Drive a stake above the dam at a distance of about six feet from the face of the plank and then obstruct the water until it rises precisely to the bottom of the gap and mark the water level on the stake. Complete the dam so that all the water will be compelled to flow through the gap and when the stream has assumed a regular flow mark the stake at this new level.
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 After the stake has been marked it may be withdrawn and the distance between the first and last marks gives the theoretical flow according to the table, page 391.
_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 20-7/8-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 ----+-------+-------+-------+-------+-------+-------+-------+------