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 conditions, 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, a weir's crest must be in good condition – without nicks or abrasions and free of clinging debris. Since Portable Weir Plate sets are usually moved from site to site, it is important to check each plate's crest before use as they may 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 weir pool's depth.
Weir Flow Rates
Weir Angle |
GPM |
L/S |
---|---|---|
22 1/2º |
3.990 - 67.53 |
0.25148 - 4.318 |
45° |
8.309 - 143.4 |
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.