Hydraulic jump length

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Hydraulic jump length

Brett
Hi

I'd like to model a hydraulic jump down a steep drop (2V:1H) in a simple rectangular channel. I am looking to determine the distance of the hydraulic jump from the base of the drop.

I have modelled this as steady flow in HEC-RAS interpolating cross sections every 0.1m.  It appears to show the flow at the base of the drop transition from supercritical to subcritical as required.

Is HEC-RAS likely to model this reliably, or is there a better method to use?

Regards

Brett
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Re: Hydraulic jump length

Chris G.
Administrator
I would be very cautious using RAS for this.  You may be able to get an answer, but you're breaking a major "rule" for RAS modeling in doing so.  10% is about the steepest you want to go when modeling a reach in RAS.  2V:1H is a 200% slope!  Sounds like a spillway to me.  Plus at slopes that steep, air entrainment, bulking of flow, cross-waves all play a major role in the ultimate water surface profile.  If you're looking for accuracy here, you'd need to go with a physical model.  Boundary layer theory is a method for computing the water surface profile, but at slopes that steep, I'd still be suspicious.  Plus, it won't do a hydraulic jump for you.  If you're jump is down on a shallower part of teh chute, you may be able to use RAS for it.  But RAS will only show a jump from one cross section to the next.  i.e. you won't be able to have a jump span multiple cross sections.  At 0.1 m spacing, I thinnk that may be a problem for you.  
Chris G.
@RASModel
www.therassolution.com