Sewers should be laid in continuous straight lines, as far as possible.

No turn greater than a right angle should be made at any one point by any sewer less than 24 inches in diameter, and any turn whatever made by such a sewer should be in a manhole, by means of a curved channel. For sewers larger than 12 or 15 inches it is advisable to use two manholes in making a bend greater than 45 degrees (see Fig. 20). Brick or concrete sewers more than 30 inches in diameter may be laid on curves, since they can be entered for inspection or cleaning.

30"

42"

30"

Each lateral sewer should take the most direct course to its main, each main the most direct course to its outlet, and the number of This serves both economy

An exception to the last rule is made where this would cause velocities of more than 6 or 8 feet per second, as in draining a steep hillside. Here the main should follow a diagonal course down the hill to prevent the high velocities with their disadvantages.

Dead ends should be made as few as possible, even at some expense of additional excavation, but not by reducing mean velocities below 2.5 feet per second; nor is it ordinarily serviceable to unite the upper ends of sewers flowing in opposite directions.

Separate sewers should be carried within reach, as regards both horizontal distance and grade, of every lot in the sewered district.

Storm sewers should have as few branches as can be made

to reach all the street-inlets; to better insure which, such inlets should be located previous to the location of the sewer lines.

Keep large or very deep sewers out of narrow business or high-value streets if possible, because of construction difficulties.

It is generally advisable to avoid crossing private property where possible, since legal complications and delays might result from such crossing. This will frequently be impossible, however, particularly near outlets.

The sewer lines can usually be laid cut directly upon a contoured working map, an approximate rough estimate of the necessary size and consequent minimum slope of each sewer being made, that deep or shallow cutting may be avoided. The direction of flow may be indicated by arrows.

In laying out sewer lines, the outfall sewer, interceptors and mains should be located first. Attention should then be given to low areas which promise to offer difficulties. Laterals in other areas may be left until all other lines have been determined.

In the separate system the storm-water sewers should usually be placed on one side of the street centers, the separate sewers being placed in the centers. The two should never be placed one above the other in the same trench unless in. contact with each other or connected by masonry.

ART. 35. VOLUME OF HOUSE SEWAGE

Since the minimum grade of a sewer is limited by its size, and the size is determined by the grade and consequent velocity, but to even a greater extent by the maximum volume of sewage to be carried, this last must be determined before either the limiting grade or size can be decided upon. If the maximum rate of water-consumption be taken at 175 gallons per day per capita, the maximum volume per second to be carried by 175DA a sewer (in cubic feet) is in which D=density 7.48X86,400'

of population and A = the area in acres. As previously de

scribed, a factor of safety of 2 is generally used for laterals, 1 for sub-mains and 1 for mains.

Beginning at the summit of each lateral, it is clear that it is unnecessary to calculate the capacity required for any section of sewer until the point is reached where the volume of sewage to be carried exceeds the capacity of the smallest sewer used at the given grade. For an 8-inch pipe flowing full with an average velocity of 2.5 feet per second, this volume is about cubic feet per second, which, allow

3.1416 X 16 X 2.5 - )0.8726

144

=

ing a factor of safety of 2, would be contributed as a maximum

domestic flow by a population of

(0.8726X7.48X86,4001611.

2X175

=

Dividing 1611 by the density of population, assumed for the area served by this lateral, gives the number of acres to be sewered before the size is increased. Below this point the size must be enlarged to the next market size of pipe or the next size of masonry sewer convenient for construction.

The amount of commercial and industrial sewage is determined in the same way, but using the units given in Art. 7. If the same district is expected to be at one time residential and at another industrial or commercial, the condition that would yield the maximum amount is made the basis of design.

The allowance for leakage into the sewer of ground water, which should be small compared to the sewage proper, may be added at intervals, according to the engineer's judgment, based on such data as he is able to obtained. In the Cincinnati plans 75,000 gallons per day per mile of sewers was used. The amount depends upon the local rainfall and character of soil as well as upon the tightness of the sewer.

In calculating the volumes of sewage, it is advisable to begin with the furthermost lateral sewer first; where this joins another the contributions of both are to be added to determine the flow below that point; and in tracing down this line, as each lateral or branch is encountered its contribution must be calculated and added. Decision having been made, after a study of the topographical map, as to the line of sewer

into which each section of undeveloped territory will drain when sewered, the sewage which this area will ultimately con

FIG. 21.-MAP USED IN CALCULATING SEPARATE SEWER SYSTEM.

tribute should be placed at the heads of the volumes of flow in this line.

An excellent method of making these calculations is as follows, the sewerage-map, Fig. 21, being used for this table.

In this case it is seen that the capacity of an 8-inch pipe at the minimum grade was reached at the junction of the Newcastle and Budd street sewers, but the line down Budd has 150 as its grade, and no increase of size is yet necessary. CALCULATION OF SEWAGE QUANTITIES AND SEWER SIZES

At the junction of Budd and Walnut the sewage amounts to 452,300 gallons, or 42 cubic feet per minute, and the sewer from there to the river must have the minimum grade allowable. The size must therefore be increased, and as the next market size, 10-inch, has a capacity at that grade, with a factor of safety of 1, of about 590,000 gallons, it is therefore sufficiently large for the rest of the line, including sewage contributed along its length and ground water. No ground water was anticipated on the hillside, but it was considered probable that on Budd below Walnut this would leak into the sewer at the rate of 2 gallons per day per foot of sewer.

ART. 36. VOLUME OF STORM SEWAGE

The rational method of calculating the run-off to storm sewers has already been described in some detail in Chapter III. In making such calculation, the inlets are located so as to intercept the rain water flowing in the gutters without necessitating too long a surface flow. Storm-sewer lines are located so as to serve all the inlets.

Inlets and sewers must