THE HYDRAULIC JUMP AS AN ENERGY DISSIPATOR 33 



formation of the jump at all stages, it will be a coincidence if the jump 

 forms over any considerable range of discharges. 



The hydraulic jump may be used for mixing liquids, and as an aera- 

 tion device. Its use for the former purpose was patented by J. W. 

 Ellms. 



The hydraulic jump as an energy dissipator. In order that the 

 hydraulic jump may function ideally as a dissipator of the excess energy 

 of water flowing over dams, it is necessary for the elevation of the water 

 surface after the jump to coincide with the normal tailwater elevation, 

 for every discharge. If the tailwater is too low, the high velocity stream 

 will continue on downstream. If it is too high, the jump will be drowned 

 out. In either case dangerous erosion is likely to occur for a considerable 

 distance below the dam. The ideal condition is to have the rating curve 

 for elevations after the jump coincide exactly with the tailwater rating 

 curve.^ It is best to accomplish this as nearly as possible in the design, 

 leaving only minor adjustments to be made at the time of the model 

 verification. These adjustments are customarily made by trial-and- 

 error placing of artificial roughnesses, blocks, or piers, as required for 

 satisfactory operation of the model. In high-head developments it is 

 best to avoid blocks or dentates of any kind, for they erode rapidly, and 

 make maintenance charges high. If the head is low, artificial rough- 

 nesses may be used more safely. This is fortunate, for it is in low-head 

 developments that adjustment to secure satisfactory operation of the 

 jump at all stages is most difficult to obtain. If a perfect jump can be 

 secured at all stages, over a smooth apron, the energy will be dissipated 

 without appreciable wear, and upkeep will be negligible. 



One possible source of damage, which must be guarded against in 

 design and construction of the apron, is that the full hydrostatic pressure 

 of the depth after the jump may gain access to the under side of the 

 apron, causing it to be lifted up under the thin sheet of water before the 

 jump. 



The position of the " jump-height " rating curve, an example of 

 which is shown in Fig. 305, may be shifted by the following changes in 

 the design : 



(1) Changing the crest length. Lengthening the crest will lower the 

 rating curve AB; shortening it will raise AB. 



(2) Changing the elevation of the apron. Lowering the apron will 

 lower the curve AB. 



(3) Sloping the apron. The apron should not be given too great a 

 slope, for the dissipation of energy becomes less efficient as the slope is 



^See " Protection Against Scour Below Overfall Dams," by E. W. Lane and 

 W. F. Bingham, Engineering News-Record, Mar. 14, 1935, pp. 373-378. 



