity of flow for small depths in small pipes is less than for the same proportionate depth in large pipes. The maximum permissible velocity of flow is fixed at about 10 feet per second in order to avoid excessive erosion of the invert. If the sewer is carefully laid this limit may be exceeded in sanitary sewers.

The method for determining the grade and diameter of sewers is best explained through an illustrative problem which is worked out in Table 20 for the profile shown on Fig. 26. The figures are inserted in the table from left to right in each line, one line being completed before the next one is commenced. The headings in the first 6 columns are self-explanatory. The elevations of the surface at the upper and lower manholes are read from the profile. The total flow is read from column (18) in Table 19. The slope of the ground surface is then computed, and with the quantity, slope, and coefficient of roughness, the diameter of the pipe and the velocity of flow are read from Fig. 15.

The following conditions may arise: