If the material is soft enough to be scoured by the stream, it is often practicable to excavate a diversion to a small section and to let it enlarge itself by scour. This operation is immensely facilitated if the old channel can be closed at the bifurcation. The question whether the scoured material will deposit in the channel downstream of the diversion must be taken into consideration; also the question whether the diversion will continue to enlarge itself more than is desirable. The velocity in the diversion will be a maximum if its section is of the “best form,” _i.e._ if its bed and sides are tangents to a semicircle whose diameter coincides with the water surface, but this may not (CHAP. IV., _Art. 6_) be the section which will give most scour. In order to prevent the enlargement of the diversion taking place irregularly, the excavation can be made as shown in fig. 24, water being admitted only to the central gullet. The side gullets should not be quite continuous, but unexcavated portions should be left at intervals, so that if the water in scouring out the channel breaks into the gullet, it will not be able to flow along it until it has broken in all along. [Illustration: FIG. 24.] If a diversion is made, not with the object of lowering the water-level but merely in order to shorten the channel, the increased velocity caused by the steepened slope may be inconvenient. In this case a weir or weirs can be added (CHAP. VIII., _Art. 4_). If the water contains sufficient silt to enable the abandoned loop to be silted up within a reasonable time, it may be desirable to do this. The silting up may, for instance, increase the value of the land. The loop should be closed at its upper end. Water entering the lower end will cause a deposit there. When the lower end is well obstructed by silt, the upper end should be opened. The set of the stream, due, for instance, to a bend at the point where a diversion takes off has very little to do with the quantity of water which goes down the diversion. The only effect of the set of the stream is a slight rise of the water-level as compared with the opposite bank. Similarly, the angle at which the diversion takes off is only of importance in giving, in some cases, a velocity of approach whose effect is generally small. The distribution of the water between the diversion and the old channel really depends on their relative discharging capacities. If the required quantity of water does not flow down a diversion it can be dredged.