In the case of Indian rivers which have soft channels, and are ordinarily of moderate width but are subject to occasional floods when the width of the stream is multiplied several times and becomes very great, it is the rule to make the span of a railway bridge far less than this greater width. The stream during floods scours out a deep channel through the bridge with great rapidity, and no heading up worth mentioning occurs. The foundations of the piers are very deep, being frequently 50 feet below the lowest point of the river bed which can be found anywhere within several miles of the bridge. The span of the bridge can be arrived at by considering a general cross-section of the river as it is when in high flood, and assuming that scour to the depth of the lowest point, found as just explained, will take place in one-third of the span of the bridge. The span can then be so fixed as to give no heading up. It is not assumed that there will be no increase in velocity through the bridge. The velocity in the deep scoured portions will be increased. The piers are protected by loose stone (fig. 50). The spans vary from 100 to 250 feet. The bridge over the river Chenab at Wazirabad had originally sixty-four spans of 145 feet each. The number of spans has since been reduced to twenty-eight. With a very long bridge, the current of the shifting stream is more likely to strike the bridge obliquely, though this is not the chief reason for reducing the length. Long spans, say 250 feet, have been found to be better than shorter spans; the cost of the stone protection round the piers is of course less (_Government of India Technical Paper_, No. 153, “River Training and Control on the Guide Bank System,” by Sir F. J. E. Spring, C.I.E., 1904). 2. =Syphons and Culverts.=--Syphons are used to pass drainage channels or other streams under canals or other lines of communication. In the case of a masonry syphon under a stream which may be dry while the syphon is full, the weight of the arch and its solid load must be not less than the upward pressure of the water passing through the syphon. The channel sometimes has a vertical drop at the upstream side (fig. 51) and a slope at the downstream side. The slope enables any solid materials to be carried through, and facilitates cleaning out and unwatering. The drop at the upstream side does not give rise to any shock on the floor when the syphon is full, but a slope is preferable if there is room for it, and it causes less loss of head. [Illustration: FIG. 51.]