Embedded CMP culvert with Slope... Stream vs. Water Slide 1

[darthsoilsguy2]

i'm looking at adding culverts to a seasonal stream to make some road improvements. The state is requiring the culverts have a minimum of 18" of stone inside the bottom of the culvert so there is a natural stream bed. we would like to accomplish this with 36" or perhaps 42" Corrugated Metal Pipe, but i'm having trouble finding information about slope limitations for doing this. There is a pipe slope where we're just creating a water slide for rocks and i would like to understand those limits better. I'm not getting much out of the manufacturer reps i've reached out to, nor anything from the state either. anybody dealt with this and found info... or studied existing ones? fyi my slopes are 5%, 10%, 15% and 30%.

 

[oldestguy]

It would seem that you need some tests or at least observation of her angle of repose for different "rock" materials so that you might be able to te that into erodability evaluations. Loose dumped angle of repose is sometimes taken as the least friction angle for the material. I'm curious how one gets to place this layer way back in the pipe.

 

[cvg]

30% slope is way too steep for anything other than large riprap
you need to look at your velocity. maybe a multiplate arch, preferably without invert would be better

 

[BridgeEI]

How are you going to put 18" of stone in a small diameter pipe?

 

[BUGGAR]

They use a series of drops for local man-made steep "streams" like this.

 

[darthsoilsguy2]

the "how" is tough, anything under 5' diameter and the contractor will likely put a lot of splices in the culverts instead of sending someone with a wheelbarrow. i'm not going to do the work so this is really a means/methods item with regards to splice locations. i will certainly use the "State-Made-This-Happen" card when the contractor inevitably proposes to not line it.

The culvert with the biggest flow should only see 30 CFS during a 25-yr storm. i'm sure the actual behaivior is some combination of these variables: corrugation geometry, gradation of stone, slope of pipe, angle of repose for the stone, velocity of water, effective weight of submerged stone... but i really would love a reference to hang the hat on. Maybe i should bring this up next time somebody comes here without a thesis....

i'm trying to convince the State into letting the 30% go without stone lining. i hope they will see that this is a rare scenario which is outside the norm by which their rules and guidelines were developed, and they shouldn't worry about setting a precedent by this.

 

[oldestguy]

Any natural streams in the area to use as a reference? If so, what is the stream bed at a given slope. Any 30% stream bed slopes and what is the bed material then?? If a proposed construction is not natural, will it be used by fish and then what purpose? How about a concrete bed to match solid rock?

 

[darthsoilsguy2]

Hey OG, i'm trying not to get into the minute details of the site because there isn't anything useful. The 30% culvert would be replacing an existing 5% culvert. About 30-40 yrs ago, somebody took an excavator and benched in a 20' wide trail against a steep mountain slope. Groundwater seeps out of that cut and forms a "stream" against the bank side until it gets to a low spot in the ATV trail profile and is routed to the other side of the trail by a small culvert. That existing 5% culvert discharges to a 1:1 slope perpendicular to trail and has been carving a notch into that slope ever since and if given time, would eventually undermine the trail. the proposed 30% culvert bypasses the slope and gets to a stable part of the streambed at the toe of the slope where grade flattens out for a bit.

 

[racookpe1978]

What will you do to prevent beaver/biologics from filling up the culvert over the gap left left by the rocks?

 

[cvg]

You wont find a reference, because this is not typically done, nor is it recommended. For natural streams, arches are sometimes used and installed below the invert of the stream, but not at 30% slope and certainly not for a culvert/ditch application like what you describe. I am interested in why the state thinks this is a good idea. if you want to try it and keep the rock from sliding out, construct a concrete cutoff wall at the end of the culvert, set the top of it 18 inches above the invert and use it to help retain the rock. Be aware that your flow will be highly supercritical and may take out all the rock anyway unless it is big enough. Typically for velocities this high, would use riprap D₅₀ of perhaps 12 inches. The only saving grace here is that your depth of flow will be very shallow.

 

[oldestguy]

If this is just a mountain trail, why not do something that works more dependably and would fit into the scene better. Apparently this is not a channel that is a fish habitat. Build a very shallow dip in the trail. Line that with a designed rip-rap blanket. It may have to be laid at an angle. Call it a ford. At the discharge build a reinforced slope using the principles of filtered rip-rap design, as one would have for an earth dam emergency overflow channel. Sure it will take more material, but it can be designed to work. You may even have to have big spaced blocks to reduce velocity. For a culvert at a steep slope what privision would there be for preventing erosion at the discharge? For the planned job would there be sufficient cutoff at the entry to prevent seepage going under the culvert and subsequent erosion there taking the whole thing out. This situation deserves a lot more thought I think. Use of culverts seems to be something not thoroughly explored. For lengthy road-side ditches on a sloped road a common way to minimize a lot of erosion ptential is to have many of them and set the discharge ends pipe out well away from the slope. Where water hits have a pile of rocks.

 

[BridgeEI]

Interesting, contractors around here would tell you to go pound sand and put some rock at the outlet and call it a day.

We've done cast-in-place concrete culverts on slopes up to 33% but keyed the culvert in every so often. I can't say any of the manufacturers I'm aware of have slope limitations other than trying to find a way to key it in. I believe we sized using our normal hydraulic procedures but then had to add an energy dissipator at the outlet. It's been a while since we did those...