Get Help

Share

Flow Characteristics

To obtain the best accuracy possible when designing and installing a weir, the following should be observed:

Weir Box (for Above Ground Mounting) with V-Notch Weir Plate

  1. The minimum head should be 0.2’ (2.4”) [6.096 cm] or greater
  2. All edges and corners of the weir must be sharp
  3. Weir edges should be straight, smooth, and free of burs
  4. Weir edges should be kept clean and free from rust, debris, fouling growth, and ice build-up
  5. The nappe of the weir must be adequately aerated so as to ensure that it does not cling to the downstream face of the weir
  6. The channel should have a free-fall of 6” [15.24 cm] downstream of the crest of the weir
  7. The upstream face of the weir must be vertical
  8. Upstream weir pools must be of the proper size, be cleaned of sediment periodically, and kept free of trash and weeds

Thin-Plate Weir Free-Flow Discharge Equations

(V-notch weirs)

Weir Angle

cfs

gpm

mgd

l/s

m3/hr

22.5°

0.497 H2.5

223 H2.5

0.321 H2.5

274 H2.5

988 H2.5

30°

0.676 H2.5

303 H2.5

0.437 H2.5

373 H2.5

1304 H2.5

45°

1.04 H2.5

465 H2.5

0.669 H2.5

571 H2.5

2060 H2.5

60°

1.44 H2.5

648 H2.5

0.9333 H2.5

797 H2.5

2870 H2.5

90°

2.50 H2.5

1120 H2.5

1.62 H2.5

1308 H2.5

4970 H2.5

120°

4.33 H2.5

1904 H2.5

2.80 H2.5

2390 H2.5

8610 H2.5

(Rectangular weirs with end contractions) (L = length (ft), H = head (ft))

cfs

3.33(L-0.2H) H1.5

gpm

1500(L-0.2H) H1.5

mgd

2.15(L-0.2H) H1.5

l/s

1840(L-0.2H) H1.5

m3/hr

6620(L-0.2H) H1.5

(Rectangular weirs without end contractions)(L = length (ft), H = head (ft))

cfs

3.33LH1.5

gpm

1500 LH1.5

mgd

2.15 LH1.5

l/s

1840 LH1.5

m3/hr

6620 LH1.5

(Cipolletti) (L = length (ft), H = head (ft))

cfs

3.37LH1.5

gpm

1510LH1.5

mgd

2.18 LH

Accuracy

Under laboratory conditions, a properly installed weir can typically achieve accuracies of 2 to 5%; under most field conditions accuracies within 5 to 15% may be expected. However, the overall accuracy obtained from the flow measurement systems (the weir and flow meter) can be affected by a number of factors, including: faulty construction or installation of the weir, improper head measuring location, incorrect zero setting of the flow meter, flow meter level measurement inaccuracies, flow meter level-to-flow conversion inaccuracies, use of the weir outside its proper range, improper maintenance of the weir, and turbulence and surges in the approach channel.

Weirs crests should be free of rust and nicks. Any form of roughness will cause the weir to discharge more water than indicated in the standard discharge tables. Additionally, air should circulate freely under the nappe. An increase in discharge as much as 25% may occur if the nappe is not properly ventilated.

Flow Velocity

The approach velocity for weirs should not exceed 0.5 ft./s [0.1524 m/s]. Above this level, site-specific calibration adjustments must be developed.

Minimum and Maximum Head

At very low heads, flow over a weir may become quite unstable and errors or inconsistencies in measurement will occur. There is general agreement among investigators that due to viscous drag and the tendency of the nappe to adhere to the crest of the weir that heads of less than 0.2’ [6.096 cm] will not produce reliable results.

The flow formulas for weirs have been developed empirically and are not necessarily suitable to extrapolation. Most of the data for weirs have been derived for heads up to 2.0’ [0.6096 m]. Although some data is available for higher heads, general agreement is that a 2.0’ [0.6096 m] head should not be exceeded for any weir if good quality results are desired.

(V-notch weirs)

Weir Angle

Minimum Head

Maximum Head

22.5°

0.2’ [6.096 cm]

2.0’ [60.96 cm]

30 °

0.2’ [6.096 cm]

2.0’ [60.96 cm]

45 °

0.2’ [6.096 cm]

2.0’ [60.96 cm]

60 °

0.2’ [6.096 cm]

2.0’ [60.96 cm]

90 °

0.2’ [6.096 cm]

2.0’ [60.96 cm]

120 °

0.2’ [6.096 cm]

2.0’ [60.96 cm]

(Rectangular Weir with End Contractions)

Crest Length

Minimum Head

Maximum Head

1’

0.2’ [6.096 cm]

0.5’ [15.24 cm]

1.5’

0.2’ [6.096 cm]

0.75’ [22.86 cm]

2’

0.2’ [6.096 cm]

1’ [30.48 cm]

2.5’

0.2’ [6.096 cm]

1.25’ [38.10 cm]

3’

0.2’ [6.096 cm]

1.5’ [45.72 cm]

4’

0.2’ [6.096 cm]

2’ [60.96 cm]

5’

0.2’ [6.096 cm]

2.5’ [76.20 cm]

6’

0.2’ [6.096 cm]

3’ [91.44 cm]

8’

0.2’ [6.096 cm]

4’ [121.9 cm]

10’

0.2’ [6.096 cm]

5’ [152.4 cm]

(Rectangular Weir without End Contractions)

Crest Length

Minimum Head

Maximum Head

1’

0.2’ [6.096 cm]

0.5’ [15.24 cm]

1.5’

0.2’ [6.096 cm]

0.75’ [22.86 cm]

2’

0.2’ [6.096 cm]

1’ [30.48 cm]

2.5’

0.2’ [6.096 cm]

1.25’ [38.10 cm]

3’

0.2’ [6.096 cm]

1.5’ [45.72 cm]

4’

0.2’ [6.096 cm]

2’ [60.96 cm]

5’

0.2’ [6.096 cm]

2.5’ [76.20 cm]

6’

0.2’ [6.096 cm]

3’ [91.44 cm]

8’

0.2’ [6.096 cm]

4’ [121.9 cm]

10’

0.2’ [6.096 cm]

5’ [152.4 cm]

(Cipolletti)

Crest Length

Minimum Head

Maximum Head

1’

0.2’ [6.096 cm]

0.5’ [15.24 cm]

1.5’

0.2’ [6.096 cm]

0.75’ [22.86 cm]

2’

0.2’ [6.096 cm]

1’ [30.48 cm]

2.5’

0.2’ [6.096 cm]

1.25’ [38.10 cm]

3’

0.2’ [6.096 cm]

1.5’ [45.72 cm]

4’

0.2’ [6.096 cm]

2’ [60.96 cm]

5’

0.2’ [6.096 cm]

2.5’ [76.20 cm]

6’

0.2’ [6.096 cm]

3’ [91.44 cm]

8’

0.2’ [6.096 cm]

4’ [121.9 cm]

10’

0.2’ [6.096 cm]

5’ [152.4 cm]

Sources:

Scott, V. and Houston, C., Measuring Irrigation Water, California Experiment Station, Circular 473, 1959.

Skrenter, R., Instrumentation Handbook Water and Wastewater Treatment Plants.

Spitzer, D. (editor), Flow Measurement: Practical Guides for Measurement and Control, 2nd Edition.

Thomas, C., Errors in Measurement of Irrigation Water, Journal of Irrigation and Drainage Division, Vol. 83, Paper 1362, September 1957.

TIP 0410-01- TAPPI.

Warren, H., Water Measurement Structures: Chapter 5, Design of Small Canal Structures, Water Resources Publications, 1978 [1995].