36 



HYDRAULIC JUMP IN RECTANGULAR CHANNELS 



forms on a very steep slope, there is little depression of the water surface 

 near where the jet plunges. The position of the beginning of the jump 

 is fixed within narrow limits by the intersection of the downstream pool 

 surface, which is very nearly level, and the surface of the swiftly flowing 

 stream on the slope. If the jump forms on a comparatively flat slope, 

 on the other hand, it does not start at this intersection, but starts some 

 distance downstream, depending upon the height of the jump. Thus 

 it is for flat slopes that we need most to know the height of the jump, and 

 fortunately, it is for these cases that the theory of the jump on a level 

 floor will apply with sufiicieut accuracy. 



Fig. 308. The Hydraulic Jump in Channels Having Different Slopes. 



The level-floor jump formulas will apply with satisfactory accuracy 

 for slopes up to 10 or 15 per cent. Above this is an intermediate range 

 about which more information is needed. The information which is 

 needed is not so much the height of the jump as its length. For slopes 

 above 10 or 15 per cent, the downstream water surface will be nearly 

 level, and the length of the jump may be measured from the intersection 

 of the level of the downstream pool with the upstream water surface. 

 The actual amount of dip of the water surface below the pool level is of 

 little practical importance, for side walls would have to be built full 

 height so as not to be overflowed at low discharges with high tailwater. 



Experimental investigation of the length of the jump in sloping chan- 

 nels is as urgently needed as investigation of the length of the jump in 

 horizontal channels. For steep slopes, the " jump " action is very im- 

 perfect, and erratic fluctuations of the high velocity jet continue far out 

 into the pool. We need to know at what slope normal jump action 

 ceases and this plunging flow begins. Because of the unknown effect 

 of viscosity, surface tension, etc., experimental studies should be made 

 on nearly full-size models. 



