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Why would a culvert be replaced due to severe erosion at outlet?

Why would a culvert be replaced due to severe erosion at outlet?

(OP)
A city has asked me to design a culvert to replace one that they say is in "bad shape".  I look at the culvert and it is a cm arch culvert with concrete walls for footings and a concrete bottom.  Its diameter is equivalent to 8' and is under a busy road, in a flowing stream.  The problem seems to be, not the pipe, but severe erosion at the outlet.  The stream has undermined the wing walls and there is a fall of about 4' from the pipe to the stream.  There seems to be a better solution of fixing the outlet situation instead of replacing the culvert.  Am I missing something?  I don't want to tell the city that we can just fix the outlet situation and they tell me why it would need replacing, thus I look like the idiot!

RE: Why would a culvert be replaced due to severe erosion at outlet?

Can you extend the wing walls and slope the invert down to the real flowline and then back up a litte to slow the flow down as it leaves the paved area?

Richard A. Cornelius, P.E.
WWW.amlinereast.com

RE: Why would a culvert be replaced due to severe erosion at outlet?

Perhaps they feel that changing to a different size culvert will reduce velocities and decrease the scour.

Does the person who is asking you to repalce the culvert have a technical background?  Or is it somebody who may not know that only the erosion needs to be addressed?  

I've found that many times, particularly in smaller cities with no true public works department, decision-makers know something needs to be fixed, but not exactly what...so they assume what needs to be done and request that.  It may be beneficial to sit down with your client and let them know your perspective and see how it compares to theirs.
 

RE: Why would a culvert be replaced due to severe erosion at outlet?

Have you walked thru the culvert and inspected it completely?  Maybe there is some type of corosion within the cmp that warrants the replacement.  Why not sit and discuss their reasoning for replacement?  There is nothing wrong with asking them why they thiink the structure needs to be replaced.  Then you can let them know that you have inspected the cmp yourself and feel that the only need for repair is the outlet and recommend a retrofit rather than an entire replacement at $$$ cost savings.  

RE: Why would a culvert be replaced due to severe erosion at outlet?

Is the erosion above or below the water line? If below or the water line, is there any sign of eddies on the lip of the culvert?

RE: Why would a culvert be replaced due to severe erosion at outlet?

(OP)
Thanks to everyone for the replies.  Your responses have given me a little more confidence that I may be on the right track here.  I'm not sure it is safe to walk the culvert and I am dealing with someone that is not techically savvy; although has a lot of common sense.  This is something new for me.  I don't have much experience with repair issues.

RE: Why would a culvert be replaced due to severe erosion at outlet?

Some one will have to inspect the culvert. CCTV or in person. How long is it? Diameter?  

Richard A. Cornelius, P.E.
WWW.amlinereast.com

RE: Why would a culvert be replaced due to severe erosion at outlet?

(OP)
The pipe is about 50' long and the equivalent of an 8' diameter pipe.  The pipe looks in good shape.  They are assuming the pipe needs to be replaced because there is severe scouring underneath the wing walls.  I think I will propose to the City to grout underneath the wing walls and place large stone at the outlet.  The City thought they could get a grant to replace the pipe.  Their money is better spent with stream bank restoration beyond this pipe.

RE: Why would a culvert be replaced due to severe erosion at outlet?

I'd walk the pipe. If you think it is unsafe, send a camera with a tilt and rotate head. The bottom of the pipe will go bad first. If the bottom is gone, then you need to rehab or replace before the road swims downstream. If you walk, don't forget your confined space issues

Richard A. Cornelius, P.E.
WWW.amlinereast.com

RE: Why would a culvert be replaced due to severe erosion at outlet?

(OP)
I helped the City come around to thinking that this culvert should not be replaced.  Your responses gave me more confidence that the City probably jumped to the wrong conclusion.  Thanks for the responses.

RE: Why would a culvert be replaced due to severe erosion at outlet?

After many years of installing and designing protection for culvert outlets, I have found that if there is a fairly uniform "scour hole" at the outlet end, the culvert is often sized too small.  (Just a visual rule fo thumb, always do a good desing).  Easily erodable soil type and deficiencies are also applicable, but often the culvert is too small.

RE: Why would a culvert be replaced due to severe erosion at outlet?

fivestring:

Do you have information on the growth rate of your scour hole?

If the hole has stabilized, then it is currently about the correct size to dissipate the energy of the flow. It may be possible to leave the depression, reinforce with riprap or other broken stone, and dissipate the energy that way.

Or, you can put energy dissipators inside the pipe to slow the water before it exits.

You should read Chapter VI of HDS no.5 "Hydraulic Design of Highway Culverts"

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