Whenever the downstream conduit is undersized with respect to the standpipe capacity, full riser barrel flow will occur if the pond water surface elevation rises high enough. In these cases, the program assumes a negligible loss through the riser barrel and sets the riser flow equal to the downstream conduit flow rate.

If there are other orifices (perforations), slots (weirs), etc, flowing into the riser or inlet box, their flow rates are set equal to zero since the upstream elevation (pond water surface) and downstream elevation (inlet box headwater elevation) are identical (i.e., drop in head equals zero across these elements).

A Note on Perforations and Slots in Risers

If components of a particular composite outlet structure contains a riser structure in addition to orifice and weirs with elevations lower then the crest elevation riser, then Bentley StormCAD treats the orifices and weirs as perforations and slots in the riser structure, and calculates the overall composite structure accordingly.

Related Topics

• "Weir Stage"-367
• "Orifice Stage"-368