Mar. 13,1916 
A New Irrigation Weir 
ii 35 
Suppressing the contractions completely or in part changes the law of 
discharge through the triangular notch, decreases its accuracy as a prac¬ 
tical measuring device, and does not insure the complete removal of sand 
and silt from the box. It is therefore an open question whether the 
advantages resulting from suppressed contractions with the triangular 
notch would not be more than counterbalanced by the inaccuracies intro¬ 
duced. The data are given without recommendation, but may be desir¬ 
able for use in special cases. 
3.80 3.70 Coefficient 3 .60 3.50 
(.56 1.55 Exponent 1.54 15$ 
fiote : Coefficients from individual curves plotted • 
Exponents •• <* n „ 0 
Fig. 15.—Coefficient and exponent values of individual discharge equations plotted against weir length. 
DERIVATION OF WEIR FORMULA 
The experimental discharge data for the standard weir conditions were 
plotted logarithmically for weirs having actual crest lengths of 1.0055, 
1.5026, 2.0057, 2.9970, and 4.0056 feet, as shown in figure 14. These 
points do not lie on a straight line. An average straight line drawn 
through the points will give values too small for medium heads and too 
large for low and high heads. This characteristic of the curve is the 
reverse of the curve for rectangular weirs with full contractions, but the 
suppression of the bottom contraction and partial suppression of the end 
contraction has tended to straighten the discharge curve. 
With full-contraction weirs and quite complete pondage, the head 
can be accurately determined and there is, therefore, ample reason for 
using a complicated formula to secure that accuracy of measurement, 
but the high velocity of water and wave action which occurs in the new 
irrigation weir preclude the possibility of determining the head accu- 
