INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

Enery calculation from pipe discharge into flowing stream.

Enery calculation from pipe discharge into flowing stream.

(OP)
I have a need to determine if the discharge from a storm drainage pipe into a stream with flowing water should have outlet protection. A 15" pipe outlet is located in a stonewall with an invert approximately 1' above the stream bed. Under design conditions, the water surface elevation in the stream is slightly above the crown of the pipe (submerged). The stream bed is somewhat rocky.

Stream flow = 155 cfs
Pipe flow = 10 cfs
Pipe outlet velocity 8.6 fps

Pipe outlet is 90 degrees to stream channel

Looking for the proper way to calculate the energy out of the pipe and sizing of any outlet protection in this situation accounting for the muting of the flowing stream.

RE: Enery calculation from pipe discharge into flowing stream.

A good measure is to compare the pipe outlet velocity to the channel velocity. We used to use a ration of 1.5 (If I recall correctly) for culvert velocity to natural channel velocity to establish if some type of dissipation is needed. Now we specifically use culvert velocity: V<5fps no protection, 5fps<V<10fps Riprap Apron, 10fps<V<15fps Wire Tied/gabions V>15fps stilling basin etc..

RE: Enery calculation from pipe discharge into flowing stream.

(OP)
Thanks for your input. Your suggestion for using the culvert outlet velocity would be useful if I were discharging into a channel with little to no channel flow, in which case I could make use of standard DOT criteria and equations. My issue is that the flow of water in the stream will mute out the discharge from the pipe. With the pipe being elevated above the stream bed, the velocity force should be lower at the stream bed than what it is when it exits the pipe. The question is how much lower.

RE: Enery calculation from pipe discharge into flowing stream.

Worse case scenario for the culvert would be to assume a pond at the outlet and factor in the outlet loss due to the submerged outlet. If this scenario works for your culvert design then don't worry about determining the velocity differential. If you need to factor in the velocity for design considerations then by all means evaluate the velocity profile and use the approximate velocity. Please consider the angle od discharge from the pipe in reference to the stream. If the stream is 90^ then the stream velocity is zero in the direction of the outlet pipe.

RE: Enery calculation from pipe discharge into flowing stream.

If possible, I would lower the outlet and use a Type II CB with the inlet 2 feet or so below the top of the CB, and allow the outflow to bubble up over the top of the CB and into the stream.

Dissipates a lot of energy and will not muddy the stream.

Mike McCann, PE, SE (WA)


RE: Enery calculation from pipe discharge into flowing stream.

(OP)
The estimated exit velocity was based on a submerged outlet. What would be the proper way to calculate the velocity and force on the stream bed 1' below the pipe invert? It would seem that the stream velocity should mute the outlet pipe velocity. The angle of the outlet pipe to the stream is approximately 90 degrees.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close