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3. RATIONAL METHOD <br />The formula Q = CIA shall be used to determine quantities of flow <br />rate, in which <br />Q = flow rate in cubic feet per second; <br />C = runoff coefficient; <br />I = rainfall intensity in inches per hour for a duration <br />equal to the. time of concentration; and <br />A = drainage area in acres. <br />ORUNOFF COEFFICIENT <br />The runoff coefficient shall be determined from Table 1 (page 141 <br />It shall be based on the ultimate use of the drainage area. For dis= <br />tinctive composite drainage areas, a weighted value of runoff coeffi- <br />cient shall be used. <br />OTIME OF CONCENTRATION <br />1. Determine overland flow time from Plate 3 (page 17) generally <br />for paved, bare soil and grassed areas. <br />2. Determine flow time over small agricultural areas with well - <br />defined divides and drainage channels from Plate 5 (page 18) <br />a. Use upper curve for well- forested areas, representing <br />Tc = 0.0136 Ko' <br />b. Use lower curve for areas with little or no cover, repre- <br />senting <br />Tc = 0.0078 K017 <br />3. In case of uncertainty, check the time of concentration by <br />dividing the estimated longest route of runoff by the appro- <br />priate runoff velocity from Table 2 (page 14). <br />ORAINFALL INTENSITY <br />The design rainfall intensity of a drainage area shall be determined <br />by the following procedure: <br />1. Select the appropriate I -hour rainfall value from Plate I and <br />2 (pages 15 & 16) for the design recurrence interval. <br />2. Enter Plate 4 (page 17) with the one hour rainfall <br />value ond the required time of concentration. Obtain the <br />design rainfall intensity in inches per hour. <br />3 <br />