Originally, I modeled the catch basins and gutter pan in HydroCAD: The catch basins has pond nodes with appropriate storage attributes to model the CB inlet and then half a street section (to see how high the flow would get in the section view), then an orifice for the inlet itself routing to the outlet pipe. I think I'm doing a good job at modeling capacity on roads with lower grades, but once you get north of 12% or so, there is a chance the velocity of flow could allow runoff to blow by the inlet. This is what I was wanting to check.