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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.  As Portable Weir Plate sets are intended to be moved from site to site, it is important to check the crest of each plate before use as it is possible that they have been nicked or damaged during transit (probably rolling around in the back of a pick-up bed).

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 Rates

Weir Angle

GPM

L/S

22 1/2º

3.990 - 67.53

0.25148 - 4.318

45°

8.309 - 39.44

0.5039 - 9.051

90º 20.07 - 346.5 1.217 - 21.86

1-foot Cipolletti

135.1 - 534.2

8.196 - 32.40

 

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

45°

1.04 H2.5

465 H2.5

0.669 H2.5

571 H2.5

2,060 H2.5

90°

2.50 H2.5

1,120 H2.5

1.62 H2.5

1,308 H2.5

4,970 H2.5

 

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]

0.62’ [18.90 cm]

 

Cipolletti

Crest Length

Minimum Head

Maximum Head

1’

0.2’ [6.096 cm]

0.5’ [15.24 cm]

Weir Flow Tables 

Free-flow discharge tables specific to the interchangeable weir plates provided with the standard Portable Weir Plate set can be found in the Discharge Tables section.