Question: How does SWMM5 solve the 1D St Venant Equation in its conceptual Link/Node Network?

Answer: See these graphs of the solved for variables in the solution.

1/ There is also an ArcEngine version of InfoSWMM call InfoSWMM SA. I will SA to show some hidden features of the 1D St Venant solution for SWMM5, InfoSWMM, and ICM SWMM Networks by showing the output graphs and reports of SA

2/ ArcEngine InfoSWMM SA 3.0 uses the 10.7 version from ESRI. InfoSWMM on the other hand uses ArcMap 10.3 to 10.8

3/ In case you always wondered how ArcMap is related to other ArcGIS ESRI software you can read ===ArcMap, for viewing and editing spatial data in two dimensions and creating two-dimensional maps; https://en.wikipedia.org/wiki/ArcGIS?oldformat=true…

4/ There is only ONE Q or Flow in the middle of the link in the SWMM5 engine but it uses the upstream and downstream hydraulic information for the link. Output possible or used in SWMM5 in the middle of the link includes Flow, Depth, Capacity, Froude Number

5/ As a consequence of having the Flow computed in the middle of the link associated data such as depth and capacity is also in the middle of the link. Capacity is the middle cross-sectional area / the full flow area

6/ In SWMM5 the velocity is a computed output based on the Flow in the middle of the link and the depth-related cross-sectional area in the middle of the link

7/ SWMM5 shows the capacity or area / full area but in InfoSWMM you can also see the Surcharged d/D which is the average of the upstream and downstream depths. Capacity is between 0 and 1 but the Surcharge d/D can be greater than 1.

8/ Flow volume is SWMM5 is a computed output parameter as is the Froude Number but the Froude Number is used along with the Keep, Dampen and Ignore parameters to drop or not use the non-linear term in the St Venant Equation solution

9/ Other computed SWMM5 output can include the upstream and downstream velocity which is computed from the middle of the link Flow or Q and the upstream and downstream cross-sectional areas

10/ Used by the 1D St Venant solution is the upstream and downstream cross-sectional areas which are found from the upstream and downstream depths and used in the nonlinear term of the solution.

11/ The upstream and downstream depths for a link in SWMM5 is based on node area rules and the upstream and downstream node depths. The depth is the node depth unless there are offset and the normal depth or critical depth