Orifices in InfoSWMM and SWMM5

Orifices are used to model outlet and diversion structures in drainage systems, which are typically openings in the wall of a manhole, storage facility, or control gate. They are internally represented in InfoSWMM and InfoSWMM SA as a link connecting two nodes. An orifice can have either a circular or rectangular shape, be located either at the bottom or along the side of the upstream node, and have a flap gate to prevent backflow.

The flow through an orifice is computed based on the area of its opening, its discharge coefficient, and the head difference across the orifice. Orifice calculations are quite rigorous in InfoSWMM InfoSWMM SAand are treated as more as weirs under low-head conditions.

The area of an orifice's opening can be controlled dynamically through user-defined Real-Time Control Rules (RTC Rules).

The principal input parameters for an orifice include:

• names of its inlet and outlet nodes
• position (bottom or side)
• shape (circular or rectangular)
• dimensions (diameter or width and height)
• height above the inlet node invert (crest height)
• discharge coefficient (default = 0.65)

Computed output consists of:

• flow rate based on:

  

  Note: If the orifice is not submerged, the hydraulic engine will simulate the condition as weir flow.

• concentration of each water quality constituent.

Diagrams:

Note: Orifices can be used as storage unit outlets under all types of flow routing. If not attached to a storage unit node, they can only be used in drainage networks that are analyzed with Dynamic Wave flow routing.

Excerpt from the EPA manual Storm Water Management Model Reference Manual Volume II – Hydraulics (PDF)

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