Small culvert feeds into larger - how to calculate profile 2

[KatieTX]

I've been told to evaluate a 500-foot long culvert. The first 180 feet is 26" I.D. HDPE on a mild slope. The next 320 feet is 34" I.D. HDPE on a steeper slope. How would I evaluate this (calculate headwater, tailwater, capacity, velocity, etc.)? I've attached a quick sketch of the culvert.

 

[KatieTX]

Also, there's a small stilling basin at the culvert outlet.

 

[PELS]

It's unclear to me why the downstream pipe diameter needs to be increased in size, though there is probably a good reason.

The steeper slope would increase the capacity of the 26" pipe due to higher velocity. What is the reason for the change in sizes?

 

[KatieTX]

PELS - That's exactly what we (my supervisor and I) said. We think it's completely illogical to increase pipe size. Yet that's what we were told to evaluate :S I think that after the first 180-feet, there is more clearance, so they are going to use it.

 

[tsgrue]

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SWMM5

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tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com

 

[PELS]

Katie:

Perhaps you treat each segment of pipe as a separate culvert, with the upper pipe having outlet control and the lower, steeper pipe having inlet control (based on your sketch). There is a program by Haestad Methods called "Culvert Master" that might be suitable for this analysis.

http://www.haestad.com/store/software/haestad-water-products.asp

Good Luck!

 

[KatieTX]

I downloaded SWMM5 - I'm a sucker for hydrology/hydraulics software (I *loved* working with HEC-HMS/HEC-RAS - like I thought it was actually fun!), but I hadn't used SWMM before. I don't think we'll buy Culvert Master just for this one problem.

What about breaking the pipe into two portions, doing pressure pipe hand calculations for the upper portion, and using those results (pressure, velocity) as input for the second portion?

 

[gbam]

Katie,
Is there another pipe entering the system at the size change location?

You can evaluate using Hydraulic grade calculation. See FHWA's HEC-22 for approach and computation.

 

[RWF7437]

SWMM 5 seems like overkill.

HEC 22 or even HEC-RAS might be simpler to use.You might also be able to use Hydraflow Storm Sewers 2008 but that might not be well suited to this problem.

Based on your sketch, it appears the upper, smaller pipe is inlet controlled, NOT outlet controlled.

Is this an existing culvert, or is it proposed ? What are the slopes of the pipes ?

What are the upstream and downstream conditions ? Is the stilling basin large enough to create tailwater ?

Like the others it appears changing the pope size is an unnecessary complication. So called "broken back culverts" like this are usually avoided if possible for just that reason. Ask the persons proposing this what they are trying to accomplish.

good luck

 

[tsgrue]

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To me, SWMM5 is way more straightforward, simpler, flexible, and user friendly than HEC-RAS. (HEC-RAS is also limited to hydraulics only, wereas SWMM5 provides hydrology and water quality analysis in one analysis system.) I would use HEC-RAS instead of SWMM if I were analyzing a complex bridge. HEC-RAS is far better than SWMM5 for complex bridge hydraulics analysis. If the bridge was in concert with other HHWQ analysis, I'd export the rating curve into SWMM for use. Try out SWMM5 and let me know if you have problems (or upload the *.inp file and I'll look it over). You can find 150+ free and open source software applications for hydrologic, hydraulic, and water quality analysis at "

http://hhwq.blogspot.com",

with a table of "what does what" at "

http://tsgrue.googlepages.com/hhwq".

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tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com

 

[cvg]

download HY8 free from the FHWA website. It is specifically designed for culvert design.

http://www.fhwa.dot.gov/engineering/hydraulics/software/hy8/

also check out chapter VI in HDS No. 5 - HYDRAULIC DESIGN OF HIGHWAY CULVERTS which discusses "broken back" culvert design

It seems that your flow will go through critical depth and become supercritical in the downstream leg. Therefor it does seem that your upstream culvert will have inlet control. That being said, it really doesn't matter if you upsize the downstream leg as it does not affect the flow rate entering the inlet. It will help to reduce the possiblilty of outlet control and bend losses.

 

[tsgrue]

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Does HY8 allow "broken back" culverts? I haven't seen that option.

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tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com

 

[KatieTX]

gbam -
There is no pipe entering the culvert where the size changes. Hand/Excel calculations (HGL) are kind of what I was thinking of trying too.

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RWF7437 -
How would you model the pipe size change in HEC-RAS? I could easily model it in HEC-RAS if it was just one size, but since it upsizes in the middle, it's problematic. It doesn't seem like HEC-RAS can handle that.

I also agree that the upper pipe is inlet controlled.

This is a proposed culvert. I wasn't told what the application is.

I wasn't given slopes either. I was told that the upper end is at 5695 (Sta. 0+00), the pipe size change is at Sta. 1+80, the lower end is at 5688 (Sta. 5+02), and the upper pipe is at a very mild slope.

All I was told about the upstream and downstream conditions was to use 100 cfs as a flow rate. The stilling basin is 10 feet deep below the outlet, but the channel downstream of that is so steep (2%), that I don't believe there will be any tailwater.

Clearly I need to get more information. It seems that people think that hydrology/hydraulics engineers are magicians and can solve problems with no data whatsoever! I always tell my husband, "My boss has such confidence in me! He thinks I'm magic!" ;-)

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tsgrue -
I agree that SWMM5 appears to be more simple than HEC-RAS. But having never used it prior to yesterday, it's a little more difficult for me to use. Especially since I'm trying to use it without going through the User's Manual or Tutorials ;-) I'll let you know if I have problems with it, and I'll try not to ask anything that is in the manual (I hate it when people ask questions when they haven't even gone through those yet!)

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cvg - It doesn't seem like HY8 can handle the upsize in the middle of the culvert. Do you know of a way that it can?

 

[cvg]

no, it does not specifically allow them - however, the design procedure is explained in HDS-5, chapter VI, Section B-3 and can be analyzed using HY8 or even using nomographs

http://www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_number=7&id=13

 

[KatieTX]

cvg - I am looking into HDS-5.

I have thought about using BCAP (broken-back culvert analysis program -

http://www.nebraskatransportation.org/roadway-design/downloads.htm),

but it also doesn't allow the pipe size change.

 

[bltseattle]

I would guesstimate that the first segment is flatter so that inlet velocities are lower, reducing scour at the inlet. Regarding the pipe size increase, could this be to provide for increased energy dissipation within the culvert barrel? FHWA HEC-14 outlines energy dissipation through flow expansion which works by increasing the size of the channel/conduit to produce a hydraulic jump. It might be worth a look as they also provide formulas to estimate flow velocities etc in this scenario... HEC-14 also provides methods to estimate performance of the stilling well.

SWMM sounds too complicated for this. I would start at the outlet, use HEC-14 to figure out what the energy dissipator is doing. If it is an open top then you can figure out the water surface using a weir equation. Then I would use nomographs to analyze the system upstream, as a first cut.

 

[tsgrue]

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As an example of how to do this in SWMM5, I made a base model for this situation as I generally understood it from your information. It took me about 10 minutes. It would probably take a new SWMM5 user about 30 minutes. I uploaded the input file to "

http://www.blwi.com/misc/Culvert_BrokenBack_01aa.zip".

You can adjust the variables to fit your situation. Hydrologic analysis can be added as well to route through the culvert system. A screenshot of this is below.

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tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com

 

[KatieTX]

tsgrue - I will take a look at your model.

Here is a screenshot of what I came up with the other day:

I've uploaded the .inp here too. I'm sure it needs some work

 

[onyeuko]

Use SWMM5. It will solve this for you as fast as possible, without any compications or doubts. It is very easy to learn and use as it has a very good GUI.
Good luck
Teddy