+———+ +———+ | \ FIG. 13 / | | \ / | | \ / | | \ / | | \ / | | \ / | | \ / | | \ / | | \ / | | \/ | | | | | | | | | | | +—————————————————+
FIG. 14.-ELEVATION OF TRIANGULAR NOTCHED GAUGING WEIR.
FIG. 15.-LONGITUDINAL SECTION, SHOWING WEIR, GAUGE-PEG, AND HOOK-GAUGE
If it is required to find the discharge in similar terms with a depth of water of 20 in, two sets of calculations are required. First 20 in depth on the notch 6 in wide, and then 4 in depth on the notch, 28 in minus 6 in, or 1 ft wide.
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(1) C\/ H^3 = 4.69/2 \/ 10^3 = 2.345 x 31.62 = 74.15
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(2) C\/ H^3 = 1.0 x 4.69 \/ 4^3 = 1.0 x 4.69 x 8 = 37.52
Total in c. ft per min = 111.67
The actual discharge would be slightly in excess of this.
In addition to the circumstances already enumerated which affect the accuracy of gaugings taken by means of a weir fixed in a sewer there is also the fact that the sewage approaches the weir with a velocity which varies considerably from time to time. In order to make allowance for this, the head calculated to produce the velocity must be added to the actual head. This can be embodied in the formula, as, for example, Santo Crimp's formula for discharge in cubic feet per minute, with H measured in feet, is written
__________________ 195\/(11^3 + .035V - H^2
instead of the usual form of
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195\/ H^3, which is used