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<br />. <br /> <br />. <br /> <br />. <br /> <br />DRAINAGE CRITERIA MANUAL <br /> <br />RUNOFF <br /> <br />3. RATIONAL METHOD <br /> <br />For basins that are not complex and have generally 160 acres or less, it <br />is acceptable that the design storm runoff be analyzed by the Rational Method. <br />This method was introduced in 1889 and is still being used in most engineering <br />offices in the United States. Even though this method has frequently come <br />under academic criticism for its simpl icity, no other practical drainage <br />design method has evolved to such a level of general acceptance by the <br />practicing engineer. The Rational Method properly understood and applied can <br />produce satisfactory results for urban storm sewer design (7,8,9, 10). <br /> <br />3.1 Rational Formula <br />The Rational Method is based on the Rational Formula: <br /> <br />Q = CIA <br /> <br />(3-1) <br /> <br />Q is defined as the maximum rate of runoff in cubic feet per second. Actually <br />Q has units of inches per hour per acre; however, since this rate of in/hr/ac <br />differs from cubic feet per second by less than one percent, the more common <br />units of ft3/s are used. C is a runoff coefficient which is the ratio between <br />the maximum rate of runoff from an area and the average rate of rainfall <br />intensity over a duration equal to the time of concentration for the area. <br />The I is the average intensity of rainfall in inches per hour for a duration <br />equal to the time of concentration. The time of concentration is typically <br />defined as the time required for water to flow from the most remote point of <br />the area to the point being investigated. <br /> <br />3.2 Assumptions <br />The basic assumptions that are often made when the Rational Method is <br />applied are: <br />1. The computed maximum rate of runoff <br />functi on of the average rai nfa 11 <br />concentration to that point. <br />2. The maximum rate of rainfall occurs during the time of <br />concentration, and the design rainfall depth during the time of <br /> <br />to the design point is a <br />rate during the time of <br /> <br />5-1-84 <br />