4. =Heavy Stone Pitching with Apron.=--On the great shifting rivers of India a system of bank protection is adopted, consisting of a pitched slope with an apron (fig. 22). The system is used chiefly in connection with railway bridges or weirs, but it has been used in one instance, that of Dera Ghazi Khan, for the protection of the bank near a town. When, as is usual, the flood-level is higher than the river bank, an artificial bank is made. In any case the bank is properly aligned. The pitching has a slope of 2 to 1, and consists of quarried blocks of stone loosely laid, the largest blocks weighing perhaps 120 lbs. The apron is laid at the time of low water on the sandbank or bed of the stream. If necessary, the ground is specially levelled for it. It is intended to slip when scour occurs. The following dimensions of the apron are given by Spring (_Government of India Technical Paper_, No. 153, “River Training and Control on the Guide Bank System,” 1904). The probable maximum depth of scour can be calculated as explained in CHAP. XI., _Art. 3_. If this depth, measured from the toe of the slope pitching is D, and if T is the thickness considered necessary for the slope pitching, then the width of the apron should be 1·5 D, and its thickness 1·25 T next the slope and 2·8 T next the river. It will then be able to cover the scoured slope to a thickness of 1·25 T. This thickness is made greater than T because the stone is not likely to slip quite regularly. The thickness T should, according to Spring, be 16 inches to 52 inches, being least with a slow current and a channel of coarse sand, and greatest with a more rapid current and fine sand; but since the sand is generally finer as the current is slower, it would appear that a thickness of about 3 feet would generally be suitable. Under the rough stone there should be smaller pieces or bricks. Along the top of the bank there is generally a line of rails so that stone from reserve stacks, which are placed at intervals along the bank, can be quickly brought to the spot in case the river anywhere damages the pitched slope. For the special protection to banks required near weirs and similar works, see CHAP. X., _Arts. 2_ and _3_. CHAPTER VII DIVERSIONS AND CLOSURES OF STREAMS 1. =Diversions.=--When a stream is permanently diverted the new course is generally shorter than the old one, and the diversion is then often called a cut-off. The first result of a cut-off is a lowering of the water-level upstream and a tendency to scour there, and to silt downstream of the cut-off. Fig. 23 shows the longitudinal section of a stream after a cut-off _A B_ has been made. The bed tends to assume the position shown by the dotted line. If both the diversion and the old channel are to remain open, the water-level at the bifurcation will be lowered still more, and the tendency to scour in the diversion will be reduced. [Illustration: FIG. 23.]