The value of d/D in InfoSWMM is calculated as Link capacity or the Midpoint Capacity. d/D is defined as the capacity or current cross-sectional area at the center of the link over the full cross-sectional capacity at full depth. The midpoint cross-sectional area is based on the mean depth in the center of the link and the average depth is the average of the upstream and downstream ends of the pipe.
Whereas the Surcharged d/D is calculated from the end node depths or
Surcharged d/D = Average depth in the middle of a link or ½ (Upstream Depth + Downstream Depth) /Maximum depth
Midpoint Capacity = the midpoint cross-sectional area (based on the average depth) / the full cross sectional area
If you look at the reports d/D or Midpoint Capacity is not
quite the same as the Surcharged d/D which is based on the upstream and downstream depths and not the Capacity (a function of Area). I hope this explains why Surcharged d/D is equal to the Depth and not the same as the d/D or Midpoint Capacity.
|Inverse Color Attribute Browser in InfoSWMM and H2OMap SWMM showing various output data.