At the junction of Budd and Walnut the sewage amounts to 441,800 gallons, or 40.9 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 when two thirds full 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 hill side, but it was considered probable that on Budd below Walnut this would leak into the sewer at the rate of two gallons per day per foot of sewer (see Art. 46).

ART. 36. VOLUME OF STORM-SEWage.

The principles stated in Articles 16-20 will be used as a basis in determining the amount of storm-water to be provided for. Decision should first be made as to whether this shall include run-off from storms of the first, second, or third class. Then the past rates of fall of such storms should be ascertained. If the records of such rates extending over a series of years are not obtainable use may be made of the rainfall data given in Art. 17. Plate No. IV shows rainfallcurves for average maximum rains of the second class, from

which may be taken the amount of rain to be expected during any given period of time in the localities named. If sufficient rainfall data for the place in question are available a similar curve for that place plotted from these data will be found. serviceable. If these data have not been kept by the city it is probable that the rates for a neighboring city can be obtained from the Weather Bureau at Washington, which now has self-registering gauges in over fifty cities of the United States.

Next to be determined is the character of surface of the streets and included areas in each section drained; that is, the amount of impervious surface. The safest course would be to assume that every street-surface is, or will be made, wholly impervious; that the space covered by each building will also be impervious; and that in residence districts the remaining areas will be 30% to 80% impervious at the time of heavy downpours, since rainfall records show that at least 25% of these are preceded by one or more hours of rainfall, which increase the natural imperviousness. These figures will be used in illustrative calculations in this work; but the judgment of the engineer, based on local conditions, may well dictate others, differing for each case considered. For instance, a closely built-up business district having paved yards and courts may be assumed as all wholly impervious.

These points having been decided, the inlets should be located on a contour-map. Also it will be well to state in figures on each city block its area and percentage of imperviousness (see Plate V).

The percentage of imperviousness may be calculated thus:
Let the average length of a city block.

/

RATE OF RAINFALL IN INCHES PER HOUR, OR CUBIC FEET PER SECOND PER ACRE.

Z4.5

800

400

WIDTH OF AREA IN FEET.

300

200

LENGTH OF AREA IN FEET.

a = the average area covered by a building:
width of street;

66

w=

i=

66

66

percentage of imperviousness of yards,

courts, etc., expressed as a decimal;

I= " percentage of imperviousness of the entire

As an example, let = 450, b = 250, ƒ=50, d= 125, a = 1200 sq. ft., w = 66, i = .60; then I = .777, or say .78. When most of the above factors must be estimated by judgment only, as for areas not yet opened up or fully developed, it may be as well to estimate I at once.

By comparing this formula with that on page 36 we see that

I = plb(a+ifd — ia)+wfd(l+b+w)

43560lbo

which formula can be used when P has already been calculated. The relation between I and P will, it is evident, vary in different cities and also in different parts of the same city. The map having been thus prepared, with the a and I on each block, the uppermost corner of the drainage-area furthest from the outlet may be taken as a starting-point. If there are beyond this any areas not included in the sewered districts, but the run-off from which flows into such districts, this runoff must be estimated and provided for. For this purpose the formula Q = AIR may be used, A being the total area, I the coefficient of imperviousness, and R the maximum rate of rainfall (of the class to be provided for) for that length of