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)

Parameter Name Description Default Value
NHYD Hydrograph number.
NAME Name of the component.
From Node The name of the from node.
To Node The name of the destinated node.
COMMENT 1, 2, and 3 Comment for the command.
Tag Optional category or classification.
Type Type of weir (Transverse/Sideflow/Vnotch/Trapezoidal/Roadway). transverse
Height [m] The vertical height of the weir opening. 1
Length [m] The horizontal length of the weir crest (or crown for V-NOTCH weir). 1
Slope Side Slope (width-to-height) of a TRAPEZOIDAL weir side wall.
Inlet Offset [m] Depth of bottom of weir opening from inlet node invert. 0
Discharge Coeff. [CMS] Discharge coefficient for the central portion of the weir. 3.33
Flap Gate Select YES if the weir contains a flap gate to prevent backflow. NO
End Contraction Number of end contractions. 0
End Coeff. [CMS] Discharge coefficient for flow through the triangular ends of a TRAPEZOIDAL weir.
Can Surcharge? Whether it can surcharge or not (Yes/No). NO
Coeff. Curve Click on … to enter Head-Coefficient data for the weir.

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