1st. Note the most advantageous points to locate the inlets and lay out the system to drain these inlets. 2nd. Determine the required capacity of the sewers by a study of the run-off from the different drainage areas. 3rd. Draw the profile and compute the diameter and slope of the pipes required.
51. Location of Street Inlets.—The location of storm sewers is determined mainly by the desirable location of the street inlets. The inlets must therefore be located before the system can be planned. In general the inlets should be located so that no water will flow across a street or sidewalk, in order to reach the sewer. This requires that inlets be placed on the high corners at street intersections, in depressions between street intersections, and at sufficiently frequent intervals that the gutters may not be overloaded. City blocks are seldom so long as to necessitate the location of inlets between crossings solely on account of inadequate gutter capacity. The capacity of a gutter can be computed approximately by the application of Kutter’s formula. Inlet capacities are discussed in Chapter VI. When the area drained is sufficiently large to tax the capacity of the gutter or inlet, an inlet should be installed regardless of the location of the street intersections.
The street inlets are located on the map as shown in Fig. 25. The sewer lines are then located so as to make the length of pipe to pass near to all inlets a minimum. Storm sewers are seldom placed near the center of a street because of the frequent crowded condition on this line.
52. Drainage Areas.—The outline of a drainage area is drawn so that all water falling within the area outlined will enter the same inlet, and water falling on any point beyond the outline will enter some other inlet. This requires that the outline follow true drainage lines rather than the artificial land divisions used in locating the drainage lines in the design of sanitary sewers. The drainage lines are determined by pavement slopes, location of downspouts, paved or unpaved yards, grading of lawns and the many other features of the natural drainage which are altered by the building up of a city. The location of the drainage lines is fixed as the result of a study of local conditions.
The watershed or drainage lines are shown on Fig. 25 by means of dot and dash lines. A drainage line passes down the middle of each street because the crown of the street throws the water to either side and directs it to different inlets. A watershed line is drawn about 50 feet west of such streets as Kentucky St., Florida St., etc., because the downspouts from the houses on those streets discharge or will discharge into the street on which they face. The location of any watershed line within 20 feet more or less is, in most cases, a matter of judgment rather than exactness. Each area is given an identifying number or mark which is useful only in design. It usually corresponds to the inlet number.
53. Computation of Flood Flow by McMath Formula.—McMath’s Formula is used as an example of the method pursued when an empirical formula is adopted for the computation of run-off, and because of its frequent use in practice. Other formulas may be more satisfactory under favorable conditions.
Computations should be kept in order by a tabulation such as is shown in Table 21, in which the quantity of storm flow discharged from the sewer at the foot of Tennessee St., on Fig. 25, has been computed by means of the McMath Formula, using the constants suggested for St. Louis conditions, i = 2.75, and c = 0.75. The solutions of the formula have been made by means of Fig. 11. The column headings in the Table are explanatory of the figures as recorded. The computation should begin at the upper end of a lateral, proceed to the first junction and then return to the head of another lateral tributary to this junction. They should be continued in the same manner until all tributary areas have been covered. Special computations will be necessary for the determination of the maximum quantity of storm water entering each inlet to avoid the flooding of an inlet or gutter. These computations have not been shown as they are so easily made by the application of McMath’s Formula to each area concerned.
The determination of the average slope ratio is a matter of judgment, based on the average natural slope of the surface of the ground and an estimate of the probable future conditions.
54. Computation of Flood Flow by Rational Method.—The rational method for the computation of storm-water run-off is described in Chapter III. An example of its application to storm sewer design is given here for the district shown in Fig. 25.[[34]] The computations are shown in Table 21. As in the preceding designs the table has been filled in from left to right and line by line. Computations have started at the upper end of laterals tributary to each junction. The column headed I represents the imperviousness factor in the expression Q = AIR. It is based on judgment guided by the constants given in Chapter III concerning imperviousness. The column headed “Equivalent 100 per cent I acres” is the product of the two preceding columns. It reduces all areas to the same terms so that they can be added for entry in the column headed “Total 100 per cent I acres.” It may be necessary to record the values for this column on several lines where the imperviousnesses of the tributary areas are different. This condition is illustrated in the last line of the table, for the length of sewer nearest the outlet. In the preceding lines the imperviousness recorded represents an average for all the tributary areas.
| TABLE 21 | |||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Computations for the Quantity of Storm Sewage at the Foot of Tennessee Street on Figure 25 | |||||||||||||||||||
| On Street | From Street | To Street | Identifying Number of Acres Drained | By McMath’s Formula | By Rational Method | Line Number | |||||||||||||
| Additional Acres Drained | Total Acres Drained | Slope of Surface | Run Off in C.F.S. | Area, Acres | I | Equivalent 100 Per Cent I Acres | Total 100 Per Cent I Acres | Time of Concentration, Minutes | R | Q | S | V | Sewer Length, Feet | Time in Sewer | |||||
| State | N. Carolina | S. Carolina | 91 and 92 | 2.35 | 2.35 | 0.005 | 5.5 | 2.35 | 0.50 | 1.17 | 1.17 | 7.0 | 4.8 | 5.6 | 0.011 | 4.6 | 300 | 1.1 | 1 |
| State | S. Carolina | Georgia | 88, 89 and 90 | 3.0 | 5.35 | .005 | 10.8 | 3.00 | .50 | 1.50 | 2.67 | 8.1 | 4.6 | 12.2 | .010 | 5.5 | 300 | 0.9 | 2 |
| State | Georgia | Florida | 85, 86 and 87 | 3.0 | 8.35 | .007 | 16.5 | 3.00 | .50 | 1.50 | 4.17 | 9.0 | 4.4 | 18.3 | .012 | 5.8 | 300 | 0.9 | 3 |
| State | Florida | Kentucky | 81, 83 and 84 | 3.0 | 11.35 | .009 | 22.0 | 3.00 | .50 | 1.50 | 5.67 | 9.9 | 4.2 | 23.9 | .009 | 6.0 | 300 | 0.8 | 4 |
| State | Kentucky | Tennessee | 79, 80 and 82 | 3.0 | 14.35 | .010 | 28.0 | 3.00 | .50 | 1.50 | 7.17 | 10.7 | 4.1 | 29.3 | .009 | 6.2 | 300 | 0.8 | 5 |
| State | Texas | Louisiana | 76 and others | 3.8 | 3.8 | .005 | 8.3 | 3.80 | .35 | 1.33 | 1.33 | 10.0 | 4.2 | 5.6 | .009 | 3.2 | 370 | 1.9 | 6 |
| State | Louisiana | Alabama | 73, 74 and 75 | 3.7 | 7.5 | .007 | 15.0 | 3.70 | .40 | 1.48 | 2.81 | 11.9 | 3.9 | 11.0 | .011 | 5.2 | 300 | 1.0 | 7 |
| State | Alabama | Tennessee | 70, 71 and 72 | 3.0 | 10.5 | .006 | 19.0 | 3.00 | .45 | 1.35 | 4.16 | 12.9 | 3.8 | 15.8 | .002 | 3.2 | 300 | 1.6 | 8 |
| Tennessee | State | Talon | 68, 69, 77 and 78 | 4.3 | 29.15 | .15 | 52 | 4.30 | .50 | 2.15 | 13.48 | 14.5 | 3.6 | 48.5 | .019 | 9.8 | 450 | 0.8 | 9 |
| Talon | Albemarle | Tennessee | 65, 66 and 67 | 2.8 | 2.8 | .018 | 8.4 | 2.80 | .40 | 1.12 | 1.12 | 8.0 | 4.6 | 5.2 | .004 | 3.0 | 210 | 1.2 | 10 |
| Tennessee | Talon | Burnside | 64 and 64a | 0.7 | 29.85 | .15 | 55 | 0.70 | .20 | 0.14 | 14.74 | 15.3 | 3.5 | 51.5 | .006 | 5.0 | 120 | 0.4 | 11 |
| Burnside | N. Carolina | S. Carolina | 57, 58 and 59 | 2.84 | 2.84 | .008 | 7.2 | 2.84 | .55 | 1.56 | 1.56 | 10.0 | 4.2 | 6.5 | .008 | 4.5 | 300 | 1.1 | 12 |
| Burnside | S. Carolina | Georgia | 54, 55 and 56 | 3.88 | 6.72 | .010 | 14.9 | 3.88 | .55 | 2.13 | 3.69 | 11.1 | 4.0 | 14.8 | .007 | 4.7 | 300 | 1.1 | 13 |
| Burnside | Georgia | Florida | 50, 52 and 53 | 3.88 | 10.60 | .012 | 22 | 3.88 | .55 | 2.13 | 5.82 | 12.2 | 3.9 | 22.7 | .011 | 5.8 | 300 | 0.9 | 14 |
| Burnside | Florida | Kentucky | 47, 48 and 51 | 3.88 | 14.48 | .013 | 30 | 3.88 | .55 | 2.13 | 7.95 | 13.1 | 3.7 | 29.4 | .016 | 7.5 | 300 | 0.7 | 15 |
| Burnside | Kentucky | Tennessee | 44, 45 and 46 | 3.88 | 18.36 | .013 | 36 | 3.88 | .55 | 2.13 | 10.08 | 13.8 | 3.7 | 37.3 | .019 | 9.2 | 300 | 0.5 | 16 |
| Tennessee | Burnside | Elm | 42 and 43 | 2.84 | 51.05 | .015 | 82 | 2.84 | .45 | 2.28 | 26.10 | 15.7 | 3.4 | 88.8 | .015 | 10.2 | 280 | 0.5 | 17 |
| Elm | Above Chetwood | Chetwood | Included in next line below | 18 | |||||||||||||||
| Elm | Chetwood | Albemarle | 31, 32 and 33 | 2.75 | 2.75 | .007 | 7.0 | 2.75 | .40 | 1.10 | 1.10 | 8.0 | 4.6 | 5.1 | .020 | 5.3 | 480 | 1.5 | 19 |
| Elm | Albemarle | Tennessee | 27, 28, 29 and 30 | 5.75 | 8.50 | .016 | 20 | 5.75 | .45 | 2.59 | 3.69 | 9.5 | 4.3 | 15.8 | .012 | 6.1 | 410 | 1.1 | 20 |
| Tennessee | Elm | Varennes | 25, 26 and 41 | 2.62 | 62.17 | .017 | 100 | 2.62 | .50 | 1.31 | 30.00 | 16.2 | 3.4 | 102 | .012 | 10.2 | 180 | 0.3 | 21 |
| Varennes | S. Carolina | Georgia | 17, 18 and 19 | 3.17 | 3.17 | .010 | 8.3 | 3.17 | .55 | 1.74 | 1.74 | 9.0 | 4.4 | 7.7 | .012 | 5.2 | 270 | 0.9 | 22 |
| Varennes | Georgia | Florida | 14, 15 and 16 | 3.17 | 6.34 | .011 | 14.5 | 3.17 | .55 | 1.74 | 3.48 | 9.9 | 4.2 | 14.5 | .010 | 5.7 | 300 | 0.9 | 23 |
| Varennes | Florida | Kentucky | 11, 12 and 13 | 3.17 | 9.51 | .013 | 21 | 3.17 | .55 | 1.74 | 5.22 | 10.8 | 4.1 | 21.4 | .017 | 7.7 | 300 | 0.6 | 24 |
| Varennes | Kentucky | Tennessee | 8, 9 and 10 | 3.17 | 12.68 | .013 | 26 | 3.17 | .55 | 1.74 | 6.96 | 11.4 | 4.0 | 27.8 | .015 | 7.8 | 300 | 0.6 | 25 |
| Tennessee | Varennes | Boulevard | 6 and 7 | 2.32 | 77.17 | .017 | 120 | 2.32 | .55 | 1.28 | 32.84 | 16.5 | 3.3 | 108 | .012 | 10.2 | 230 | 0.4 | 26 |
| Tennessee | Boulevard | Outlet | 1, 2, 3, 4, and 5 | 4.72 | 81.89 | .017 | 122 | 0.18 | .80 | 0.14 | Area No. 1 | 27 | |||||||
| 1.38 | .50 | 0.69 | Area No. 2 | 28 | |||||||||||||||
| 2.80 | .55 | 1.54 | Areas No. 3 and 4 | 29 | |||||||||||||||
| 0.36 | .75 | 0.27 | 35.48 | 16.9 | 3.3 | 117 | Areas No. 1–5 inclusive | 30 | |||||||||||