(a) The head (H) must be measure upstream from the weir at a
distance equivalent to at least 2.5 times H.
(b) The depth of the water below the notch must be at least 2.5H.
(c) The end contractions on either side of the notch (a) must be not
less than 3/4 L (width of the notch at the water surface with maximum head).
NOTE:
Table 5-2 may be used to convert a measured head in inches to discharge in
gallons per minute. This table is based on the formula: Q = 1130 H2.47,
where Q = quantity of water in gallons per minute and H = measured head in
feet.
Head (H) Discharge (Q)
Head (H) Discharge (Q)
Head (H) Discharge (Q)
Inches
(gpm)
inches
(gpm)
inches
(gpm)
__________________________________________________________________________
1"
(2)
7"
300
16"
2,310
1/4
(4)
1/4
332
1/2
2,480
1/2
(6)
1/2
354
17"
2,670
3/4
(9)
3/4
383
1/2
2,880
2"
(12)
8"
413
18"
3,080
1/4
(17)
1/4
451
1/2
3,290
1/2
24
1/2
486
19"
3,530
3/4
30
3/4
520
1/2
3,760
3"
37
9"
555
20"
4,000
1/4
45
1/4
594
1/2
4,250
1/2
53
1/2
632
21"
4,500
3/4
65
3/4
684
1/2
4,770
4"
76
10"
725
22"
5,060
1/4
88
1/2
815
1/2
5,340
1/2
100
11"
910
23"
5,640
3/4
114
1/2
1,040
1/2
5,950
5"
130
12"
1,130
24"
6,280
1/4
149
1/2
1,250
1/2
6,580
1/2
166
13"
1,380
25"
6,900
3/4
183
1/2
1,520
6"
204
14"
1,650
1/4
225
1/2
1,810
1/2
247
15"
1,970
I3/4
276
1/2
2,120
Table 5-2. Discharge of water over 90 o v-notch weir.
(2) Floating stick method. The floating stick method of measuring stream
flow is an approximate, simple method that may be employed by the preventive
medicine specialist with no equipment other than a measuring device (tape measure or
yard stick) and a watch with a second hand. The principle involved is to measure the
velocity of a stream by means of timing a floating stick as it moves along the surface,
MD0160
5-6