Hydraulic jump length

[Anonymous]

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

[Chris G.]

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.

[Author]

Posts