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Weir Accuracy

Laboratory conditions have shown thin-plate weirs to be accurate to within +/-2%.  However, unlike flumes, which are more forgiving to variances in upstream conditions and device condition, weirs are quite susceptible to these variations.  Also, weirs require more maintenance than flumes.  Under field conditions, accuracies of +/- 5-15% are more common.

In particular, the crest of a weir must be kept in good condition – without nicks or abrasions and free of clinging debris.  

The upstream weir pool must also be of the proper size and kept free of sediments, debris, and trash that may change the depth of the weir pool.

Weir Flow Equations

V-notch Weirs [H = head (ft)]

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

1,304 H2.5

45°

1.04 H2.5

465 H2.5

0.669 H2.5

571 H2.5

2,060 H2.5

60°

1.44 H2.5

648 H2.5

0.9333 H2.5

797 H2.5

2,870 H2.5

90°

2.50 H2.5

1,120 H2.5

1.62 H2.5

1,308 H2.5

4,970 H2.5

120°

4.33 H2.5

1,904 H2.5

2.80 H2.5

2,390 H2.5

8,610 H2.5

 

Rectangular Weirs with End Contractions [L = length (ft), H = head (ft)]

cfs

3.33 (L-0.2H) H1.5

gpm

1,500 (L-0.2H) H1.5

mgd

2.15 (L-0.2H) H1.5

l/s

1,840 (L-0.2H) H1.5

m3/hr

6,620 (L-0.2H) H1.5

 

Rectangular Weirs Without End Contractions [L = length (ft), H = head (ft)]

cfs

3.33LH1.5

gpm

1,500 LH1.5

mgd

2.15 LH1.5

l/s

1,840 LH1.5

m3/hr

6,620 LH1.5

 

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

cfs

3.37LH1.5

gpm

1,510LH1.5

mgd

2.18 LH

Weir Minimum / Maximum Head

V-notch Weirs

Weir Angle

Minimum Head

Maximum Head

22.5° - 120º

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]

Weir Flow Tables 

General free-flow discharge tables for thin-plate Weirs can be found in the Discharge Tables section.