Need to calculate if a large boulder can be moved by a river!! 1

[DanielRonn]

Hi. For environmental purposes, we need to calculate the minimal water/ice SPEED needed from the river to be able to move this large boulder laying on top of the rocky riverbed. The river is in Canada and fed by rainwater only. This is a non-profit project. Please advise on a formula to use.

It is a white water river during a few months a year, no vegetation only bare rocks, a very rough surface. Below data has been calculated with a photometric 3D scan of the boulder and 3D software. The boulder is pointy at the edge facing the waterflow:

Boulder weight:
17.47 metric tons.

Boulder footprint (area touching the riverbed):
6.4 square meters.

Boulder upstream surface in the direction of the water flow, exposed to the push of water and ice. This is the flat 2D surface. The real surface is larger but angled as its the pointy edge of the boulder that faces the riverflow:
2.9 square meters.

 

[LittleInch]

Can't you just drill and blast it out of the way?

Or do that boulder cracking technique you see people use?

I've tried to do similar things with pipelines crossing rivers / peat / mud flows and it's difficult and the answers have a wide range.

Your biggest unknown is not the water flow pressure / force, but the impact (literally) of other boulders / trees / ice on the boulder.

Once it starts moving it could easily just keep going for long way and that "jolt" from a smaller boulder or tree could be the cause of it starting to move. That force is very difficult to simulate.

I'll be interested to see if there is any sort of guidance / formula for this.



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[DanielRonn]

We don't need to remove it, we just need to figure out how it got there as this is a 19th-century mining area in general. Either it's put there by man (4 oxen would move it), or by the river. It is unique to the area of the river. No such large boulders exist here and the river is shallow.

 

[hokie66]

My guess is that nature put it there, along with all those other rocks. But you really need a geologist for this kind of advice.

But why does the boulder bother you? It doesn't seem to be doing any harm.

 

[DanielRonn]

The boulder does not bother me.. it is a very nice boulder. I know the landowners beside it and they are trying to get to know more about the area. There is a possible X cut into the shore-side of the boulder and people say the boulder is too big to be moved by the river.

We just want to get an idea of how difficult it would be for water to move it.. aka get a number on the speed needed for water/ice to push and move it. The old miners have dug deep trenches with machines around here so it could have been moved to this place.

 

[SlideRuleEra]

Maybe it is a glacier erratic, from the last ice age.

http://www.SlideRuleEra.net

 

[LittleInch]

"For environmental purposes, we need to calculate the minimal water/ice SPEED "

What environmental purposes exactly??

I thought you were worried about it moving downstream and hitting something.

If you've got ice flows or the ability for ice to firm under the rock then maybe 10m/sec?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[oldestguy]

US Corps of Engineers may have some info on rip-rap and water velocity.

 

[r13]

What is the river width, depth, channel shape, slope, flow rate, maximum flood in the past....?

 

[IRstuff]

As stated, "normal" "flow" of the river probably didn't move the rock, but rivers often have far from "normal" flows, so it's possible that some massive flood moved it into position.

Nevertheless, the majority of the rocks in the picture appear to be relatively unweathered, so ice age movement seems likely, since that was only about 14,000 years ago

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[cvg]

I can pretty much guarantee that 4 oxen did not put that boulder there. it is natural and was placed precisely by a large flood. the fact that it is round / sub-rounded proves it has been subjected to water flow for a very long time. it may have been moved downstream from an area where there are rocky canyon walls where large rocks could slide and fall into the river over time

 

[CarlB]

Perhaps it was moved along with the help of an adhered ice sheet, along with water flow.

https://www.usatoday.com/story/news/nation/2014/08/28/newser-death-valley-rock-mystery/14736847/

 

[cvg]

very large rocks can be moved easily by large floods. especially if those floods carry large amounts of debris. the boulder in this photo was moved by a debris flow. the normal flow that you can see is only a few feet deep.

 

[r13]

Also, don't forget the buoyancy.

 

[LittleInch]

I've seen boulders the size of houses moved down rivers in mega flood big enough to bring down bridges.

That boulder could have moved there hundreds of years ago out much longer of it was ice movement.

It is a nice boulder alright.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[DanielRonn]

ok.. thanks for all the input but let me re-phrase the question...

We know it will take 9.6 metric ton-force to get the boulder moving if it had a flat underside and the riverbed was also flat (friction coefficient 0.55). We calculated this. So the question is, what kind of speed does the river need to achieve this power on the boulder surface to move it?

Can one use bridge foundation formulas or anything alike?

 

[r13]

Can one use bridge foundation formulas or anything alike?

This article may help. Link

 

[LittleInch]

A lot.

Great question but a river flow like you have isn't like that and neither is friction like that. Far took many variables to get an answer worth having.

Like I said before 10 m/sec won't be far off. Maybe a bit less maybe a bit more. Throw in a few rocks, a tree stump or two to create a bit more drag and maybe 7-8 m/sec.

Like a few others here as the water level rises the friction reduces as the buoyancy increases.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[cvg]

its a function of flow velocity, flow depth and density
you can calculate the shear force of the flowing water and that can be used to estimate the force applied to the rock. dont forget the bouyant force

http://www.fsl.orst.edu/geowater/FX3/help/8_Hydraulic_Reference/Shear_Stress.htm

 

[cvg]

debris flow
note that the rocks dont slide, they roll or "float" due to bouyancy

https://youtu.be/Fsh5E9m3PrM