────────────┬─────────────────────────┬──────────────────────────────── Name of │ Conditions for which │ Formula Originator │ Formula is Suitable │ ────────────┼─────────────────────────┼──────────────────────────────── E. S. Dorr │ │_i_ = 150⁄(_t_ + 30) A. N. Talbot│Maximum storms in Eastern│_i_ = 360⁄(_t_ + 30) │ United States │ A. N. Talbot│Ordinary storms in │_i_ = 105⁄(_t_ + 15) │ Eastern United States │ Emil │Heavy rainfall near New │_i_ = 120⁄(_t_ + 20), etc. Kuichling │ York City │ L. J. Le │For San Francisco. See T.│ Conte │ A. S. C. E. v. 54, p. │_i_ = 7⁄_t_^½ │ 198 │ Sherman │Maximum for Boston, Mass.│_i_ = 25.12⁄_t_^{.687} Sherman │Extraordinary for Boston,│_i_ = 18⁄_t_ ^½ │ Mass. │ Webster │Ordinary for │_i_ = 12⁄_t_^{0.6} │ Philadelphia, Pa. │ │Ordinary storms for │ Hendrick │ Baltimore. Eng. & │_i_ = 105⁄(_t_ + 10) │ Cont., Aug. 9. 1911 │ J. de │Ordinary storms for │_i_ = 163⁄(_t_ + 27) Bruyn-Kops│ Savannah, Ga. │ C. D. Hill │For Chicago, Ill. │_i_ = 120⁄(_t_ + 15) Metcalf and │Louisville, Ky. Am. Sew. │_i_ = 14⁄_t_^½ Eddy │ Prac., Vol I. │ W. W. Horner│St. Louis, Mo. Eng. News,│_i_ = 56⁄(_t_ + 5)^{.85} │ Sept. 29, 1910 │ R. A. │For Spokane, Wash. Eng. │_i_ = 23.92⁄(_t_ + 2.15) + 0.154 Brackenbuy│ Record, Aug. 10, 1912 │ Metcalf and │New Orleans │_i_ = 19⁄_t_^½ Eddy │ │ Metcalf and │For Denver, Colo. │_i_ = 84⁄(_t_ + 4) Eddy │ │ │Central Park, N. Y. │ Kenneth │ 51–Year Record. Eng. │_i_ = 400⁄(2_t_ + 40)[23] Allen │ News-Record, April 7, │ │ 1921, p. 588 │ ────────────┴─────────────────────────┴──────────────────────────────── =30. Time of Concentration.=—By the time of concentration is meant the longest time without unreasonable delay that will be required for a drop of water[24] to flow from the upper limit of a drainage area to the outlet. Assuming a rainfall to start suddenly and to continue at a constant rate and to be evenly distributed over a drainage area of 100 per cent imperviousness and even slope towards one point, the rate of run-off would increase constantly until the drop of water from the upper limit of the area reached the outlet, after which the rate of run-off would remain constant. In nature the rate of rainfall is not constant. The shorter the duration of a storm the greater the intensity of rainfall. Therefore the maximum run-off during a storm will occur at the moment when the upper limit of the area has commenced to contribute. From that time on the rate of run-off will decrease.