Sewerage and Sewage Treatment

by Harold E. Babbitt

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.