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