How can I model a situation in StormCAD (or the GVF-Rational solver in SewerGEMS or CivilStorm) where an inlet is in sag, but flow can overtop the sag and pass on to the next downstream inlet? In other words, the inlet acts as in-sag with flow ponding up to a certain overflow/curb depth, above which all the additional incoming flow passes downstream to the next inlet. Illustration below:
There is no built-in method to tell StormCAD that an inlet is in sag but can "overtop" and have overflow that passes downstream.
One solution is to set the inlet type to "inflow-capture curve", set the Inlet Location to On Grade, then configure the table of inlet flow vs. captured flow to mimic this inlet's performance.
First, run a series of tests with a regular in-sag inlet (or a worksheet in FlowMaster) to determine the inflow above which the overtopping condition occurs (based on the computed gutter depth)
Next, in the inflow-capture curve, set both the inlet flow and capture flow equal for the first couple rows in the inflow-capture curve. Above that, you could keep the "captured flow" portion equal to the flow that was captured right at the overflow point, and the remainder will then become bypass and flow down the downstream gutter link element. In the example screenshot below, the overtopping condition occurs above a flow of 5 CFS. Ensure that you set the catchbasin "Inlet Location" to "On Grade". Otherwise if you keep it as "In Sag", it will always have a capture efficiency of 100% instead of using the rating table.
Note: you can also use the "Flow to Inlet vs. Flow Captured" Inlet Type in the Inlet Catalog, for the same functionality.