Storm Water Energy Dissipator with limited length

[AnalyzingAlan]

We've experienced contractor error which led to a 36" outlet pipe discharging out of a CRM wall (16' high) with way too steep of a slope. I originally designed a drop inlet structure before the wall (not constructed) which led to adequate exit velocity. The property line is 5' from the outside face of wall, limiting our ability to use common dissipator types. I was thinking of using an impact basin but turning it 90 degrees at the pipe outlet. The water (63 cfs, 20 fps) would slam into a wall and turn parallel the wall, thus giving baffle/riprap length as needed. I have no idea how to calculate the final velocity or the effect of the wall impact. Any advice or direction is much appreciated, (fyi I've got the FHWA HEC14 manual handy).

Thanks,
AnalyzingAlan

 

[3feethighandrising]

To counter act the forces from the 90 degree turn you are going to need thrust blocking or wingwalls to keep this structure from moving. The structure could not be tied to the existing wall, unless the structural engineer would approve it. Thrust blocking would have to be installed opposite the direction into impact wall and you would probably be over the property line.

There are some culvert roughing techniques in section 7-1 and 7-2 of HEC 14 that may be useful, or maybe convert the pipe to a broken back culvert design, but if the pipe is already installed it would just be easier to tear it out and build it to plan, than to install the roughness features, or change the slope.

 

[AnalyzingAlan]

Thanks man, the thrust is definitely an issue. I've calculated the force and will be getting the structural involved. I'm hoping he can dowel into the wall but its made of boulders. Will not be easy getting this past him. In the meantime the flow calcs are still escaping me. I've changed the concept to be basically a junction box, thus just calculating the velocity based on box conditions after the 90 degree bend. This is not conclusive enough for my boss so the analyzing continues...

 

[beej67]

I've designed things like this before but you need to really, really think it through. If you flume the water 90 degrees and then carry it along and drop it with the same basic cross sectional area, it's going to have the same velocity purely based on the continuity relationship. You want to bleed not just velocity, but flow itself, as it runs parallel to the property line. Consider doing a very long cast in place concrete wall with orifices all along its invert, to let a little water out here and a little out there, until the flow is evenly distributed along a wide length. Like a level spreader, but more controlled.

That's worked for me in your scenario. Of course cross your Ts and dot your Is on structural and geotechnical issues.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

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[Twinkie]

Not to get off topic, but if this is contractor error, can't you have him rip out what he built and build according to the original design?

 

[AnalyzingAlan]

Wow this is my first forum thread, I'm impressed with the responses. The force issue and the flow dispersion issue are the two that I'm most concerned with. And we just did a level spreader on another project so this concept is being adapted at present. To Twinkie's point, the contractor should have to redo per plan but the wall is built, and a house pad is poured above it already. I saw a thread here about a compact outfall weir but the linked articles/photos were dead. That may be an option. In the end, I feel the structural will not sign off as 3feethighandrising stated, but we will have explored the options as requested by the owner. He will then be forced to reconsider obtaining an easement into the neighboring property, allowing us to do a normal energy dissipator straight away.

Thanks again everybody

 

[dskzeus]

I have another option for you. On many embankments with 2 to 1 slopes, 20 to 30 feet high. The down drains have a conventional CMP "T" fitting attached to the end of the pipe. You could use this concept with a 36" by 36" x 36" T and attach a 36 inch flared and section to each side outlet to help reduce your velocity. youYou do have a thrust force problem.There are several ways to anchor your T. You can anchor to the wall, anchor to the pipe upstream of the wall,or cast a "dead man footing" to take the thrust force. You need to calculate your thrust force using the Momentum Equation.

One thing I caution you on and that is the velocity leaving your site must be equal to or less than the velocity before your project began. This is a fundamental drainage If your materials are not CMP you will need to customize.
Please look me up if you need more help on the details