Energy Dissipation in Buried Culverts?

[Margavich]

I'm designing a couple culverts (one 8x4 box, one double 72") for stream crossings and here in the state of Georgia the culverts must be 20% buried in the stream bed. Is energy dissipation required with buried culverts? I considered designing a rip rap basin at each outfall but began to wonder what the point of that would be since there really is no culvert to stream bed transition. Also the point of burying the culvert is for fish/wildlife passage and rip rap may interfere with that.

Any thoughts? I'm in early design stages but I need to nail this down to make sure I stay under 300 LF of stream disturbance per ACOE Nationwide permit requirements.

Also, a secondary question, if energy dissipation is needed what storm event would it typically be designed for? The pipe capacities are designed for the 100 year storm.

 

[Ryb01]

Maragavich,

You may be missing the bigger picture.

True, there would not be creek invert tranistion if the inverts are burried but there is velocity increase accosiated with flows through the culvert barrel in comparison to velocities expereinced further downstream of the crossing where flows are at normal depth. The velocities through the culvert barrel would increase based on head. This velocity increase could have the potential to erode the downstream watercourse depending on a number of factors including head on the culvert inlet, watercourse grade, soils and vegetation characteristics, watercourse geometry, backwater conditions, etc.

Good rule of thumb....if you have the slightest inclination to provide erosion protection/energy dissipation.....the cost of doing so is usually a drop in the bucket in comparison to the total cost of the project....put it in. It's easier to put it in now vs a few years down the road.

Depending on your state guidelines (beej67 would be able to elaborate further), rip-rap may or may not be permitted. If there is fish habitat present, river run stone may be more appropriate.

The depth of burry(i.e 20%) would include the depth of stone you are using to provide erosion protection. .

The design event for the dissipation would typically be the more frequent events up to and including bankfull flow. Beyond that, you'll have to make the call to see what makes sense for your project.

Hope this helps

 

[Margavich]

Thanks for the reply, yes what you're saying makes sense as the velocities will increase in the culvert regardless of having a buried bottom or not. On top of that the topo on the project site is very steep; the longitudinal slopes of the streams are 2-4% at the crossings and the contributing basins are 30-50% so velocities are high to begin with. So with all that some kind of energy dissipation seems logical. I found a small section on this in the GA DOT manual but again they make no mention of energy dissipation, I guess this is one area they really are leaving it up to engineer (I assume for liability reasons).

 

[gbam]

I really don't know the fish habitat requirements, no streams where I am located, although a very interesting topic. I do have a couple of questions:

Have you considered spanning the creek with a bottomless arch culvert? Wouldnt that eliminate the issue? You could minimize the impact to the stream as well. Do you have embankment height needed for this?

Have you evaluated the amount of scour within and downstream of the culvert to determine if the scour for your design flood is damaging? Your point may be moot of there is no significant damage resulting from the scour.

 

[Margavich]

@gbam, we are proposing bottomless culverts at a couple other locations but they are much more expensive. The nationwide permit allows for 300 lf of stream disturbance so we want to maximize this allowed disturbance to minimize costs.

 

[cvg]

if you are placing any fill in the creek, then there is a transition between the stream bed and the culvert. Unless your culvert completely spans the entire creek.

also consider that during the storm events, the flow may contain a significant amount of sediment. an abrupt (headwall) transition to a culvert may cause the sediment to drop out. Alternatively, a tapered (wingwall) transition will increase the velocity and cause scour at the wingwalls or the channel banks. That will require riprap to protect against scour.

At the downstream end, you could just provide a headwall with a cutoff to protect the culvert, but you will get a scour hole and significant sediment movement downstream. this may not meet the general conditions of the 404 permit either. riprap can be designed so there is very little effect on the movement of either fish or other wildlife.

 

[beej67]

Hey. Yep, I've done this sort of thing before.

On an iPad right now and don't have much time to elaborate, but the first thing I'd say is consider going with a smaller partially buried culvert to make the Corps happy about their fish, and a larger overflow culvert set at a higher elevation to convey the event storms. Lay the smaller one in at a slope flat enough to where velocities won't scour out your bed material, and consider muddying in a slot weir of some kind on the downstream end to help keep the soil in the pipe. Then riprap the larger pipe's discharge in accordance with its larger conveyance.

A lot of the design criteria for these partially buried pipes hasn't been fully thought out by the Corps in my opinion, so try and think the whole design through when you're doing them.

The last time I did one of these, the locals (I respectfully won't mention who) required me to do a full HEC-14 design instead of your usual Green Book thing, and the energy dissipator was twice the size of the culvert crossing. I rigged the downstream rip rap elevation at the far side of the plunge pool to force a certain backwater through the pipes for fish passage. Worked great. Spent a ton of money on rip rap though, which was fine(ish) I guess because the project itself was for the municipality.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[beej67]

Echo the scour analysis comments, particularly on your bottomless arches. I've seen those fail when the foundations weren't deep enough to handle the scour holes caused by flow turning that corner where the embankment comes into the archway. What sometimes happens on those is the civil has excluded scour analysis in his contract, thinks the arch supplier is going to do it. Arch manufacturer thinks the footing guy is going to do it. Footing guy thinks the civil is going to do it. Nobody does it. Thing fails. Everyone points. You dont want to be in that situation, so if you're not handling the scour analysis at the bottomless arches, get something in writing that the guy designing the strip footing is going to do it. It's no big deal if your span is entirely out of the 100 year floodplain, but that's not always the case when you're trying to save money.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com