Note:  Attached is an example SWMM 5 model that has all 57 culvert types possible in SWMM 5 in one model.  The culverts are 57 small individual networks consisting of an inflow node, an upstream open channel, upstream node for the culvert, culvert link with culvert code, downstream node of the culvert, downstream open channel and finally an outfall node.  The culvert code and the shape of the culvert determine which FHWA equation is used to determine the flow INTO the Culvert during the simulation:

1.   The flow from the St Venant Equation or

2.   The flow from the FHWA equation

The minimum flow is used by the program. 

via Blogger http://www.swmm5.net/2013/08/all-possible-culverts-example-model-in.html