The time of concentration can be measured fairly well by observing the moment of the commencement of a rainfall, and the time of maximum run-off from an area on which the rain is falling. A prediction of the time of concentration is more or less guess work. As the result of measurements some engineers assume the time of concentration on a city block built up with impervious roofs and walks, and on a moderate slope, is about 5 to 10 minutes. This is used as a basis for the judgment of the time of concentration on other areas. For relatively large drainage areas such a method cannot be used. The procedure is to measure the length of flow through the drainage channels of the area, to assume the velocity of the flood crest through these channels and thus to determine the time of concentration. Table 14 shows the flood crest velocities in various streams of the Ohio River Basin under flood conditions. The velocity over the surface of the ground may be approximated by the use of the formula[25] _V_ = 2,000_I_√(_S_), in which _V_ = the velocity of flow over the surface of the ground in feet per minute; _I_ = the percentage imperviousness of the ground; _S_ = the slope of the ground. For areas up to 100 acres where natural drainage channels are not existent this formula will give more satisfactory results than guesses based on the time of concentration of certain known areas. Having determined the time of concentration, the rate of rainfall _R_ to be used in the Rational Method is found by substitution in some one of the rainfall formulas given in Table 13. TABLE 14 FLOOD CREST VELOCITIES IN OHIO RIVER BASIN IN MARCH, 1913