Enery calculation from pipe discharge into flowing stream.

[RW11]

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.

 

[gbam]

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..

 

[RW11]

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.

 

[gbam]

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.

 

[msquared48]

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)

 

[RW11]

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.