While a dam is in course of construction arrangements must be made to deal with flood water. Generally the construction of some part of the dam has to be deferred to let the water pass. In the case of a masonry dam it does not much matter what part is thus deferred provided the usual procedure of stepping the work back is followed. In the case of an earthen dam it is best to defer a portion, not in the lowest ground where the dam is highest, but to one side of it, thus allowing the highest part of the dam to be brought up continuously. Temporary embankments and weirs can be constructed to cause the water to traverse the desired route without doing damage. Stepping of the earthwork should be avoided as far as possible. If it has to be adopted, the steps should be small. Sometimes the flood water is conveyed away by means of a “by-wash,” by an entirely different route. [Illustration: FIG. 57.] In Indian reservoirs the discharge over the waste weir may at times be great. The waste weir is sometimes in the position shown in fig. 57, _a e_ being the weir. In such a case a special hydraulic problem arises. In a case where a stream whose velocity is V issues from a reservoir or takes off at right angles from a larger stream there is (_Hydraulics_, CHAP. II., _Arts. 19_ and _20_) a fall in the water of about V^2/2_g_. The same thing occurs downstream of a weir, at least when there is a clear fall which is vertical or nearly so, so that the water after falling has no horizontal velocity. The water has to be started afresh on its course. In the case represented by the figure, the width of the channel is often restricted because of high ground beyond _f_, and the velocity in the channel may be very high. Suppose the channel below _e f_ to be of brickwork with vertical sides, and to have a 20-foot bed, a slope of 1 in 500, and a depth of water of 10 feet. The velocity may be 15 feet per second, and V^2/2_g_ is 3·49 feet. If the water has a clear fall over the weir at _e_, allowance must be made for a depth of water of 13·49 feet, not 10 feet, in the channel at _e_. Ordinarily the length _a e_ will be much greater, relatively to _e f_, than shown in the figure. Suppose that _a e_ is 300 feet and that the slope of the floor of the channel is carried on at 1 in 500 from _e f_ up to _a_, _b_, _c_, and _d_, following in each case the lines marked on the figure which represent the directions of flow. The length _f a_ will be about 310 feet, and the floor level at _a_ will be about ·62 feet higher than at _e f_. The water-levels below the weir will be in each case 13·49 feet above the floor. This should be allowed for in the design. It is true that the stream on first starting into horizontal motion below the weir moves more or less at right angles to it, and has thus a large sectional area and a low velocity; but it very soon has to turn parallel to the weir and acquire the full velocity of 15 feet per second, and there must be the