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SECTION 2 <br /> HYDROLOGIC CRITERIA <br /> 2 . 1 Design Storm <br /> Storm drainage facilities shall be sized to accommodate the peak <br /> flow rate from a storm having a return frequency of once in 10 <br /> years . The 10 year rainfall intensity-duration curve is shown <br /> on Figure 2-1. <br /> 2 .2 Peak Flow Rate <br /> The rational Formula shall be used to determine the peak flow <br /> rate . This formula is : <br /> Q = C IA (2-1) <br /> where Q = peak rate of flow in cubic feet per second <br /> C = coefficient of runoff having a value between <br /> 0 .0 and 1 .0 depending on surface characteristics . <br /> I = the average intensity of rainfall in inches per <br /> hour for a duration equal to the critical time, <br /> usually the time of concentration. <br /> A = the tributary area in acres corresponding to the <br /> critical time above . <br /> The procedures for determining the values of C, A and I are given <br /> in the following sections . <br /> 2 .2 . 1 Runoff Coefficient , C <br /> Table 2-1 shows the basic coefficient, C, to be used for various <br /> types of surfaces . Where a tributary area contains more than one <br /> type of surface the value of C shall be the weighted average of the <br /> respective values shown in Table 2-1 based on the percentage of <br /> each surface area to the total area . <br /> 2-1 <br />