Diversion link not being "designed" / ignored by the GVF-Convex / GVF-Rational solver in SewerGEMS / SewerCAD / StormCAD
There are a couple of reasons why the diversion link is not “designed” by the GVF-Convex / GVF-Rational Solver. The diversion link is predefined to carry a certain amount of flow as per the rating curve specified. For the GVF Convex / GVF-Rational Solver the diversion link is treated as a virtual conduit. Hence it is not considered in designing of both size and inverts.
Basically, the program cannot design diversion links because if the start and stop elevations of the diversion link were to be changed it would change the approach flow vs. diverted flow relationship defined and since constraint based design is not able to alter the rating table the design would fail. Hence, the program assumes that if you have established a specific split flow rating table, then the outgoing pipes must be in a fixed position in order to achieve that specific rating table.
However, if you do wish to design (size) the diversion link, you can split the diversion into a very short pipe with a negligible length and the downstream pipe which is your actual pipe (with the actual length). This way your actual pipe could be sized as per the diverted flow since it is not a virtual pipe like the diversion link.
Here is a typical setup to achieve this;
In the above snapshot it can be seen that conduit CO-5 was a diversion link initially but was split using a transition element (T-1) into two conduits where CO-5(1) is the diversion link (with negligible lenght) and CO-5(2) which is not a diversion link (with actual length).
Modeling a flow split (diversion) with the SewerCAD or StormCAD numerical solver
Troubleshooting Constraint Based Design Results