Need to calculate if a large boulder can be moved by a river!!
Need to calculate if a large boulder can be moved by a river!!
(OP)
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.
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.
RE: Need to calculate if a large boulder can be moved by a river!!
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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RE: Need to calculate if a large boulder can be moved by a river!!
RE: Need to calculate if a large boulder can be moved by a river!!
But why does the boulder bother you? It doesn't seem to be doing any harm.
RE: Need to calculate if a large boulder can be moved by a 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.
RE: Need to calculate if a large boulder can be moved by a river!!
www.SlideRuleEra.net
RE: Need to calculate if a large boulder can be moved by a river!!
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?
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RE: Need to calculate if a large boulder can be moved by a river!!
RE: Need to calculate if a large boulder can be moved by a river!!
RE: Need to calculate if a large boulder can be moved by a river!!
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)
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RE: Need to calculate if a large boulder can be moved by a river!!
RE: Need to calculate if a large boulder can be moved by a river!!
https://www.usatoday.com/story/news/nation/2014/08...
RE: Need to calculate if a large boulder can be moved by a river!!
RE: Need to calculate if a large boulder can be moved by a river!!
RE: Need to calculate if a large boulder can be moved by a river!!
That boulder could have moved there hundreds of years ago out much longer of it was ice movement.
It is a nice boulder alright.
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RE: Need to calculate if a large boulder can be moved by a river!!
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?
RE: Need to calculate if a large boulder can be moved by a river!!
This article may help. Link
RE: Need to calculate if a large boulder can be moved by a river!!
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.
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RE: Need to calculate if a large boulder can be moved by a river!!
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_Hydrau...
RE: Need to calculate if a large boulder can be moved by a river!!
debris flow
note that the rocks dont slide, they roll or "float" due to bouyancy
https://youtu.be/Fsh5E9m3PrM
RE: Need to calculate if a large boulder can be moved by a river!!
V = √[H/(ρwA)]
H = (Wrock - U)Tanø
A = Horizontal project area of the rock
RE: Need to calculate if a large boulder can be moved by a river!!
TTFN (ta ta for now)
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RE: Need to calculate if a large boulder can be moved by a river!!
- Why did the boulder not end up infront of the meander? As noted, the normal water level in this river is about 2 to 3 feet. It is fast during spring, but not very fast.
Retired13 posted a formula above. Could you please elaborate on all the variables? I'm not a math guy.
RE: Need to calculate if a large boulder can be moved by a river!!
RE: Need to calculate if a large boulder can be moved by a river!!
RE: Need to calculate if a large boulder can be moved by a river!!
It might need a 1 in 500 yr flood to do this, but that isn't really a big boulder in real terms. It might have got "flung out" if the main flood path is different.
River channel width might have been a lot narrower leading to higher water velocities.
I've got no problems envisaging this being carried downstream in a serious flash flood.
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RE: Need to calculate if a large boulder can be moved by a river!!
V - velocity (m/s)
H - buoyant force (N)
ρw - density of water (kg/m3)
A - horizontal project area of the rock (m2)
Wrock- weight of the rock (N)
U - net uplift (N), U = unit weight of water x
depth of watervolume of rockTanø -
soilfriction coefficientRock weight = 174.7 kN
Rock base area = 6.4 m2; assume rock height = 1.5 m
U = 10*6.4*1.5 = 96 kN
H = (174.7-96)*0.55 = 43.3 kN
Rock horizontal project area = 2.9 m2
Density of water = 1000 kg/m3
V = (43.3*1000/(1000*2.9))0.5 = 3.9 m/s
RE: Need to calculate if a large boulder can be moved by a river!!
Will you please post other pictures (if any ) , along the river bed to see the steep hills and mountain valleys around ? ..This could happen after heavy storm flood , and the reason is rolling of the large boulders together with sand and gravel at river bed along the steep slope. You can see even more larger boulders at OMAN river beds.
RE: Need to calculate if a large boulder can be moved by a river!!
It may well have originally been more curved / round or rolled on what is now its side, not slid down the river bed.
If you rotated that boulder 90 degrees and rotated it on its axis 90 degrees so the front face was higher then it would start rolling at a much lower water speed and then just keep rolling along....
Now it might not have been as round as this one, but I think you get the drift... https://www.youtube.com/watch?v=ou8v8zPAcOM
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RE: Need to calculate if a large boulder can be moved by a river!!
RE: Need to calculate if a large boulder can be moved by a river!!
RE: Need to calculate if a large boulder can be moved by a river!!
RE: Need to calculate if a large boulder can be moved by a river!!
V - velocity (m/s)
This is the velocity of the water as I understand it?
ρw - density of water (kg/m3)
I will find.
A - horizontal project area of the rock (m2)
Is this the total "flat" area facing the waterflow? Not counting for the pointy edge? Or is it the total area including the slanted sides on the pointy edge?
Wrock- weight of the rock (N)
Not in ton or kg? How do I get this in N?
U - net uplift (N), U = unit weight of water * height of rock.
What is unit weight of water?
Tanø - friction coefficient
Do you know this for a rough rock surface?
RE: Need to calculate if a large boulder can be moved by a river!!
I made correction on my definition of terms, and provided calculation. If my math is correct, the velocity shall be greater than 3.9 m/s in order to move the rock. See my previous post on above for other parameters/assumptions.
The friction coefficient is 0.55 as provided by you.
RE: Need to calculate if a large boulder can be moved by a river!!
Would it be possible for you to post the values you used? The coefficient 0.55 is for a flat dry rock surface on both the floor and the boulder. I have no other info on coefficients.
RE: Need to calculate if a large boulder can be moved by a river!!
I would guess the river bed consists of silt loam, for which ø = 25°, and the approximate friction coefficient will be Tan(0.7ø) = 0.32 (geotechnical guys should chime in here). The adjusted H = (174.7-96)*0.32 = 25.2 kN, then revised V = (25.2*1000/(1000*2.9))0.5 = 2.94 m/s.
Derivation of equations:
RE: Need to calculate if a large boulder can be moved by a river!!
So COF for even simple siding could be 0.8 or 0.9 and that's before it runs into another boulder.
So if that's 4m/sec, I reckon my 7-8 m/sec in real life isn't too far away.
That's a LOT of water, but flash floods can be intensely damaging and fast flowing.
A quick google for flash floods found this https://www.sciencedirect.com/topics/social-scienc...
"In 1976, Colorado’s Big Thompson Canyon experienced a flash flood with a channel velocity of 30 ft per second. At that speed, the floodwater moved 250 ton (227,000 kg) boulders (FEMA, 2006)"
So your measly little 17,000 kg boulder stands no chance.
And look at this and fig 94 http://assets.wwf.org.uk/downloads/flood_risk___pa...
Looks like "normal" flash floods are about 3m/sec, so that's why the boulder doesn't move very often, but an extreme event up to 9 or 10 is possible.
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RE: Need to calculate if a large boulder can be moved by a river!!
RE: Need to calculate if a large boulder can be moved by a river!!
RE: Need to calculate if a large boulder can be moved by a river!!
https://www.youtube.com/watch?v=tWGK4CiWxeM
Then tell me the river couldn't shift your boulder??
I'm still struggling as to why you want / need to know.
Is someone trying to say a crowd of yetis dumped it there? Or big foot or whatever mythical huge hairy creature you have in northern canada ( and I don't mean the lumberjacks)
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RE: Need to calculate if a large boulder can be moved by a river!!
RE: Need to calculate if a large boulder can be moved by a river!!
The weight you say is in kN, is that just a transformation from metric tons?
You say the riverbed consists of silt loam, but what we see at the site the surface under the boulder is big 300 to 500 kg rocks forming a very rough surface. There are some football-sized rocks as well but no small loose gravel.
RE: Need to calculate if a large boulder can be moved by a river!!
1 metric ton = 1000 kg (mass)
1 kg mass weights 10 N
So 1 metric ton mass weights 10000 N, or 10kN
The friction thing is very difficult, if not impossible, to evaluate. But you are right, the friction coefficient can go up much higher. So you may say my estimate is the lower bound estimate. 0.55 could be middle in the road.
RE: Need to calculate if a large boulder can be moved by a river!!
RE: Need to calculate if a large boulder can be moved by a river!!
Well this is an Engineering forum so:
1kg (mass) at sea level on Earth creates a force commonly know as weight equal to mass x gravity (9.8m/s2)
So it is really 9.80665N
Sure 10 is a nice round number to use, but that 2% can become important....
Given the assumptions and lack of accuracy of everything else, 10 is a good number, so 1 tonne = 10kN for these purposes.
He doesn't want to move or break up the rock, for reasons he's not really telling us, he just wants to know if the river could have moved the rock.
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RE: Need to calculate if a large boulder can be moved by a river!!