27. Résumé of Method for Determination of Quantity of Dry weather Sewage.—The steps in the determination of the quantity of sewage are: determine the period in the future for which the sewers are to be designed; estimate the population and tributary area at the end of this period; estimate the rate of water consumption and assume the sewage flow to equal the water consumption; determine the maximum and minimum rates of sewage flow; and finally, estimate the maximum rate of ground water seepage and add it to the maximum rate of sewage flow to give the total quantity of sewage to be carried by the proposed sewers.
| TABLE 12 | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Data on the Infiltration of Ground Water into Sewers | |||||||||
| Abstracted from paper by J. N. Brooks in Transactions Am. Society of Civil Engineers, Vol. 76, p. 1909. | |||||||||
| Place | Shape | Diameter or Dimensions in Inches | Material | Wet Trench, Per Cent of Total Length | Avg. Head of Ground Water, Fee | Character of Subgrade | Gallons per 24 Hours | ||
| Per Foot of Joint | Per Inch Diameter Per Mile of Pipe | Per Mile of Pipe | |||||||
| Boston, Mass. | Circ. | 8 to 36 | V.P. | 2.6 | 1,818 | 40,000 | |||
| East Orange, N. J. | 10 | Q. | 22,400 | ||||||
| East Orange, N. J. | 8 to 24 | V.P. | 0.8 | 540 | 8,650 | ||||
| Joint trunk sewer, New Jersey | G. & Q. | 25,000 | |||||||
| Rogers Park, Ill. | 6 | 0.3 | 207 | 1,240 | |||||
| Altoona, Pa. | 30 | 5.0 | 2,890 | 86,592 | |||||
| Concord, Mass. | 18 | 8 | 43,000 | ||||||
| Malden, Mass. | Circ. | V.P. | 60 | 50,000 | |||||
| Westboro, Mass. | 15 | V.P. | 100 | 88,100 | 1,320,300 | ||||
| Fond du Lac, Wis. | Circ. | 24 | V.P. | 100 | 5 | C. | 1.5 | 1,010 | 24,370 |
| East Orange, N. J. | Circ. | 10 to 24 | V.P. | 100 | 4.7 | 2,540 | 43,250 | ||
| Ocean Grove, N. J. | Circ. | 4 to 12 | V.P. | 100 | 3 | S.C. | 2.7 | 1,890 | 15,126 |
| Ocean Grove, N. J. | Circ. | 4 to 12 | V.P. | 100 | 4 | S.C. | 7.9 | 5,480 | 43,764 |
| East Orange, N. J. | Rect. | 24 × 36 | Brick | 100 | 570,000 | ||||
| Westboro, Mass. | Brick | 415,850 | |||||||
| Altoona, Pa. | Rect. | 33 × 44 | B. & C. | 5,390 | 264,000 | ||||
| Columbus, Ohio. | H.S. | 42 × 42 | Concrete | 120 | 6,340 | ||||
| Bronx Valley, N. Y. | Circ. | 44 to 72 | Concrete | G. | 123 | 7,266 | |||
| Cincinnati, Ohio. | Estimated in design. Data not from Brooks | 67,500 | |||||||
| Milwaukee, Wis. | Residential districts, gals. per acre per day. Not taken from Brooks | 1460 to 2200 | |||||||
| Abbreviations: H.S. = horseshoe shaped; B. & C = Brick and concrete; V.P. = vitrified pipe; G. = gravel; Q. = quicksand; S. C. = sand clay; C. = clay. | |||||||||
Quantity of Storm Water
28. The Rational Method.—The water which falls during a storm must be removed rapidly in order to prevent the flooding of streets and basements, and other damages. The quantity of water to be cared for is dependent upon: the rate of rainfall, the character and slope of the surface, and the area to be drained. All methods for the determination of storm-water run-off, whether rational or empirical, depend upon these factors.
The so-called Rational Method can be expressed algebraically, as,
Q = AIR,
in which Q = rate of run-off in cubic feet per second; A = area to be drained expressed in acres; I = percentage imperviousness of the area; R = maximum average rate of rainfall over the entire drainage area, expressed in inches per hour, which may occur during the time of concentration.
The area to be drained is determined by a survey. A discussion of R and I follows in the next two sections. An example of the use of the Rational Method is given on page [95].
29. Rate of Rainfall.—Rainfall observations have been made over a long period of time by United States Weather Bureau observers and others. Continuous records are available in a few places in this country showing rainfall observations covering more than a century. Such records have been the bases for a number of empirical formulas for expressing the probable maximum rate of rainfall in inches per hour, having given the duration of the storm. Table 13 is a collection of these formulas with a statement as to the conditions under which each formula is applicable. The formula most suitable to the problem in hand should be selected for its solution.[[22]]
| TABLE 13 | ||
|---|---|---|
| Rainfall Formulas | ||
| Name of Originator | Conditions for which Formula is Suitable | Formula |
| E. S. Dorr | i = 150 t + 30 | |
| A. N. Talbot | Maximum storms in Eastern United States | i = 360 t + 30 |
| A. N. Talbot | Ordinary storms in Eastern United States | i = 105 t + 15 |
| Emil Kuichling | Heavy rainfall near New York City | i = 120 t + 20, etc. |
| L. J. Le Conte | For San Francisco. See T. A. S. C. E. v. 54, p. 198 | i = 7 t½ |
| Sherman | Maximum for Boston, Mass. | i = 25.12 t.687 |
| Sherman | Extraordinary for Boston, Mass. | i = 18 t ½ |
| Webster | Ordinary for Philadelphia, Pa. | i = 12 t0.6 |
| Hendrick | Ordinary storms for Baltimore. Eng. & Cont., Aug. 9. 1911 | i = 105 t + 10 |
| J. de Bruyn-Kops | Ordinary storms for Savannah, Ga. | i = 163 t + 27 |
| C. D. Hill | For Chicago, Ill. | i = 120 t + 15 |
| Metcalf and Eddy | Louisville, Ky. Am. Sew. Prac., Vol I. | i = 14 t½ |
| W. W. Horner | St. Louis, Mo. Eng. News, Sept. 29, 1910 | i = 56 (t + 5).85 |
| R. A. Brackenbuy | For Spokane, Wash. Eng. Record, Aug. 10, 1912 | i = 23.92 t + 2.15 + 0.154 |
| Metcalf and Eddy | New Orleans | i = 19 t½ |
| Metcalf and Eddy | For Denver, Colo. | i = 84 t + 4 |
| Kenneth Allen | Central Park, N. Y. 51–Year Record. Eng. News-Record, April 7, 1921, p. 588 | i = 400 2t + 40[[23]] |