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e e 5.4 STORM FREQUENCY, RUNOFF AND DATUM A. Storm Frequency All drainage improvements shall be designed for the following storm frequencies. Type of Facility Road Side Ditches and Culverts Storm Sewers Ditches & Culverts Draining 100 acres and more Bridges Creeks Detention Facilities 3 year 3 year 25 year 100 year 100 year Refer to Section 5.5 B. Storm Runoff Design flow of storm water runoff is to be calculated using the Rational Method. The Rational Method is based on the direct relationship between rainfall and runoff, and the method is expressed by the following equation: Q = CIA, where Q is the storm flow at a given point in cubic feet per second (c.f.s.). C is a coefficient of runoff (see Table 1). I is the average intensity of rainfall in inches per hour for a period equal to the time of flow from the farthermost point of the drainage area to the point under consideration. (See Figure 1, I-D-F Curves and Figure 2, Detenninination of Time of Concentration) A is the drainage area in acres The size and shape of the watershed must be determined for each installation. The area of each watershed may be determined through the use of planimetric-topographic maps of the area, supplemented by field surveys in areas where topographic data has changed or where the contour interval is insufficient to adequately determine the direction of flow. The outline of the drainage area contributing to the system being designed and outline of the sub-drainage area contributing to each inlet point shall be determined. When calculating the peak flow rate of storm water runoff, rainfall intensity will be determined from the rainfall intensity, duration and frequency curves, shown in Figure 1. The storm frequency used for this determination will be according to the facility to be designed as listed in Section A. 1. Runoff Coefficients and Time of Concentration Stonnwater Criteria 3 of 12