Culvert design for shallow channels on flat grades.

[Akel]

Hello,

This may sound like a beginner's question but please bear with me.

I have no problem designing a culvert when the channel has a depth that is comparable to the height of the road. The narrowing of the cross-section at the entrance of the culvert causes water to back up and I design the culvert as outlined in FHWA's HDS-5, maintaining an acceptable headwater elevation and avoiding the overtopping of the road.

But what do I do when the channel is shallow (say 0.5m deep) and running through nominally flat ground and then encounters a road that is, say, 2m higher than the top of channel?. Any backing up of water behind the culvert will cause the channel to flood onto the adjacent flat areas. Should I provide a culvert that is at least as wide as the channel itself to avoid flow constriction? And if I do, should I design it as a culvert, as outlined in HDS-5, or design it as an open channel and size it using Manning?

Thank you.

 

[LincolnPE]

You need still need to design as a culvert per HDS-5... Sounds like you'll need multiple barrels (or a box if they can afford it).

 

[gbam]

First, Questions:
1. Can HECRAS be used to model the pre-culvert and post culvert condition for diorect comparision of backwater?
2. How wide is the approaching channel and can a arch culvert (bottomless) be used to span the flow?
3. Can Arch Pipes be used to increase capacity of shallow flows?
Maybe some additional site information would be helpful. However, I agree wuith Lincoln that you must still follow accepted practices to evaluate the condition. HECRAS just makes a direct comparison easy.

 

[Akel]

Thank you for your input, LincolnPE.

1 - The site is currently an empty land. All roads, channels and culverts are proposed, so there are no pre-culvert conditions to model in HEC-RAS. HEC-RAS can be used to model the post culvert condition.

2 - The approaching channel is a proposed prismatic trapezoidal concrete channel with a bottom width of 0.5m, a depth of 0.5m and with 2H:1V side slopes. The channel will be used exclusively for draining the adjacent road. Bottomless arch culverts are not allowed.
I'm only allowed to use either concrete pipe, box or arch culverts. Box culverts will certainly be rejected because it's unwarranted for such a small drainage channel. Arch culverts (with concrete bottom) come only in a single standard size, and that's with a span of 1.5m and a rise of approximately 1.4m.

3. I'm not allowed to use arch pipes.

Here is a sketch of the situation:

Thank You.

 

[gbam]

Sounds like your pipe arch fits your channel well. Check the hydraulics and make sure its good and roll with it. If the flow is contained within the channel, the flow will never encounter the culvert inlet right?

 

[Akel]

Though using the arch culvert does solve the problem, it is huge compared to the size and flow of the open channel and I feel it would be a waste of money.

The open channel will terminate at the face of the culvert thus the flow will encounter the channel bottom. But this brings me back to my earlier question: if the culvert is large enough (open channel is 0.5 m wide while the culvert is 1.5 m) as not to cause any flow constriction at the face of the culvert, why do I need to design it as a culvert instead of modeling it as an open channel?

 

[cvg]

your flow is 0.5 meters deep so your top width is 0.5 + 1.0 + 1.0 = 2.5 meters. your arch bottom width is 1.5 meters, so it is too narrow. It is likely that you will have significant headwater backing up at the inlet to the culvert. Suggest you use a box or two-barrel culvert. you will need to estimate entrance and exit losses for the culvert regardless of what size, shape or number of barrels.

 

[Akel]

Thank you, gbam and cvg.