Weirs
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 GeoSWMM 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 across the weir as listed in Table 8.
Table 8: Available types of Weirs
Weir Type | Cross Section Shape | Flow Formula |
|---|---|---|
Transverse | Rectangular | CwLH3/2 |
Side Flow | Rectangular | CwLH5/3 |
V-Notch | Triangular | CwSH5/2 |
Trapezoidal | Trapezoidal | CwLH3/2 + CwsSH5/2 |
Cw Weir discharge coefficient, L = Weir length, S = Side slope of V-notch or trapezoidal weir, H = head difference across the weir, Cws= Discharge coefficient through sides of trapezoidal weir | ||
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 Control Rules. This feature can be used to model inflatable dams.
The principal input parameters for a weir include:
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Names of its inlet and outlet nodes
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Shape and geometry
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Crest height or elevation above the inlet node invert
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Discharge coefficient