A weir is a small dam built across a river to control the upstream water level. Unlike large dams which create reservoirs, the goal of building a weir across a river isn’t to create storage, but only to gain some control over the water level. One feature of a weir is that water flows over its crest. Using the width of the weir and the water depth as it flows over the crest, the volume of flowing water can be calculated.
The shape of the weir construction will determine the formula to be used. Below is a table of water flow for a rectangular 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 |
*cfs = cubic feet per second
*gpm = gallons per minute
Use 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, use the table to estimate the flow.
For example:
Water depth = 1.5 inches
Weir width = 3 feet
Flow estimate = 3 x 66 or 198 gallons per minute
*from: King's "Handbook of Hydraulics", Fourth Edition, Table 36.
