How do I measure the gradient of a winding culvert?

[pilesmakesmiles]

We are working on a drainage investigation, a significant part of which involves calculating the flow capacity of a 100m long culvert beneath a road and some houses to see if it is likely to surcharge.

We want to determine the gradient of the culvert to verify someone else's guesstimate which seems quite steep compared to the fall of the land.

In an ideal world we would measure the level at the upstream end and the downstream end, the distance between the two and get a slope. Unfortunately there is no access to the upstream end.

We could start at the downstream end and measure the level from inside the culvert. Unfortunately the culvert is approximately 1m x 1m in section and had a bend every 5-10m so this would be very time consuming. There are no intermediate access chambers on the culvert.


Can anyone suggest any methods for working out the gradient of the culvert to a reasonable degree of accuracy?

 

[gbam]

Are there as-built plans available? If not, can it be excavated and survey the top of the pipe? Maybe use ground penetrating radar to determine alignment or metal detector to map alignment. Just a couple of ideas.

 

[msquared48]

Pretty primitive, but, if there is significant flowing water in the culvvert what about tying a light line to a small floating stick and sending it through the culvrt. Mark the upstream end when it appears at the downstream outlet, and measure the string.

If the culvert is dry, you could do the same thing with a small robotic vehicle.

Mike McCann
MMC Engineering

 

[msquared48]

On thinking further, a round styrofoam ball with fishing line attached to would probably work best.

Mike McCann
MMC Engineering

 

[bimr]

A laser profiler will make short work of this project:

http://www.cuesinc.com/pdf docs/Laser Profiling M0608.pdf

 

[beej67]

Hire a midget!

More seriously, what do you mean "no access to the upstream end?" How do you know where it ends? I'm a bit confused by the scenario. How do you know it's 100 meters upstream if you can't access it?

The obvious answer is find the upstream end, knock a hole in it, and survey it.

If you somehow had access to the upstream end "observationally" but not with survey equipment, you could dam up the downstream end during a period of no flow and fill the culvert up with water. When the upstream end goes wet, you know the delta because of how deep the downstream end is. Then it's just a matter of measuring the length of the culvert.

If you're really going to do an analysis of this, you should probably include the junction loss at the pipe bends in your analysis, which would mean crawling through the whole thing anyway.


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

http://www.campbellcivil.com

 

[MiketheEngineer]

Take your kids RC model car and drive it up the pipe - when it is dry??

 

[cvg]

BIMR got it right, laser profiler / CCTV inspection is what you need

http://www.pro-pipe.com/laser.htm

 

[pilesmakesmiles]

Thanks for the replies.

Three of us have crawled through the culvert already as far as we can, up to 80m and have traced the route on the surface.

We cannot get a crawler CCTV camera through the culvert due to the condition (debris, loss of invert). RC car is unlikely to work for the same reason, and we can only get about 30m range from UHF walkie talkies inside the culvert (normally they are good for 1km or so).

I wondered about using a pipe laser level in reverse (i.e. set it up to follow the invert and then read off the gradient). Something like this:

http://www.leica-geosystems.com/en/Construction-Lasers-Pipe-Lasers_4247.htm

Works in the same way as the profiler attached to the CCTV camera?

beej67 - We can see the upstream end from a distance but we can't get access with survey equipment (not even a level staff).
We can't get 'out' of the upstream end because it has been piped and there has been a collapse about 20m from the upstream end.

I like the idea of damming up the downstream end and measuring the depth.

 

[beej67]

Well, I hate to bring it up but I'm sure OSHA has something to say about sending someone into a 100 meter long culvert and then flooding it with water. Make sure you don't do anything dangerous.

cvg and BIMR - how do laser profilers determine changes in the invert elevation of the pipe?



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

http://www.campbellcivil.com

 

[MiketheEngineer]

Also - air quality!!!

 

[gbam]

How expensive is uaing a laser profiler?

 

[bimr]

beej67, the laser projects a ring of light around the pipe interior, while software processes the information it receives from analyzing the ring.

The laser profiler will provide hard data and dead-accurate readings within a fraction of a percentage point.

 

[beej67]

How does it know what grade it's travelling at as it traverses the pipe?

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

http://www.campbellcivil.com

 

[bimr]

beej67, here is a link to a technical paper that discusses laser profiling and accuracy.

http://www.uta.edu/ce/abolmaali/Accuracy of Laser Profiling of Flexible Pipes Using CUES System.pdf

 

[gbam]

Does the laser profiler create a 3d image of the pipe that is spacially correct?

 

[dicksewerrat]

If you can't get to the upstream end, how do you expect to repair/replace the pipe. It is bound to surcharge if the pipe has already collapsed. Who owns the pipe?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[Ryb01]

pilesmakesmiles,

Following the past posts, a few things of note...

1)beej67, thank you for your earlier post. Crawling through a culvert, is not a good idea. I understand the intent, however, not something I'be recommending anytime soon. If there has already been a culvert collapse, what's to say that the culvert won't callapse with you in it? Not to mention the legal ramifications if something happened.

2) How do you calculate a culvert capacity if you can't see the inlet or know what the inlet configuration looks like? If the inlet is inaccessible or blocked the culvert has no capacity unless flows are entering iternal sections of the pipe via storm sewer. This leads into my next question...

3) If the culvert is located beneath a road and houses, with the inlet blocked....How is water getting into the pipe? Has there ever been a surcharge condition? Have you spoke to the homeowners(i.e any wet basements?)


4) A vac truck rented for a day of two might save a few headaches moving forward.

Thank you all for the information on the laser profiling, iteresting stuff.

 

[pilesmakesmiles]

We managed to find an access grating near the upstream end of the culvert yesterday, so managed to get a level in the end.

The landowner at the upstream end was refusing us access so we weren't able to get to the inlet. We had seen it from a distance so had an idea of the arrangement.

We are more interested in the downstream half of the culvert, which is in much better condition. We noted the collapses when we were there previously and have reported them to the owner(s).

 

[chicopee]

Your clue is with Beej67 by daming the downstream end during minimal flow and wait until the upstream gets wet from relatively stagnant water. You the have your difference in elevation and the length for gradient calculation is the straight line of sight between both ends of the culvert regardless how many bends you have in it.