4. =Various Types of Weirs.=--The type of weir shown in fig. 32 may be varied by steepening or flattening the slopes of one or both faces. Flattening increases the cost but gives a greater spread for the foundations. It may, however, be combined with a decrease in the width of the crest. Flattening of the downstream slope reduces the shock of the water on the floor, but the slope itself, especially the lower portion, has to stand a good deal of wear, and the length exposed to this is increased. Flattening the upstream slope facilitates the passage of floods. The same result is obtained by making the crest slope upwards (fig. 34). In a small stream or in an irrigation distributing channel, a weir may be a simple brick wall with both faces vertical and corners rounded. [Illustration: FIG. 34.] Weirs in America are often built of crib-work filled with stones. Weirs are also made of sheet piling filled in with rubble, and the top may be protected by sheet iron. A weir made on the Mersey in connection with the Manchester Ship Canal works was so made. There were three rows of piles and the filling in the back part was of clay. Sometimes the downstream faces of weirs used to be made curved (figs. 35 and 36), the object being to reduce the shock of the falling water, but the advantage gained is not very appreciable, and this type of weir is not very common. [Illustration: FIG. 35.] [Illustration: 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. [Illustration: 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. [Illustration: FIG. 38.]