The most common method of measuring the rate of flow in an open channel is through the use of hydraulic structures. Hydraulic structures are specially shaped, static devices over or through which flow is directed that under free-flow conditions generate a know relationship between the water level (head) at a single specified location and the flow rate.
The hydraulic structures used in measuring open channel flows are known as primary devices and may be roughly divided into two categories: flumes and weirs.
A flume is a specially shaped structure with an area and/or slope that is different from the channel in which it is placed. The result is an acceleration and change in the head of the flow through the flume. Acceleration is provided by converging the sidewalls, raising the bottom, or a combination of both. A flume normally consists of a converging section, a throat section, and a diverging section.
Flume head loss is less than about one-fourth of that needed to operate a sharp-crested weir having the same control width. Another advantage compared to most standard weirs is that for a properly designed and installed flume, the velocity of approach is a part of the calibration equations. Unauthorized altering of the dimensions of constructed flumes to obtain an unfair share of water is difficult and, therefore, not likely. Unlike weirs, most flume styles allow for the ready passage of sedimentation and floating debris – reducing the time and effort associated with maintaining a flume.
Additional information on flumes may be found here.
A weir is essentially a dam across an open channel over which water flows, typically through a notch or opening. Weirs divided into sharp-crested (thin plate) and broad-crested types. Sharp-crested weirs are further classified according to shape and geometry of the notch (i.e. 45° V-notch, 12" [30.48 cm] Rectangular with End Contractions, 36" [941.44 cm] Cipolletti/Trapezoidal). The simple design and low fabrication (although not necessarily installation) cost has lead to the widespread use of weirs in open channel flow measurement.
The discharge through a weir is proportional to the head on the crest and is affected by: the upstream weir pool, the condition of the crest, the contraction, the approach velocity, and the elevation of the water surface downstream of the weir. To perform accurately, the body of water upstream of the weir (the weir pool) must be carefully formed and [for sharp-crested (thins plate) weirs] the maximum head (Hmax) should not exceed 24" [60.96 cm].
The obstructive nature of weirs may result in sedimentation and inadvertent screening of floating debris. Additionally, as the crest (flow surface) is critical to the proper operation of the weir, vegetative debris, biological growth, and damage to the crest may all result in inaccurate discharges over a weir.
Pre-fabricated weirs are almost exclusively of the sharp-crested (thin plate) type and are provided as either portable, interchangeable weir plate carriers (for stream gauge and runoff monitoring) or installed in weir boxes (for piped flows just below grade or at/above grade).
Additional information on weirs may be found here.