A Method of determining the Cross-section of an Open Channel 
by Ganguillet and Kutter’s Formula. 
BY 
Seinen Yokota. 
With Plate I—IV and two Figures in the Text. 
A problem that oeenrs when arranging the distribution of irrigation 
water is how to find the best form and size for an open channel, being 
given the discharge per unit time. The form of its cross section com- 
monly adopted is a trapezoid with its sloping sides more or less inclined 
to the vertieal. 
To solve the problem, the bottom breadth is usnally guessed at and 
the inclination of the sides fixed according to the nature of the earth 
in which the channel is to be eycavated. The depth of water, being 
another variable, is also assumed at a convenient amount, and the cross- 
section A and the wetted perimeter p are then caleulated. The former 
divided by the latter is called the hydraulie mean radius or depth. Then 
the mean velocity of flow is ealeulated by Ganguillet and Kutter’s For- 
mula, as being the most trustworthy among several. This mean velocity 
multiplied by the cross section gives the discharge per unit time. If 
this discharge does not give what is wanted, the bottom breadth or the 
depth of water is varied, and the above process repeated until the dis- 
charge comes out to be in agreement with requirements. 
This tentative process is sometimes rather laborious, even with the 
aid of tables giving the coefticients of “R.J for several values of n. 
J & R in the Ganguillet and Kutter’s Formula, viz.:— 
