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Water Flow Estimate Over a Weir

Dishcharge per Foot Dishcharge per Foot
Depth of Weir Width Depth of Weir Width
(inches) cfs gpm (inches) cfs gpm

1/2 .03 13 7 1.48 605
3/4 .05 24 7 1/4 1.56 702
1 .08 36 7 1/2 1.65 738
1 1/4 .11 50 7 3/4 1.73 776
1 1/2 .15 66 8 1.81 814
1 3/4 .19 83 8 1/4 1.90 853
2 .23 102 8 1/2 1.98 890
2 1/4 .27 122 8 3/4 2.07 930
2 1/2 .32 142 9 2.16 971
2 3/4 .36 164 9 1/4 2.25 1012
3 .42 187 9 1/2 2.35 1053
3 1/4 .47 211 9 3/4 2.44 1096
3 1/2 .53 236 10 2.53 1136
3 3/4 .58 262 10 1/4 2.63 1179
4 .64 287 10 1/2 2.73 1223
4 1/4 .70 315 10 3/4 2.82 1268
4 1/2 .77 343 11 2.92 1312
4 3/4 .83 372 11 1/4 3.03 1358
5 .90 402 11 1/2 3.12 1401
5 1/4 .97 433 11 3/4 3.23 1448
5 1/2 1.03 463 12 3.33 1495
5 3/4 1.10 495
6 1.18 528
6 1/4 1.25 562
6 1/2 1.33 596
6 3/4 1.41 631

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*cfs = cubic feet per second*gpm = gallons per minuteUse the above table to estimate the water flow over damboards or similar devices. In a stream without such a device, a wooden board (1 inch x 6 inches or 1 inch x 8 inches) can be placed on edge in the stream from bank to bank so that the water flows over the board. Once the average depth and the width of the water over the weir is measured with a ruler (Figure 1), use the table to estimate the flow.For example:water depth = 1.5 inchesweir width = 3 feetflow estimate = 3 x 66 or 198 gallons per minute
from: King’s “Handbook of Hydraulics”, Fourth Edition, Table
36.

Written By

Photo of John Dorner, IVJohn Dorner, IVArea Agent, Information Management (828) 290-9876 john_dorner@ncsu.eduMadison County, North Carolina
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