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tap into box culvert with RCP

tap into box culvert with RCP

tap into box culvert with RCP

I'm working on intersection drainage improvements.  A double 6 by 6 culvert is running under the north leg of the intersection.  Just to the east of the intersection, the box has a 45 degree turn in it.  Flow is from northeast, through the turn, and to the west.

I am considering tapping into the side of the east-west portion of the box with a 30" RCP just below the 45 degree turn.  I'm concerned that water coming from the northeast in the box culvert will be pushed into the outlet of the 30" RCP limiting the flow through it.  My question is, how can I quantify the effects of the box culvert flow flowing into the end of my 30" RCP?

So far I'm thinking I need to figure out the pressure head in the box based on inlet and outlet water surface elevations.  I'm also thinking that velocity head needs to be taken into account.  Is this as easy as summing the pressure head and velocity head, adding that head to the invert elevation of the box, and using that elevation as my starting hydraulic grade line condition in the 30" RCP?  Am I missing anything?

RE: tap into box culvert with RCP

Apart from the hydraulics, you'll also need to consider the structural effects of "tapping" this box culvert. RC Boc culverts usually have their highest stresses at the corners of the box so a 30" hole at mid height would do the least structural damage.  The safest thing to do would be to cut a lerger hole and reinforce it tying to the existing rebar in the culvert.  Have a structural engineer analyze or model your proposed design using good finite element software.

Back to the hydraulics.  I believe you may be able to analyze several scenarios one of Haestad's many ( and expensive !) programs. Search their site.  http://www.haestadmethods.com

good luck

RE: tap into box culvert with RCP

Looking at the hydraulics is pretty straight foward.  Just use standard HGL procedures to determine the hgl @ your proposed outlet.  Then use that elevation to run up your pipe.  HEC-22 (FHWA) has a table that may help, 7-3 Frequencies for Coincidental Occurence.  This situation is somewhat typical in my area.  Good Luck!

RE: tap into box culvert with RCP

I would probably also run standard HGL analysis on the existing conditions, as I think you've outlined.  Use that as a static headwater for the proposed conditions, but only if the proposed 30" is inline with existing flow.  If it comes in counter to (or even perpendicular to) ex. flow...I'm not sure what I would do, but you would need some factor for the turbulence...maybe an extra-conservative k value on the velocity loss term of 1.0?  Software will NOT automatically make these judggement calls, so use care.

Do you have a little extra cover and/or will a smaller dia pipe suffice?  That will allow you to lift the invert in (even a few inches will help a lot) and minimize the existing static head submergance of your prop. 30", as well as minimizing turbulence through the structure.

RE: tap into box culvert with RCP

Thanks for the advice.  I'll do an HGL analysis on the box culvert and go from there.

RE: tap into box culvert with RCP

There will be some junction headloss, due to turbulence, where the two flows combine.  I have a nomograph that was apparently prepared by Baltimore County Dept. of Public Works if you're interested.

RE: tap into box culvert with RCP

Like RWF7437 said, you really should contact a structural engineer or the manufacturer of the box culvert.  I have run into problems in the past where I wanted to tap into a curvert and it turned out that the culvert was reinforced 12 inches o.c. and there was no way to get a lateral into the box without excavating, demolishing, and rebuilding part of the culvert.  If you intend to tap into an elbow, you very well may run into this situation.

RE: tap into box culvert with RCP

Move the tap downstream far enough so the intersection forms a 45 degree angle with the flow. This will  give the 30" pipe a 'venturi' effect and not restrict the flow out of the additional pipe. To make the connection to the box, saw cut the box rather than jack hammer the opening. Remember to measure the outside diameter of the pipe. It will be about 37 inches and the angle will make the horizontal measurement about 52".

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