Since the overflow will provide low head pushing out, I would remove the orifice flow constraint by modeling a broad crested weir with a length = to the higher of the frame's inside perimeter, or teh perimeter of the area at the grade break, if the inlet is sumped.  The breadth would be the width of that frame or grade break.

Make sure it is Secondary discharge, or the overflow will go to the device specified, or keep adding to the final outflow if Primary discharge is specified.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

The key is to determine what will control the overflow.  As LHA suggests, the CB overflow is often modeled as a weir.  In some cases this may be at the rim of the CB, but in other cases it may be a higher low-point in the surrounding pavement.

If the control point is above the rim, you may also want to define the additional above-grade storage (as part of the CB/Pond) since it can have a significant effect on the hydrograph routing.

For further details please see www.hydrocad.net/pipes.htm