Steep stormline terminating in manhole. Energy dissipation nozzle? 1

[khu]

We have a very steep site and will be sending stormwater down a hill to a swale and wanted to utilize a manhole as an energy dissipator at the bottom. I've seen installations like this around the city, but have never designed something like this myself. The site does not have room to accommodate a full riprap energy dissipation outlet design, so I was told to utilize a manhole instead. Essentially the water will flow down into the manhole and then overflow into the swale.

It would be 18" line at 25% for ~16' down the hill, which looks to be about 19fps based on my runoff calcs. Would there be a need for some sort of energy dissipation nozzle within the manhole for that kind of velocity? Do those exist?

 

[cvg]

use a grated basin (bubbler), not a manhole. water will fill the basin and back up into the storm drain. a nozzle may plug with debris

 

[khu]

use a grated basin (bubbler), not a manhole.

I was planning on specifying an open grate lid for the manhole to allow for the overflow, but maybe a bubbler CB would be more common? I was struggling to find the max pipe size allowed for CBs. The city's detail shows a max pipe size of 12", so I went the MH route. I was wondering about the riprap in the base also. Does that interfere with cleaning operations?

 

[cvg]

the catch basin was sized so that the velocity through the grating was reasonably low and assuming the grate was partially plugged. The basin was much larger than a standard manhole. A manhole grated lid would probably be too small.

the example had riprap and filter material in the bottom allowing the water to soak away after storms, otherwise it would stay full of water for days

 

[nackra]

I would think that flows of 19-fps will "move" 6-inch rock in the base. Fines will likely blind off the "percolation" unless someone maintains the rip rap base.
Will the soils percolate? A geotechnical report should have some comments on percolation.

 

[khu]

Geotech didn't do any infiltration testing it seems. It's not really a big deal if this basin stays flooded either, it's off in a field and out of sight. I'm more concerned with scour or other effects of the high velocity coming down that hill.

 

[cvg]

here is another option, although it might be overkill for your situation. it is relatively maintenance free

 

[fel3]

cvg's photo shows what is called a baffled outlet. You can find design info about them in USBR's "Design of Small Canal Structures":

https://www.usbr.gov/tsc/techreferences/mands/mands-pdfs/SmallCanals.pdf

BTW, the reason they are called baffled outlets is that they are often confused.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[khu]

the catch basin was sized so that the velocity through the grating was reasonably low

How would I go about calculating the velocity of my water exiting the catch basin? I'm assuming it's an instance of the energy equation, but I'm struggling to recall how to set up this type of analysis. If I'm going from an upstream 18" pipe (that would be closed conduit flow) to a 24" grate catch basin, I would imagine my velocity at the grate would be lower compared to the 18" pipe due to the increase in cross-sectional area. I'm not sure how it works going closed conduit to open channel though.

 

[Twinkie]

Can we look at this another way?

How about putting a deep manhole or structure at the upstream end and flattening the pipe?

With your concept you are trying to essentially design a level spreader, which would need to be massive for that velocity.

 

[khu]

How about putting a deep manhole or structure at the upstream end and flattening the pipe?

I'm looking at this again and I think that would actually work. Originally we had a sanitary line running deep below this, but that has since been relocated, so now there is room to drop the upstream manhole quite a bit. I can get that slope down to ~ 10% with a 10ft deep manhole. Cuts the slope roughly in half, so that would help the velocities significantly.

 

[cvg]

if you go the bubbler route, than you need a grate much wider than 24". the example I posted had a 12'x3' grate