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Adk <br /> flow time. In complex drainage situations more than one computation <br /> may be required in order to determine the combination of contributing <br /> area and flow time which results in the maximum flow rate. <br /> - 2 .2 .4 Rainfall Intensity, I_ <br /> After determining the critical flow time the Intensity-Duration <br /> curve on Figure 2-1 or Table 2-3 shall be used to determine the <br /> rainfall intensity for that time . <br /> 2 . 3 Retention Basins <br /> Retention basins other than those shown on the Master Storm <br /> Drainage Map may be used for revisions to the Master Plan or for <br /> stage development only upon approval of the Director of Public <br /> Works . <br /> The volume of storage shall be computed from the following basic <br /> formula : <br /> V = CAR <br /> (2-2) <br /> where V is the volume in acre feet <br /> C is the runoff coefficient (see Section 2 .2 . 1) <br /> A is the contributing area in acres (see Section 2 .2 .2) <br /> R is the total rainfall in inches for the storm period <br /> In urbanized areas R shall be 3 . 12 inches . (Equivalent to a <br /> 10 year, 48 hour storm. ) <br /> In rural areas R shall be 2 .40 inches . (Equivalent to a 10 year, <br /> 24 hour storm. ) <br /> The volume of retention basins shall be determined with no <br /> allowance for percolation or outlet facilities . <br /> 2-6 <br />