Flow Characteristics of Weir Boxes

Weir Accuracy

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

In particular, a weir's crest must be 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 weir pool's depth.

Weir Flow Equations

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

Weir Angle	cfs	gpm	mgd	l/s	m³/hr
22.5°	0.497 H^2.5	223 H^2.5	0.321 H^2.5	274 H^2.5	988 H^2.5
30°	0.676 H^2.5	303 H^2.5	0.437 H^2.5	373 H^2.5	1,304 H^2.5
45°	1.04 H^2.5	465 H^2.5	0.669 H^2.5	571 H^2.5	2,060 H^2.5
60°	1.44 H^2.5	648 H^2.5	0.9333 H^2.5	797 H^2.5	2,870 H^2.5
90°	2.50 H^2.5	1,120 H^2.5	1.62 H^2.5	1,308 H^2.5	4,970 H^2.5
120°	4.33 H^2.5	1,904 H^2.5	2.80 H^2.5	2,390 H^2.5	8,610 H^2.5

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

cfs	3.33 (L-0.2H) H^1.5
gpm	1,500 (L-0.2H) H^1.5
mgd	2.15 (L-0.2H) H^1.5
l/s	1,840 (L-0.2H) H^1.5
m3/hr	6,620 (L-0.2H) H^1.5

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

cfs	3.33LH^1.5
gpm	1,500 LH^1.5
mgd	2.15 LH^1.5
l/s	1,840 LH^1.5
m3/hr	6,620 LH^1.5

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

cfs	3.37LH^1.5
gpm	1,510LH^1.5
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]

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]

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]

Weir Flow Tables

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

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Flow Characteristics