Stormwater outfall water æprojectionÆ or æthrowÆ distance

[dkntec]

How do I calculate the ‘discharge distance’ from a storm water Outfall pipe for a certain elevation drop?
I’m not sure what the correct term is for this - have heard it referred to as ‘waterfall’, outflow jet’, ‘outflow projection’, launch, or ‘throw’ distance.

The application is outflow discharge approximately 3’ above a creek water level.
Due to adjacent factors, a riprap slope below the Outfall is essentially fixed at 1.5H:1V slope (66.66…%).
I need to calculate at what distance from the pipe outlet the waterfall ‘curve’ will contact or land on this slope, or as I suspect, beyond this slope.

I am in Canada & normally use SI (metric) units, but no problem in work in Imperial (feet) and simply convert.
As I assume most of the members on this site are in the USA, I will post the information in feet;
- 24” (2’) diameter storm pipe
- Gravity flow with pipe slope of 2.00% (0.02 feet/foot)
- Manning N=0.012 (using ADS N-12 HDPE smooth interior pipe)

Assume ½ full flow in pipe.

Using Manning’s equation at ½ diameter flow, I calculate;
Q (flow) = 17.33 cfs
V (velocity) = 11.03 fps

I anticipate the distance I am trying to determine will be of significance.
This ‘discharge distance’ is needed to establish an appropriate set back distance to stay clear of the creek
Review of alternate pipe slopes, riprap slope angles, and discharge heights above creek water level may be needed as a result of these calculations to find a solution.

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I found the following post in searching the forum

http://www.eng-tips.com/viewthread.cfm?qid=30012

“Waterfall laminar flow to mist” started 21 Aug 02
Is the formula in this link appropriate in my application?
I assume NOT, and assume this is for ‘level water’ (i.e. behind a weir or reservoir spillway), and NOT for flowing water discharging from a pipe slope with velocity, as in my case.

Also, is this formula in feet or metric units?

Thank you for your advice.
This is my first post here.
DKN

 

[bimr]

The Department of Transporation Hydraulics Engineering has many documents on line:

http://www.fhwa.dot.gov/engineering/hydraulics/library_listing.cfm

In particular, you may be interested in:

Title: Hydraulic Design of Energy Dissipators for Culverts and Channels

http://isddc.dot.gov/OLPFiles/FHWA/010492.pdf

Title: Hydraulic Design of Highway Culverts

http://isddc.dot.gov/OLPFiles/FHWA/015808.pdf

 

[bimr]

http://www.fhwa.dot.gov/engineering/hydraulics/pubs/06086/hec14ch04.cfm

 

[coloeng]

x=v/sqrt(g/2z)

x= horizontal distance
v= initial velocity
g= gravity
z= vertical distance

 

[dkntec]

Thank you!
It works.
Have 2' to spare on the slope + the scour protection width at bottom.
You have made my day.
DKN

 

[gbam]

Just remember that the "z" or fall distance is to the centroid of Flow in your pipe. It may or may not make a difference.

 

[dkntec]

gbam
Yes, it does make a difference.
Thanks
DKN