Weirs in SWMM5 and InfoSWMM

Weirs, like orifices, are used to model outlet and diversion structures in a drainage system. Weirs are typically located in a manhole, along the side of a channel, or within a storage unit. They are internally represented in InfoSWMM and InfoSWMM SA as a link connecting two nodes, where the weir itself is placed at the upstream node. A flap gate can be included to prevent backflow.

Four varieties of weirs are available, each incorporating a different formula for computing flow as a function of area, discharge coefficient and head difference across the weir:

• Transverse (rectangular shape)
• Sideflow (rectangular shape)
• V-notch (triangular shape)
• Trapezoidal (trapezoidal shape)

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

The height of the weir crest above the inlet node invert can be controlled dynamically through user-defined Real-Time Control Rules (RTC Rules). This feature can be used to model inflatable dams.

Input parameters for a weir include:

• names of its inlet and outlet nodes
• shape parameters
• height above the inlet node invert
• discharge coefficient.

Computed results consist of:

• Flow rate
• Concentration of each water quality constituent.

Diagrams: 

Note: Weirs can be used as storage unit outlets under all types of flow routing. If not attached to a storage unit, 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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