Max HW/D = 1.5 for culverts - Why? 1

[Mike17650]

A number of culvert design standards say that the maximum headwater depth is limited to 1.5 x diameter of the culvert. Can anyone explain the basis for this criteria?

We're designing a replacement culvert for an old 3-ft diameter culvert that collapsed. If we replace it with another 3-ft diameter pipe, the headwater will not overtop the road (not even close), but if we use the criteria that HW/D cannot exceed 1.5, then a 4-ft diameter pipe is needed. I've seen this design criteria in several publications, but haven't found any reasoning or justification for it. If anyone knows, I would appreciate an answer.

 

[Highway27]

We've been discussing this around the office today, and can't really find a reason for 1.5 specifically.

In the jurisdictions that we design for, 1.5 (or 1.2 some places) is not a limit, but a design guideline for selecting the size of the pipe for the application. If you do not have overtopping, you can certainly go higher, especially with a 3' diameter pipe. If it was something like wanting to use a 1' diameter pipe where you should be using a 3', that would be kinda iffy. If your HW/D is getting up around 4 or 5, I'd definitely look at the larger pipe, but if it's 2, and you're thinking that 1.5 is a hard line, I wouldn't think you would need to increase the pipe size.

My guess for why 1.5 is used as a guideline is that it scales with drainage area, and for areas where design flows are not maximum flows, it gives you a pipe that won't be too badly undersized for the maximum flow, but will still provide a high enough velocity for normal and design flows.

Of course, either your client or the local permitting authority will be the final word about whether they'd accept the computations.

 

[bpattengale]

I know that 1.5 HW/D is the commonly accepted point where the culvert transitions to orafice flow. Maybe that has something to do with it?

 

[cvg]

with sufficient tailwater, outlet control can be maintained beyond 1.5 so orifice control is not the controlling factor. A ratio below 1.5 will generally maintain subcritical, relatively low velocity flows while providing some safety factor for clogging or for larger flows. ratios of 4 or 5 (which I would say are generally unacceptable) would generally give extremely high velocities which causes supercritical flows conditions, potential for hydraulic jumps, excessive erosion etc. Just bad practice for a highway culvert. This is generally only allowed for outlet conduits in dams.

 

[RWF7437]

CVG's answer is probably right. But you can check it easily yourself. Just calc flow and velocity for your culvert assuming inlet control and 1.0 D, 1.5 D, 2.0 D and 3.0 D. Then plot the results.

good luck

 

[LCruiser]

cvg's definitely right. Sometimes. However, every situation is different.

What's the difference between orifice control and inlet control anyway?

 

[RWF7437]

If there is a difference, my guess would be that inlet control simply means that the capacity of the culvert is determined SOLELY by the inlet geometry; including the effects of wingwalls, rounded entrances, projecting entrances and the like. Orifice flow does not occur until the headwater depth exceeds the soffit of the culvert by some amount. At very low flows the culvert acts more like a weir. At the transition from weir to orifice flow there is turbulence which makes the flow hard to predict exactly.

 

[LCruiser]

I thought about that, but then thought that that tipping point (excuse me while I gag) would vary with inlet conditions, pipe size, etc.

 

[mdelorme]

Another reason to avoid excessively high headwaters is that overtopping the endwall may create a large hydraulic gradient between outlet and inlet sides of the fill. Then you run the risk of piping (eroding the fill by creating a flow path along the outside of the culvert pipe).

See page 175 of this text:

http://isddc.dot.gov/OLPFiles/FHWA/012545.pdf

There's also some comment on flotation that may be appropriate, depending on your endwall.

 

[Mike17650]

Thank you for all the replies...you certainly shed some light on it for me.

Highway27 - Your reasoning makes sense to me, especially since the regulatory agency's design manual says nothing about the HW/D, just basing allowable headwater on being some distance below the road. I'm going to run comps for various diameter pipes to compare HW/D, then discuss it with our client and the agency.

Regarding inlet control vs. orifice control, I believe RWF7437 is correct. I took a look at my old copy of HEC-5 and it shows culvert flow profiles, one of which is for unsubmerged inlet control, where the HW is below the top of the pipe.

Thanks again.