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Calculate Composite Curve Number With An Unconnected Impervious Area And Total Impervious Area of less than 30%:
*Referencing TR-55, Appendix F, this calculation is to be used for calculating the composite CN for area with an unconnected impervious area and a total impervious area of less than 30%.
 
Calculation:
Designer/Checker:
   
Input:  
 
 
Pervious runoff curve number (CNp)
Percent imperviousness (Pimp - %)
Ratio of unconnected impervious area to total impervious area, (R) 
 
   
 
Output:
 
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Composite runoff curve number, CNc =
 
 
 
 



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