Max HW/D = 1.5 for culverts - Why?
Max HW/D = 1.5 for culverts - Why?
(OP)
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.
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.





RE: Max HW/D = 1.5 for culverts - Why?
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.
RE: Max HW/D = 1.5 for culverts - Why?
RE: Max HW/D = 1.5 for culverts - Why?
RE: Max HW/D = 1.5 for culverts - Why?
good luck
RE: Max HW/D = 1.5 for culverts - Why?
What's the difference between orifice control and inlet control anyway?
RE: Max HW/D = 1.5 for culverts - Why?
RE: Max HW/D = 1.5 for culverts - Why?
RE: Max HW/D = 1.5 for culverts - Why?
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.
RE: Max HW/D = 1.5 for culverts - Why?
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.