Where can I find documentation on the computation scheme for the StormCAD (GVF-Rational solver) profile calculations?
StormCAD (GVF-Rational solver) uses the Direct Step Method for flow profile calculations, and does not use the Bernoulli equation. The Direct Step Method is discussed in the textbook Stormwater Conveyance (section 7.8, page 275).
If you need to account for dynamic effects in profile calculations, use the Implicit dynamic or Explicit dynamic (SWMM) numerical solvers in SewerGEMS or CivilStorm.
Textbook: "Stormwater Conveyance Modeling and Design"