SCOURING AND SLUICING. 269 



renders sluicing much more difficult, because once aggregation 

 has taken place a much higher velocity is required to remove 

 the deposit. 1 



The sluices should face as nearly as possible directly along 

 the course to be scoured, and they should be distributed equally 

 throughout the whole cross-section in order to produce a uniform 

 current. 



The water should issue from the sluicing apertures hori- 

 zontally. A mass of water projected downwards will follow 

 the same laws as a solid body falling from the same height, with 

 the result of the bottom being torn up, whilst the water being 

 deflected upwards on striking the bottom will produce upper 

 or surface currents. 



Under ordinary circumstances, it is possible to effect sluicing, 

 provided the depth of water in the basin or channel is moderate, 

 and that a sufficient quantity of water is obtainable to give the 

 general velocity not bottom velocity merely necessary to move 

 the material to be dealt with. 



The bottom velocity will always be less than the top velocity, 

 except in the immediate proximity of the sluicing apertures, 

 when the water will issue and flow for some distance with a 

 velocity due to the head of water with which it is propelled, 

 but there are causes which prevent that action being continued 

 for any considerable distance. In the first place, there is the 

 enormous friction between the moving water and the immovable 

 bottom over which it flows ; and, secondly, a volume of water 

 moving with a high velocity through a body of water in a nearly 

 quiescent state encounters a very large resistance. In deep 

 water, the effect of these causes will be augmented. Under such 

 conditions, the water will rise, and a succession of waves and 

 eddies be produced. The deposit will be moved away for a short 

 distance, but as the water loses its velocity it will be collected 

 again in banks and shoals. 



When the volume of water discharged is great compared 

 with the channel through which it has to pass, the stagnant 

 water in the channel does not to the same extent destroy the 

 momentum, as when the scouring has to be produced by a sudden 

 finite impulse. In the one case, the scouring power depends 

 cceteris paribus simply on the relation subsisting between the 

 quantity liberated in a given space of time and the sectional 

 1 M.P.I.C.E., vol. Ixx. p. 3C. 



