InfoSWMM (d/D v. Surcharge d/D)

Subject:   InfoSWMM (d/D v. Surcharge d/D)

 What is the difference between the output variables d/D and Surcharge d/D in InfoSWMM and H2OMap SWMM 

The d/D is calculated as link capacity based on the midpoint depth of water in the link or Link depth/ Link Maximum Depth

            Since the depth in the link is restricted to the Maximum Depth the d/D value is always between 0 and 1

The Surcharged d/D is calculated from the end node depths at each end of the link

            The two node depths are averaged and the value of Surcharge d/D is the Average Node Depth / Link Maximum Depth,

The value of Surcharge d/D varies from 0 to a large number depending on the maximum depths of the nodes and the possible surcharge depth of the nodes 

The value of d/D is based on the middle of the link and the value of Surcharge d/D is based on the average of the node depths at the end of the link.  They may be and often are different.   However, if you have a Surcharge d/D greater than 1 it will indicate at least one end of the link is surcharged.  A Surcharge d/D may be greater than 1 with a d/D less than 1 due to the ends of the node being surcharged and not surcharged. 

·         A Surcharged d/D indicates that at least one end of the link is Full, but

·         A d/D value less than 1 does not preclude that one end may be Surcharged.


Figure 1.  Plot of d/D and Surcharged d/D in InfoSWMM.