Length of culvert's apron 1

[7058]

Is there any rool regarding length of the apron for box (and pipe) culverts size 1-5m.?
Is it trade rool to have apron as long as wing walls or it may be shorter? I made hundreds of those but do not remember a single apron shorter than wing walls. Now it seems I have to design so.
Please let me have some opinions.
Dragan Andrejevic

 

[cvg]

apron or protected surface of some kind should extend as far as necessary until the velocity in the channel is non-erosive or matches the existing condition velocity in the stream. There is no rule.

 

[francesca]

There is no rule but there are nomographs. I know the Nashville storm water manual has a few based on flow rate, velocity and pipe size.

 

[jev2740]

take a look at the NYSDEC erosion control handbook with expamples referencing nomographs for discharges from pipes and the dicaharge velovity.

http://www.dec.ny.gov/docs/water_pdf/section5b_perm_str.pdf

 

[bltseattle]

FHWA HEC-14 is the sacred tome for stable outfall design but may be overkill if you can find a DOT or County standard to use....

http://www.fhwa.dot.gov////////engineering/hydraulics/library_listing.cfm

 

[sam74]

I've got a program I downloaded off the FHWA website a while back called HY8Energy Model V2.0 which is supposed to automate what is described in HEC no. 14. And if it works it really does. I've done one by hand before and it sucks. I can't seem to find a place to download it anywhere now though. So that makes me wonder if it really works or not.

Has anybody else heard of this program or its demise. It is not the old DOS version either. It is a windows friendly version with documentation dated May 2000. The FHWA website mentions they are adding energy dissipation to their next release in the fall but this version is not to be found anywhere currently.

 

[tsgrue]

.
Two notes in this one (a bit of a late response)...

1) A link to the latest version of HY8 can be found at "

http://hhwq.blogspot.com"

as well as links to other software to aid in energy dissipation analysis. The site “

http://tsgrue.googlepages.com/hhwq”

has a table of what software application does what stuff. HY8 is excellent software for culvert analysis.

2) Somewhat over 10 years ago I pretty much stopped using rip-rap aprons for energy dissipation at culvert/pipe outlets (as part of a general process to move away from rip-rap except where absolutely necessary). You might consider using a device that I call a "compact weir outfall". It sounds fancy, but is really not much more than a yard inlet at the end of the culvert. You can find a detail for one of these at “

http://www.blwi.com/misc/CompactWeirOutfall_20070917aa.pdf”.

A picture of one of these operating during a storm event is at “

http://www.blwi.com/misc/CompactWeirOutfall_20070917aa.jpg”.

The picture is of a compact weir outfall at the edge of a yet to be stabilized construction site.

You can see how the system works by checking out the picture. Some advantages of this outlet type include:

1) You can “turn the flow” (redirect the water) in a very small space, which is perfect for culverts exiting at large intersection angles to downstream channels.
2) The cost is generally less than a rip-rap apron (depending upon your location and market, of course).
3) It works well where aesthetics are important - it is easily hidden behind shrubbery or placed in an open yard/lawn and doesn't need weeding and trash removal.
4) It takes up much less space than a rip-rap apron - which can be huge!
5) The outlet water is “instantly subcritical” with minimal horizontal velocity.

When first mulling these creatures over, some folks are concerned about clogging. The systems are self-flushing of sediment. You might can imagine low velocity flow outside the pipe and higher velocity in the pipe which results in the sediment being fluidized - stirred up - in the pipe. Some sediment will drop out at the tail end of the runoff curve (hydrograph), but that gets swept out at the rising limb of the next stormwater runoff event. I don't suggest placing these where lots of sediment is expected to enter or pass, but I have been surprised at these haven't clogged even under very high sediment loadings (fully denuded watershed being graded). As long as the inlet openings are about 1/2 the size of the minimum outlet opening (read as “use a grated inlet of some sort”), then debris clogging is not a problem either. I have not seen any of these fail to date (10+ years in the ground).
.


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

http://hhwq.blogspot.com

 

[newnickname]

tsgrue,

Please correct me if I'm misunderstanding the concept here. This releases water once the head builds up to the weir elevation. Is there a low flow orifice somewhere to drain the structure?

 

[tsgrue]

.

newnickname,

You seem to have the concept. Drainage holes with gravel are provided at the structure base.

.



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

http://hhwq.blogspot.com

 

[newnickname]

Thank you for all the great info.

Do you use these structures as a method to account for partial recharge volume requirements? Is it necessary to get soil borings at each outfall location to ensure recharge is possible and the water table is low enough to be acceptable?

 

[tsgrue]

.

These structures can provide a significant amount of recharge (depending upon volume in pipe, of course). Subsurface information is always advisable to obtain - and you would want to make sure a high water table or restrictive layer was not preventing outflow. In some situations, you might should undertake soil water mounding analysis.

.



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

http://hhwq.blogspot.com