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Distance water jets
2

Distance water jets

Distance water jets

(OP)
Here's what I'm looking for, a formula that tells you how far water travels after exiting an HDPE pipe at a certain velocity.  Working on a rip rap design on a steep slope but don't know how far I need to extend the rip rap.    

RE: Distance water jets

You can also calculate the distance the water travels by applied physics, if you assume that the water jet behaves as a projectile.  Since d=at^2/2, where a is acceleration, then if we replace a = g, and d = H, then, H=gt^2/2, and t=(2H/g)^(1/2), and the horizontal distance X the jet travels is X=Vt.

I usually calculate the distance for the top of the jet, and provide a margin of error in the rip rap length to allow for possible errors in the velocity calculation.

RE: Distance water jets

If the slope is so steep that a hydraulic jump is not formed (flow is supercritical in the channel) then you should line the entire channel or provide a cutoff to prevent headcut.  If there is a hydraulic jump, then calculate the location and size of the jump and provide erosion protection for some distance past the end of the jump.  Again, provide a cutoff.  As indicated by bltseatle, HEC14 should provide some guidance.  Also, calculate scour depths and provide protection against scour.  You could start with HEC6 for this.

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