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Under controlled conditions, thin-plate weirs can be accurate to +/-2 to 5%.   Field conditions can vary widely, as can weir accuracy (+/-5 to 15%).

Faulty construction, poor maintenance, and too small upstream weir pools all contribute to field measurement errors.

Weirs are designed for head measurements between 0.2-feet [6.096 cm] and of 2-feet [60.96 cm].  Below 0.2-feet and the nappe (body of water flowing over the weir edge) tends to cling to the downstream face of the weir (making the installation underindicate the actual level/flow rate).

Minimum Guidelines for Flow Measurement with a Weir  

  • The weir pool must be properly sized and kept free of debris, trash, weeds, and sediment (refer to Upstream Conditions for additional details)
  • The weir crest must be vertically and horizontally straight
  • The weir crest must be square, straight, smooth, and free of burs, nicks, or abrasions
  • The weir crest must be kept clear of rust, debris, grow, and ice
  • The nappe of the weir must spring clear of the crest and not adhere to the downstream face of the weir
  • There should be at least 2-inches [5.08 cm], and preferably 6-inches [15.24 cm] of distance from the lowest most portions of the weir crest and the downstream water level.
  • The approach velocity should not exceed 0.5 ft/s [0.1524 m/s].

Thin-Plate Weir Free-Flow Discharge Equations

 

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

 

 

 

(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]