Hydraulic Area - Converting a box culvert to a series of circular culverts

[CanuckPE]

I'm not much of a hydraulics guy, but I do have a mech degree and I practice a bit in piping.

A client asked if we could provide services/plans to convert a box culvert (old, wooden, bad condition during inspection) to a series of circular CMP culverts. In lieu of a more complete study of the expected hydraulic flow rates, I assume we could replace the hydraulic capacity with something that's, "in kind" with the existing arrangement. By my eye, the more open cross section of the box culvert would offer less drag when compared to a series of smaller circular openings. Does anyone have any general recommendations for this sort of conversion from rectangular openings to circular openings? I was thinking of A.circular = ~2*A.rectangular, but looking for guidance of those more experienced.

 

[ACtrafficengr]

You can download HY-8 software for simple culvert analysis from the US FHWA website. From your username, I'm assuming USGS StreamStats won't help with flow rates. Does Canada have something similar?

The main issues we have with multibarrel culverts are compaction between the pipes, and clogging from tree branches getting caught on the fill between the pipes.

My glass has a v/c ratio of 0.5

Maybe the tyranny of Murphy is the penalty for hubris. -

http://xkcd.com/319/

 

[gbam]

I agree with HY-8 this would be a good way to compute the culvert hydraulics. I would add that in lieu of a hydrologic study to determine the runoff flowrate, one can assess the current wooden box based on a specific headwater elevation and make sure you provide the same culvert capacity for the proposed condition. Of course, the appropriate approach would be to compute flow rates for the design storm and then properly analyze existing and proposed hydraulics based on the governing agency policy/criteria to design the culvert. If this goes beyond your ability, I recommend that your client hire a drainage engineer/hydrologist to perform the design services.

 

[Ekenny_G]

My recommendation is to use HY-8 to determine existing culvert capacity and Headwater depth.
Then using the same HY-8, convert the existing rectangular culvert total opening to an equivalent circular Area.
For example:
-Determine the total area of the existing box culvert, lets say is a 12ft x 12ft rect culvert = 144 sq.ft.
-Calculate the equivalent circular pipe diameter, which total area would be equal or > than 144 sq.ft. In this case, pipe diameter of 13.5 feet or more will be equivalent.
-Update the HY-8 calculation using the equivalent circular pipe diameter.

The ideal result should maintain the headwater depth as the existing rectangular culvert without water overtopping the roadway. This is just general approach, and would recommend a drainage engineer to do a more detail analysis and design.