§ 7.3.2. Time of Concentration and Discharge

The rational formula should be used to determine the discharges through the storm drain system. The time of concentration is the time required for water to travel from the most hydraulically distant point in the watershed to the point under consideration. The time of concentration is comprised of overland flow, sheet flow, and gutter flow to the first inlet. Then from the first inlet there is additional time in the underground drainage system to the design points which should be considered. With the total time of concentration to the point of interest in the storm drain system the designer can select the intensity value for use in the rational formula.