Fig. 36.
The Okla weir ([fig. 37]) across the river Jumna near Delhi was built about thirty-eight years ago on the river bed, which consisted of fine sand. The depth of water over the crest in floods is 6 to 10 feet. The material, except the face-work and the three walls, is dry rubble.
Fig. 37.
When the reach of channel downstream of a weir has a bed-level much lower than that of the upstream reach—this is often the case in irrigation canals,—the work is known as a “fall” or “rapid.” At a fall the water generally drops vertically, and a cistern ([fig. 38]) is provided. The falling water strikes that in the cistern and the shock on the floor is greatly reduced. An empirical rule for the depth of the cistern, measured from the bed of the downstream reach, is K = H + ∛H √D,
where H is the depth of the crest of the fall below the upstream water-level, and D is the difference between the upstream and downstream water-levels. At some old falls on Indian canals the water, as it begins to fall into the cistern, is made to pass through a grating which projects with an upward inclination from the crest of the weir at the downstream angle. This splits up the water and reduces the shock, but rubbish is liable to collect.
Fig. 38.
In the usual modern type of canal fall in India the weir has no raised crest, and the water is held up by lateral contraction of the waterway just above the fall. The opening through which the water passes is trapezoidal ([fig. 39]), being wide at the water-level and narrow at the bed-level. In a small channel there is only one opening, but in a large canal there are several side by side, so that the water falls in several distinct streams. The curved lip shown in the plan is added to make the water spread out and cause less shock to the floor. The dimensions of the openings are calculated so that however the supply in the canal may vary, there is never any heading up or drawing down. The detailed method of calculation for finding C F and the ratio of A B to B C is given in Hydraulics, Chap. IV. In cases where it is only necessary for the notch to be accurate when the depth of water ranges from B C to three-fourths B C, it will suffice to calculate as follows:—Let b be the bed width of the canal, and let Q be the discharge and B the mean width of the stream when the depth of water is B C. Decide on the number of notches, and let W be the width of a notch calculated as if it were to be rectangular, i.e. by the ordinary weir formula. Increase the width to W´ = 1·05 W. Then make the notch trapezoidal, keeping the mean width W´, and making the bottom width w (or C F), such that w/W´ = b/B. The top width of the notch is of course increased as much as the bottom width is reduced.
