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41. The Plan.—Good practice demands that a comprehensive plan for a sewerage system be provided for the needs of a community for the entire extent of its probable future growth, and that sewers be constructed as needed in accordance with this plan.

Sewerage systems may be laid out on any one of three systems: separate, storm, or combined. A separate system of sewers is one in which only sanitary sewage or industrial wastes or both are allowed to flow. Storm sewers carry only surface drainage, exclusive of sanitary sewage. Combined sewers carry both sanitary and storm sewage. The use of a combined or a separate system of sewerage is a question of expediency. Portions of the same system may be either separate, combined, or storm sewers.

Some conditions favorable to the adoption of the separate system are where:

a. The sanitary sewage must be concentrated at one outlet, such as at a treatment plant, and other outlets are available for the storm drainage.

b. The topography is flat necessitating deep excavation and steeper grades for the larger combined sewers.

c. The sanitary sewers must be placed materially deeper than the necessary depth for the storm-water drains.

d. The sewers are to be laid in rock, necessitating more difficult excavation for the larger combined sewers.

e. An existing sewerage system can be used to convey the dry weather flow, but is not large enough for the storm sewage.

f. The city finances are such that the greater cost of the combined system cannot be met and sanitary drainage is imperative.

g. The district to be sewered is an old residential section where property values are not increasing and the assessment must be kept down.

Some additional points given in a report by Alvord and Burdick to the city of Billings, Montana, are:

The separate system of sewerage should be used, where:

1st. Storm water does not require extensive underground removal, or where it can be concentrated in a few shallow underground channels.

2nd. Drainage areas are short and steep facilitating rapid flow of water over street surfaces to the natural water courses.

3rd. The sanitary sewage must be pumped.

4th. Sewers are being built in advance of the city’s development to encourage its growth.

5th. The existing sewer is laid at grades unsuitable for sanitary sewage, it can be used as a storm sewer.

A combined system must be relatively larger than a separate storm sewer as the latter may overflow on exceptional occasions, but the former never.

A combined system of sewerage should be used where:

1st. It is evident that storm and sanitary sewerage must be provided soon.

2nd. Both sanitary and storm sewage must be pumped.

3rd. The district is densely built up.

42. Preliminary Map.—The first step in the design of a sewerage system is the preparation of a map of the district to be served within the limits of its probable growth. The map should be on a scale of at least 200 feet to the inch in the built up sections or other areas where it is anticipated that sewers may be built, and where much detail is to be shown a scale as large as 40 feet to the inch may have to be used. The adoption of so large a scale will usually necessitate the division of the city or sewer district into sections. A key map should be drawn to such a scale that the various sections represented by separate drawings can all be shown upon it. In preparing the enlarged portions of the map it is not necessary to include these portions of the city in which it is improbable that sewers will be constructed, such as parks and cemeteries.

The contour interval should depend on the character of the district and the slope of the land. In those sections drawn to a scale of 200 feet to the inch for slopes over 5 per cent, the contour interval need not be closer than 10 feet. For slopes between 1 and 5 per cent the contour interval should be 5 feet. For flatter slopes the interval should not exceed 2 feet, and a one foot interval is sometimes desirable. In general the horizontal distances between contours should not exceed 400 feet and they should be close enough to show important features of the natural drainage. Elevations should also be given at street intersections, and at abrupt changes in grade. For portions of the map on a smaller scale the contours need be sufficiently close to show only the drainage lines and the general slope of the land.

The following may be shown on the preliminary map: the elevation of lots and cellars; the character of the built up districts, whether cheap frame residences, flat-roof buildings, manufacturing plants, etc.; property lines; width of streets between property lines and between curb lines; the width and character of the sidewalks and pavements; street car and railroad tracks; existing underground structures such as sewers, water pipes, telephone conduits, etc.; the location of important structures which may have a bearing on the design of the sewers such as bridges, railroad tunnels, deep cuts, culverts, etc.; and the location of possible sewer outlets and the sites for sewage disposal plants.

Fig. 24 shows a preliminary map for a section of a city, on which the necessary information has been entered. The map is made from survey notes. All streets are paved with brick. The alleys are unpaved. The entire section is built up with high-class detached residences averaging one to each lot. The lots vary from 1 to 3 feet above the elevation of the street.

43. Layout of the Separate System.—Upon completion of the preliminary map a tentative plan of the system is laid out. The lines of the sewer pipe are drawn in pencil, usually along the center line of the street or alley in such a manner that a sewer will be provided within 50 feet or less of every lot. The location of the sewers should be such as to give the most desirable combination of low cost, short house connections, proper depth for cellar drainage, and avoidance of paved streets. Some dispute arises among engineers as to the advisability of placing pipes in alleys, although there is less opposition to so placing sewers than any other utility conduit. The principal advantage in placing sewers in alleys is to avoid disturbing the pavement of the street, but if both street and alley are paved it is usually more economical to place the sewer in the street as the house connections will be shorter. On boulevards and other wide streets such as Meridian Avenue in Fig. 24, the sewers are placed in the parking on each side of the street, rather than to disturb the pavement and lay long house connections to the center of the street.

All pipes should be made to slope, where possible, in the direction of the natural slope of the ground. The preliminary layout of the system is shown in Fig. 24. The lowest point in the portion of the system shown is in the alley between Alabama and Tennessee Streets. The flow in all pipes is towards this point, and only one pipe drains away from any junction, except that more than one pipe may drain from a terminal manhole on a summit.

44. Location and Numbering of Manholes.—Manholes are next located on the pipes of this tentative layout. Good practice calls for the location of a manhole at every change in direction, grade, elevation, or size of pipe, except in sewers 60 inches in diameter or larger. The manholes should not be more than 300 to 500 feet apart, and preferably as close as 200 to 300 feet. In sewers too small for a man to enter the distance is fixed by the length of sewer rods which can be worked successfully. In the larger sewers the distances are sometimes made greater but inadvisedly so, since quick means of escape should be provided for workmen from a sudden rise of water in the sewer, or the effect of an asphyxiating gas. In the preliminary layout the manholes are located at pipe intersections, changes in direction, and not over 300 to 500 feet apart on long straight runs at convenient points such as opposite street intersections where other sewers may enter.

No standard system of manhole numbering has been adopted. A system which avoids confusion and is subject to unlimited extension is to number the manholes consecutively upwards from the outlet, beginning a new series of numbers prefixed by some index number or letter for each branch or lateral. This system has been followed with the manholes on Fig. 24.

Fig. 24.—Typical Map Used in the Design of a Separate Sewer System.

Fig. 25.—Typical Map Used in the Design of a Storm Sewer System.

45. Drainage Areas.—The quantity of dry weather sewage is determined by the population rather than the topography. Lot lines and street intersections or other artificial lines marking the boundaries between districts are therefore taken as watershed lines for sanitary sewers. The quantity of sewage to be carried and the available slope are the determining factors in fixing the diameter of the sewer. Since there may be no change in diameter or slope between manholes the quantity of sewage delivered by a sewer into any manhole will determine the diameter of the sewer between it and the next manhole above. In order to determine the additional amount contributed between manholes a line is drawn around the drainage area tributary to each manhole. This line generally follows property lines and the center lines of streets or alleys, its position being such that it includes all the area draining into one manhole, and excludes all areas draining elsewhere. An entire lot is usually assumed to lie within the drainage area into which the building on the lot drains. In laying out these areas it is best to commence at the upper end of a lateral and work down to a junction. Then start again at the upper end of another lateral entering this junction, and continue thus until the map has been covered.

The areas are given the same numbers as the manholes into which they drain. The dividing lines for the drainage areas on Fig. 24 are shown as dot and dash lines, and the areas enclosed are appropriately numbered. If more than one sewer drains into the same manhole the area should be subdivided so that each subdivision encloses only the area contributing through one sewer. Such a condition is shown at manhole C2. The areas are designated by subletters or symbols corresponding to the symbol used for the sewer into which they drain. For example, the two areas contributing to manhole C2 are lettered C2_K and C2_D. The sewer from manhole C3 to C2 receives no addition, it being assumed that all the lots adjacent to it drain into the sewer on the alley. There is therefore no area C2. Likewise there is no area A1_C.

46. Quantity of Sewage.—The remaining work in the computation of the quantity of sewage is best kept in order by a tabulation. Table 19 shows the computations for the sewers discharging from the east into manhole No. 142. The computation should begin at the upper end of a lateral, continue to a junction, and then start again at the upper end of another lateral entering this junction. Each line in the table should be filled in completely from left to right before proceeding with the computations on the next line. In the illustrative solution in Table 19, computations for quantity have not been made between manholes where it was apparent that there would be an insufficient additional quantity to necessitate a change in the size of the pipe.

In making these computations the assumptions of quantity and other factors given below indicate the sort of assumptions which must be made, based on such studies as are given in Chapter III. The density of population was taken as 20 persons per acre, the assumption being based on the census and the character of the district. The average sanitary sewage flow was taken as 100 gallons per capita per day. The per cent which the maximum dry weather flow is of the average was taken as M = 500 P^⅕, in which P is the population in thousands. The per cent is not to exceed 500 nor to be less than 150. The rate of infiltration of ground water was assumed as 50,000 gallons per mile of pipe per day.

In the first line of Table 19, the entries in columns (1) to (6) are self-explanatory. There are no entries in columns (7) to (10), as no additional sewage is contributed between manholes 3.5 and 3.4. In column (11), 2250 persons are recorded as the number tributary to manhole No. 3.5 in the district to the north and west. These people contribute an average of 100 gallons per person per day, or a total of 0.346 second foot. This quantity is entered in column (13). The figure in column (14) is obtained from the expression M = 500 P^⅕. Column (15) is .01 of the product of columns (13) and (14). Column (16) is the product of the length of pipe between manholes 3.5 and 3.4, and the ground water unit reduced to cubic feet per second. Column (17) is the sum of column (16), and all of the ground water tributary to manhole 3.5, which is not recorded in the table. Column (18) is the sum of columns (15) and (17).

No new principle is represented in the second and third lines.

In the fourth line the first 10 columns need no further explanation. The (11th) column is the sum of the (10th) column, and the (11th) column in the third line. It represents the total number of persons tributary to manhole 3.4 on lateral No. 8. Column (13) in the fourth line is the sum of column (13) in the third line and the (12th) column in the fourth line, and the (15th) column in the fourth line is the product of the 2 preceding columns in the fourth line. Note that in no case is the figure in column (15) the sum of any previous figures in column (15). With this introduction the student should be able to check the remaining figures in the table, and should compute the quantity of sewage entering manhole No. 142 from the west, making reasonable assumptions for the tributary quantities from beyond the limits of the map.

TABLE 19
Computations for Quantity of Sewage For a Separate Sewerage System
On Street	From Street	To Street	From Manhole	To Manhole	Length Feet	Mark of Added Areas	Area, Acres	Population per Acre	Number of Persons	Total Persons Tributary	Avg. Sanitary Flow, C.F.S.	Cumulative Avg. Sanitary Flow, C.F.S.	Per cent Max. Sanitary is of Average	Total Max. Sanitary, C.F.S.	Increment of Ground Water, C.F.S.	Cumulative Ground Water, C.F.S.	Total Flow, C.F.S.	Line Number
Nebraska St.	Map margin	Alley S. Grant St.	3.5	3.4	338					2250	0.0000	0.346	425	1.47	0.005	0.0187	1.66	1
Alley S. of Grant St.	Railroad	E. of Missouri St.	8.3	8.2	328	8.2	2.7	20	54	54	.0084	.0084	500	0.041	.0048	.0048	0.046	2
Alley S. of Grant St.	E. of Missouri St.	E. of Kansas St.	8.2	8.1	355	8.1	3.41	20	68	122	.0106	.0190	500	0.095	.0052	.010	0.105	3
Alley S. of Grant St.	E. of Kansas St.	Nebraska St.	8.1	3.4	340	3.48	2.68	20	54	176	.0084	.0274	500	0.137	.0050	.015	0.152	4
Nebraska St.	Alley S. of Grant St.	Alley S. of Meridian	3.4	3.3	380					2428	.0000	.373	423	1.58	.0058	.208	1.79	5
						7.1
Alley S. of Meridian	Railroad	Nebraska St.	7.2	3.3	800	3.37	7.14	20	142	142	.0221	.0221	500	0.111	.0117	.0117	0.123	6
Nebraska St.	Alley S. of Meridian	Alley S. of Smith Av.	3.3	3.2	304					2568	.0000	.395	414	1.63	.0045	.224	1.85	7
						6.1
Alley S. of Smith Ave.	Railroad	Nebraska St.	6.2	3.2	609	3.26	3.82	20	76	76	.0119	.0119	500	0.060	.0089	.0089	0.069	8
Nebraska St.	Alley S. of Smith Ave.	S. of Cordovez St.	3.2	3.1	300					2644	.0000	.407	414	1.68	.0044	.237	1.92	9
S. of Cordovez St.	Railroad	Nebraska St.	4.1	3.1	410	3.14	3.10	20	62	62	.0096	.0096	500	0.048	.006	.006	0.054	10
S. of Cordovez St.	Map margin	Nebraska St.	5.1	3.1	380	3.15	2.69	20	54	54	.0084	.0084	500	0.042	.0056	.0056	0.048	11
Nebraska St.	S. of Cordovez St.	Long St.	3.1	148	172					2760	.0000	.425	409	1.74	.0025	.251	1.99	12
Long St.	Map margin	Nebraska St.	149	148	380	148	1.53	20	31	31	.0048	.0048	500	0.024	.0056	.0056	0.030	13
Long St.	Nebraska St.	N. Carolina St.	148	147	492					2791	.0000	.430	409	1.76	.0072	.264	2.02	14
Long St.	N. Carolina St.	Georgia St.	147	146	430					2791	1.000[33]	.430	409	1.76	.0064	1.27	3.03	15
Long St.	Georgia St.	Harris St.	146	145	419	146	0.81	20	16	2807	.0025	.433	407	1.76	.0061	1.28	3.04	16
						2.1
Long St.	Harris St.	Tennessee St.	145	143	725	143–145	6.6	20	132	2936	.0205	.454	403	1.83	.024	1.30	3.13	17
Column No. (1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)	(11)	(12)	(13)	(14)	(15)	(16)	(17)	(18)

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.