Some would prefer to drive the stake with its top precisely level with the bottom edge of gap in dam so that the depth of water in stream may be measured with a rule or steel square placed upon top of this stake at any time after the flow of water has reached its average depth over dam. However the marks upon the stake are preferred by most experts. After the stake has been marked it may be withdrawn and the distance between the first and last marks gives the theoretical flow according to the table, page 391.
Fig. 676.
Measurement in an open stream by velocity and cross section. Measure the depth of the water at from 6 to 12 places across the stream at equal distances apart. Add together all the depths in feet and divide by the number of measurements made; this will be the average depth of the stream, which, multiplied by its width, will give its area or cross section. Multiply this by the velocity of the stream in feet per minute which gives the cubic feet per minute of the stream.
Cubic Feet of water per minute that will flow over a Weir one-inch wide and from 1⁄8 to 207⁄8-inches deep.
| INCHES | 1⁄8 | 1⁄4 | 3⁄8 | 1⁄2 | 5⁄8 | 3⁄4 | 7⁄8 | |
|---|---|---|---|---|---|---|---|---|
| 0 | .00 | .01 | .05 | .09 | .14 | .19 | .26 | .32 |
| 1 | .40 | .47 | .55 | .64 | .73 | .82 | .92 | 1.02 |
| 2 | 1.13 | 1.23 | 1.35 | 1.46 | 1.58 | 1.70 | 1.82 | 1.95 |
| 3 | 2.07 | 2.21 | 2.34 | 2.48 | 2.61 | 2.76 | 2.90 | 3.05 |
| 4 | 3.20 | 3.35 | 3.50 | 3.66 | 3.81 | 3.97 | 4.14 | 4.30 |
| 5 | 4.47 | 4.64 | 4.81 | 4.98 | 5.15 | 5.33 | 5.51 | 5.69 |
| 6 | 5.87 | 6.06 | 6.25 | 6.44 | 6.62 | 6.82 | 7.01 | 7.21 |
| 7 | 7.40 | 7.60 | 7.80 | 8.01 | 8.21 | 8.42 | 8.63 | 8.83 |
| 8 | 9.05 | 9.26 | 9.47 | 9.69 | 9.91 | 10.13 | 10.35 | 10.57 |
| 9 | 10.80 | 11.02 | 11.25 | 11.48 | 11.71 | 11.94 | 12.17 | 12.41 |
| 10 | 12.64 | 12.88 | 13.12 | 13.36 | 13.60 | 13.85 | 14.09 | 14.34 |
| 11 | 14.59 | 14.84 | 15.09 | 15.34 | 15.59 | 15.85 | 16.11 | 16.36 |
| 12 | 16.62 | 16.88 | 17.15 | 17.41 | 17.67 | 17.94 | 18.21 | 18.47 |
| 13 | 18.74 | 19.01 | 19.29 | 19.56 | 19.84 | 20.11 | 20.39 | 20.67 |
| 14 | 20.95 | 21.23 | 21.51 | 21.80 | 22.08 | 22.37 | 22.65 | 22.94 |
| 15 | 23.23 | 23.52 | 23.82 | 24.11 | 24.40 | 24.70 | 25.00 | 25.30 |
| 16 | 25.60 | 25.90 | 26.20 | 26.50 | 26.80 | 27.11 | 27.42 | 27.72 |
| 17 | 28.03 | 28.34 | 28.65 | 28.97 | 29.28 | 29.59 | 29.91 | 30.22 |
| 18 | 30.54 | 30.86 | 31.18 | 31.50 | 31.82 | 32.15 | 32.47 | 32.80 |
| 19 | 33.12 | 33.45 | 33.78 | 34.11 | 34.44 | 34.77 | 35.10 | 35.44 |
| 20 | 35.77 | 36.11 | 36.45 | 36.78 | 37.12 | 37.46 | 37.80 | 38.15 |
Example showing the application of the above table
Suppose the Weir to be 66 inches long, and the depth of water on it to be 115⁄8 inches. Follow down the left hand column of the figures in the table until you come to 11 inches. Then run across the table on a line with the 11 until under 5⁄8 on top line you will find 15.85. This multiplied by 66, the length of Weir, gives 1046.10, the number of cubic feet of water passing per minute.