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 SWMM 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.
Five 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)
- Side flow (rectangular shape)
- V-notch (triangular shape)
- Trapezoidal (trapezoidal shape)
- Roadway (broad crested rectangular weir used to model roadway crossings).
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.
Weirs can either be allowed to surcharge or not. A surcharged weir will use an equivalent orifice equation to compute the flow through it. Weirs placed in open channels would normally not be allowed to surcharge while those placed in closed diversion structures or those used to represent storm drain inlet openings would be allowed to.
The input parameters for a Weirs are listed in the table below:
| Field | Description |
| Name | User-assigned weir name. |
| Start Node | Name of node on inlet side of weir. If the start node is not a Storage Unit, the routing model must be set to "Dynamic Wave" in model options. |
| End Node | Name of node on outlet side of weir. |
| Type | Weir type: TRANSVERSE, SIDEFLOW, V-NOTCH, TRAPEZOIDAL or ROADWAY. |
| Height | Vertical height of weir opening (feet or meters) |
| Length | Horizontal length of weir opening (feet or meters) |
| Side Slope | Slope (width-to-height) of side walls for a V-NOTCH or TRAPEZOIDAL weir. |
| Inlet Offset | Depth or elevation of bottom of weir opening from invert of inlet node (feet or meters). |
| Discharge Coeff. | Discharge coefficient for flow through the central portion of the weir (for flow in CFS when using US units or CMS when using SI units). Typical values are: 3.33 US (1.84 SI) for sharp crested transverse weirs, 2.5 - 3.3 US (1.38 - 1.83 SI) for broad crested rectangular weirs, 2.4 - 2.8 US (1.35 - 1.55 SI) for V-notch (triangular) weirs. Discharge over Roadway weirs with a non-zero road width is computed using the FHWA HDS-5 method. |
| Flap Gate | YES if the weir has a flap gate that prevents backflow, NO if it does not. |
| End Coeff. | Discharge coefficient for flow through the triangular ends of a TRAPEZOIDAL weir. See the recommended values for V-notch weirs listed above. |
| End Contractions | Number of end contractions for a TRANSVERSE or SIDEFLOW weir whose length is shorter than the channel it is placed in. Either 0, 1, or 2 depending if no ends, one end, or both ends are beveled in from the side walls. |
| Can Surcharge | YES if the weir can surcharge (have an upstream water level higher than the height of the opening) or NO if it cannot. If YES, when the upstream water level is higher than the opening, the weir will be treated as an orifice. |
| Coeff. Curve | Name of an optional WEIR type curve that allows the central Discharge Coefficient to vary as a function of head (in feet or meters). Does not apply to Roadway weirs. |
| ROADWAY WEIR | |
| Road Width | Width of roadway and shoulders (feet or meters) |
| Road Surface | Type of road surface: PAVED or GRAVEL. |