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.
COMPUTATIONS FOR THE QUANTITY OF STORM SEWAGE AT THE FOOT OF TENNESSEE STREET ON FIGURE 25
─────────┬──────────┬──────────┬───────────┬───────────────────────────────── On Street│ From │To Street │Identifying│ By McMath’s Formula │ Street │ │ Number of │ │ │ │ Acres │ │ │ │ Drained │ ─────────┼──────────┼──────────┼───────────┼──────────┬───────┬───────┬────── │ │ │ │Additional│ Total │ Slope │ Run │ │ │ │ Acres │ Acres │ of │Off in │ │ │ │ Drained │Drained│Surface│C.F.S. │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ ─────────┼──────────┼──────────┼───────────┼──────────┼───────┼───────┼────── State │N. │S. │ 91 and 92 │2.35 │ 2.35│ 0.005│ 5.5 │ Carolina│ Carolina│ │ │ │ │ State │S. │Georgia │88, 89 and │3.0 │ 5.35│ .005│ 10.8 │ Carolina│ │ 90 │ │ │ │ State │Georgia │Florida │85, 86 and │3.0 │ 8.35│ .007│ 16.5 │ │ │ 87 │ │ │ │ State │Florida │Kentucky │81, 83 and │3.0 │ 11.35│ .009│ 22.0 │ │ │ 84 │ │ │ │ State │Kentucky │Tennessee │79, 80 and │3.0 │ 14.35│ .010│ 28.0 │ │ │ 82 │ │ │ │ State │Texas │Louisiana │ 76 and │3.8 │ 3.8│ .005│ 8.3 │ │ │ others │ │ │ │ State │Louisiana │Alabama │73, 74 and │3.7 │ 7.5│ .007│ 15.0 │ │ │ 75 │ │ │ │ State │Alabama │Tennessee │70, 71 and │3.0 │ 10.5│ .006│ 19.0 │ │ │ 72 │ │ │ │ Tennessee│State │Talon │68, 69, 77 │4.3 │ 29.15│ .15│ 52 │ │ │ and 78 │ │ │ │ Talon │Albemarle │Tennessee │65, 66 and │2.8 │ 2.8│ .018│ 8.4 │ │ │ 67 │ │ │ │ Tennessee│Talon │Burnside │ 64 and │0.7 │ 29.85│ .15│ 55 │ │ │ 64_a_ │ │ │ │ Burnside │N. │S. │57, 58 and │2.84 │ 2.84│ .008│ 7.2 │ Carolina│ Carolina│ 59 │ │ │ │ Burnside │S. │Georgia │54, 55 and │3.88 │ 6.72│ .010│ 14.9 │ Carolina│ │ 56 │ │ │ │ Burnside │Georgia │Florida │50, 52 and │3.88 │ 10.60│ .012│ 22 │ │ │ 53 │ │ │ │ Burnside │Florida │Kentucky │47, 48 and │3.88 │ 14.48│ .013│ 30 │ │ │ 51 │ │ │ │ Burnside │Kentucky │Tennessee │44, 45 and │3.88 │ 18.36│ .013│ 36 │ │ │ 46 │ │ │ │ Tennessee│Burnside │Elm │ 42 and 43 │2.84 │ 51.05│ .015│ 82 Elm │Above │Chetwood │ Included in next line below │ │ │ Chetwood│ │ │ │ Elm │Chetwood │Albemarle │31, 32 and │2.75 │ 2.75│ .007│ 7.0 │ │ │ 33 │ │ │ │ Elm │Albemarle │Tennessee │27, 28, 29 │5.75 │ 8.50│ .016│ 20 │ │ │ and 30 │ │ │ │ Tennessee│Elm │Varennes │25, 26 and │2.62 │ 62.17│ .017│ 100 │ │ │ 41 │ │ │ │ Varennes │S. │Georgia │17, 18 and │3.17 │ 3.17│ .010│ 8.3 │ Carolina│ │ 19 │ │ │ │ Varennes │Georgia │Florida │14, 15 and │3.17 │ 6.34│ .011│ 14.5 │ │ │ 16 │ │ │ │ Varennes │Florida │Kentucky │11, 12 and │3.17 │ 9.51│ .013│ 21 │ │ │ 13 │ │ │ │ Varennes │Kentucky │Tennessee │8, 9 and 10│3.17 │ 12.68│ .013│ 26 Tennessee│Varennes │Boulevard │ 6 and 7 │2.32 │ 77.17│ .017│ 120 Tennessee│Boulevard │Outlet │1, 2, 3, 4,│4.72 │ 81.89│ .017│ 122 │ │ │ and 5 │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ ─────────┴──────────┴──────────┴───────────┴──────────┴───────┴───────┴──────