I am working on the design of a storm sewer system that will utilize a unique inlet. The inlet will function as a curb inlet on the road side and an area inlet on the back side. A depiction of the scenario is shown above. Two drainage areas drain to catch basin AA7 shown. The first drainage area is from a road that runs through the site. The second drainage area is a grassed area behind the inlet. The site will be graded such that the road is crowned and one lane will drain to the catch basin (AA7) and the grassed area is graded to drain to the back side of catch basin AA7. The road side of catch basin AA7 will be a curb inlet, and the back side of inlet AA7 will function as an area inlet.
I am looking for the best way to split the flow from these two areas up. The runoff from the road area needs to be applied to the catch basin in an "on grade" setting such that inlet capture, bypass, and spread are checked. The runoff from the grassed area needs to be applied to the inlet in an "on sump" setting such that it is all captured by the inlet and sent to the system (as the back side of this catch basin will not overflow and go into the roadside gutter. I am not sure how to apply a catchment to this inlet so that all the flow goes into the system and is not treated like a curb and gutter section.