Two Pass InfoSewer Solution
The d/D you are Mapping is from the 1st pass or the solution in which the manhole loads are calculated and the program estimates the flow in the links – the flow gives the value of d/D which you are mapping.
The HGL plot is plotting the adjusted d/D which is the result of the 2nd pass of the solution, this pass estimates backwater, surcharge and pressure and often the value of the adjusted d/D is higher than the 1st pass value of d/D.
The adjusted d/D is a better way of finding those pipes that are more than 0.75 full.
via Blogger http://www.swmm5.net/2013/06/two-pass-infosewer-solution.html