You can use the orifice equation if both sides of the pipe are submurged, flowing full, and not over 60ft long. Over 60 ft you have factor friction loss. Otherwise you need to do it as a culvert.
jgailla (Geotechnical) has the right answer. You really dont have a sheet flow depth over 0.1'. You would have some sort of shallow concentrated flow that would be produced at some point.
It can be modeled like a pond. Your flow would be a weir calc or an orifice depending on the head on the inv. Model it like a riser only scenario in your modeling software.
