Weir has almost the same purposes as Orifice, located in the outlet; however, it functions differently. Weir raises the water level and forces it to spill over.
VOSWMM encompasses five types of weirs, each utilizing a distinct formula for calculating flow based on area, discharge coefficient, and head difference across the weir:
Transverse (rectangular shape)
Side flow (rectangular shape)
V-notch (triangular shape)
Trapezoidal (trapezoidal shape)
Roadway (broad-crested rectangular weir for modeling roadway crossings)
Weirs can operate as outlets for storage units across various flow routing scenarios. In the absence of attachment to a storage unit, they are exclusive to drainage networks analyzed using Dynamic Wave flow routing.
The dynamic control of the weir crest’s height above the inlet node invert is possible through user-defined Control Rules, offering a means to simulate inflatable dams.
Weirs may be permitted or restricted from surcharging. A surcharged weir employs an equivalent orifice equation for flow computation. Generally, weirs in open channels are not allowed to surcharge, whereas those in closed diversion structures or mimicking storm drain inlet openings may be authorized to do so.
Key parameters for a weir encompass:
- Names of its inlet and outlet nodes
- Shape and geometry
- Crest height above the inlet node invert
- Discharge coefficient