Modeling a Culvert with 2 different slopes

[Cap07]

Hi,
I am wondering what is the best way to model this situation:

I have a roadside ditch which discharges into a culvert. Midway along the culvert, the slope changes from 0.5% to 1.5%. The culvert then discharges to another ditch. Should I model the culvert as two separate culverts in series?

 

[psmart]

If there's a junction between the two pipes, such as a manhole, I would model as two zero-storage ponds, each with a culvert outlet.

If the pipe is sealed at the slope transition, you could use the average slope. This will retain the correct invert elevations at the inlet and outlet.

Putting two culvert outlets in series (on the same pond) will depend on the flow regime within the culverts. Just remember that this solution will evaluate the pipes separately and then use the lesser flow at each headwater elevation.

You should probably try a couple of alternatives and compare the results.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Cap07]

Hi Peter,
Thanks for getting back to me so quickly. There is no junction between the culverts. I'll try your suggestions and see how they compare.

Thanks again.

 

[Cap07]

Another question - if I have two culverts in series, what should be specified for the entrance of the second culvert? The culvert is CMP...none of the 4 options seem to apply to the case where it's connected to another culvert upstream.

 

[psmart]

When you say "two culverts in series", do you mean two separate ponds, or two culverts on the same pond?

You can enter the Ke value directly without selecting the entrance geometry - but this really depends which approach you're talking about.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Cap07]

I think it would be two culverts on the same pond. I've attached a sketch of what I want to model. Runoff from the project site outfalls to the upstream ditch, then flows through the two culverts to another ditch.

 

[psmart]

I would use a very low Ke (close to zero) for the lower pipe, thus eliminating the normal entrance losses at the transition.



Peter Smart
HydroCAD Software

http://www.hydrocad.net