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ОглавлениеM = 1 + 14 4 + √P,
which was used in the design of the Ten Mile Creek intercepting sewer at Toledo, Ohio. For rough estimates and for comparative purposes the ratio of the average to the minimum flow can be taken the same as the ratio of the maximum to the average flow, unless direct gaugings or other information show it to be otherwise.
Fig. 10.—Daily and Hourly Variations of Sewage Flow.
1.Toledo, O.; Manufacturing average.2.Toledo, O.; Manufacturing, Monday.3.Toledo, O.; Manufacturing, Sunday.4.Toledo, O.; Residential, average.5.Toledo, O.; Residential, Monday.6.Toledo, O.; Residential, Sunday.7.Cincinnati, O., Industrial, average.8.Cincinnati, O.; Residential, average.9.Cincinnati, O.; Commercial, average.10.Average of 7 cities.
The fluctuations of flow in commercial and industrial districts are so different from those in residential districts that the formulas given should not be used in the design of sewers other than those draining residential areas. It is reasonable to suppose that fluctuations in rates of flow from industrial districts are dependent upon the character of the tributary industries. A study of these industries will give valuable light on the maximum and minimum rates at which sewage will be delivered to the sewers.
Hourly, daily, and seasonal fluctuations in rates of sewage flow are of interest in the design of pumping stations to give knowledge of the rates at which the pumps must operate at various periods. The fluctuations in rates of sewage flow during various hours and days in different cities and districts are shown in Fig. 10. Fluctuations in rate of flow of sewage lag behind fluctuations in rate of water consumption, the time being dependent on the distance through which the wave of change must travel in the sewer.
26. Effect of Ground Water.—Sewers are seldom laid with water-tight joints. Since they usually lie below the ground water level it is inevitable that a certain amount of ground water will enter. Various units have been suggested for the expression of the inflow of ground water in an attempt to include all of the many factors. Some of these units are: gallons per acre drained by the sewer per day, gallons per mile of pipe per day, gallons per inch diameter per mile of pipe per day, etc. Since the ground water enters pipe sewers at the joints, the longer the joints the greater the probability of the entrance of ground water. The last unit is therefore the most logical but the accuracy of the result is scarcely worthy of such refinement and the unit usually adopted is gallons per mile of pipe per day.
No definite figure can be given for the amount of ground water to be expected in sewers since the character of the soil and the ground water pressure must be considered. Relatively normal infiltration may be found from 5,000 to 80,000 gallons per mile of pipe per day. The minimum is seldom reached in wet ground and the maximum is frequently exceeded. Table 12 shows the amount of ground water measured in various sewers as given by Brooks.[21]
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. |
