Fig. 10.1 Ogee fall
The falls on the Western Yamuna canal were in the form of ‘rapids’ (Fig. 10.2) which were gently sloping floors constructed at a slope of about 1 in 10 to 1 in 20.These rapids worked satisfactorily and permitted the movement of timber logs also. But, these structures were very expensive due to their longer length.
U/S BED
D/S BED
Fig. 10.2 Rapids
The realisation of the importance of a raised crest wall at the location of fall in reducing the drawdown resulted in the design of trapezoidal notch fall (Fig. 10.3). The fall structure had a number of trapezoidal notches in a high breast wall constructed across the channel having smooth entrance and a flat lip projecting downstream to spread out the falling jet. Such falls were very popular till simpler and economical falls were developed.
354 IRRIGATION AND WATER RESOURCES ENGINEERING
Plane of notch profile
Setback
Lip
Plan
(a) Trapezoidal notch fall
2
H
1
L
Q
(b) Dimensions of trapezoidal notch fall
(c) Depth-discharge relation
Fig. 10.3 Trapezoidal notch fall
After World War I, the Sarda and glacis falls were developed in UP and Punjab, respectively. Of these two, the former proved to be more successful. The glacis fall gave trouble because of the resulting increase in discharge per unit width on account of fluming and, hence, the increased amount of kinetic energy to be dissipated downstream of the fall.
