Unfortunately, I don't have anything that will do what you are looking for. However, I do have a couple of Mathcad Prime 3.0 worksheets and an Excel spreadsheet that will calculate the hydraulic capacity of a street cross-section, in case you or anyone else is interested. They are available here:
BTW, the .zip file includes .pdf print-outs of the two Mathcad worksheet for users of other versions of Mathcad or who just want to see how the calculations are set up.
Good luck in your quest.
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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill
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.
If the inlets are on grade, you are not modeling the capture correctly by just an orifice (even on milder slopes). It would be prudent to read HEC-22 that jgailla posted.
FHWA Hydraulic Toolbox software can compute gutter flow capacity and inlet capture on grade.
