Weir control

A weir is a raised section of the bed of a channel, which may be a narrow plate (sharp-crested weir) or a block (broad-crested weir). Weirs are commonly used as flow control devices to raise upstream water levels in channels, and as measuring devices.

Weir flows can occur naturally in channels. In urban stormwater drainage systems, they occur at detention basin embankments, at overtopped culverts or bridges, and at obstructions encountered by overland flows, such as road crowns.

A weir creates a hydraulic control that determines water levels throughout a channel. For a weir with a rectangular shape or for flow over a long crest, the flowrate Q is related to the depth just upstream of the control by a relationship of the form:

Where Q is flowrate (m^₃/s), C is a weir coefficient, w is the width (m) of the weir section over which flow occurs, h_w is the depth (m).

Slightly different relationships apply to triangular V-notch and other non-rectangular weirs.

Weir coefficients depend on the efficiency of the weir, which is related to the smoothness of flow paths over it. A weir is more efficient if it requires less head to convey a given flowrate. Possible values for weir coefficients are shown below:

Generally, sharp-crested weirs, such as plates or walls, will have lower coefficients than broad-crested weirs (a broad-crested weir is one in which the flow passes through critical depth somewhere along the crest).

In the unsteady flow calculations in DRAINS, weir controls must be provided at sag pits, with the weir being the obstruction that is causing water to pond over a sag pit. This may be a road crown or a wall. Details of this are provided in the Overflow Route property sheet.

A weir control occurs at stormwater pits and other entrances to piped systems, as long as the upstream depth is not too high. If flow increases or a backwater effect raises water depths, water may cover the inlet and a vortex will form, sucking air into the pipes. A point will be reached where the inlet is covered and no air is sucked in. At this point, orifice flow is established. There is a change in the depth-flowrate relationship when the flow state changes from weir to orifice control. This can be abrupt with smaller inlets such as those used to drain flat roof areas, but is less well-defined in larger inlet pits.

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