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

DanielRonn (Civil/Environmental)

(OP)

23 Sep 20 09:43

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.

https://files.engineering.com/getfile.aspx?folder=1978b187-8052-47aa-b6bd-4

LittleInch (Petroleum) · Answer 1 · 2020-09-23 10:14:16

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.

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

DanielRonn (Civil/Environmental) · Answer 2 · 2020-09-23 10:55:10

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 (Structural) · Answer 3 · 2020-09-23 11:33:34

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 (Civil/Environmental) · Answer 4 · 2020-09-23 11:52:14

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 (Structural) · Answer 5 · 2020-09-23 12:55:03

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

www.SlideRuleEra.net idea

LittleInch (Petroleum) · Answer 6 · 2020-09-23 14:11:15

"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 (Geotechnical) · Answer 7 · 2020-09-23 14:55:35

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

r13 (Civil/Environmental) · Answer 8 · 2020-09-23 16:05:35

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

IRstuff (Aerospace) · Answer 9 · 2020-09-23 16:37:07

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)
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cvg (Civil/Environmental) · Answer 10 · 2020-09-23 16:41:56

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 (Civil/Environmental) · Answer 11 · 2020-09-23 17:29:54

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...

cvg (Civil/Environmental) · Answer 12 · 2020-09-23 17:54:57

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 (Civil/Environmental) · Answer 13 · 2020-09-23 18:24:13

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

r13 (Civil/Environmental) 23 Sep 20 18:24

Also, don't forget the buoyancy.

LittleInch (Petroleum) · Answer 14 · 2020-09-23 18:50:28

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 (Civil/Environmental) · Answer 15 · 2020-09-23 20:16:43

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 (Civil/Environmental) · Answer 16 · 2020-09-23 21:26:44

Quote:
Can one use bridge foundation formulas or anything alike?

This article may help. Link

LittleInch (Petroleum) · Answer 17 · 2020-09-23 21:31:33

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 (Civil/Environmental) · Answer 18 · 2020-09-23 21:36:32

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_Hydrau...

cvg (Civil/Environmental) · Answer 19 · 2020-09-23 21:44:21

debris flow
note that the rocks dont slide, they roll or "float" due to bouyancy
https://youtu.be/Fsh5E9m3PrM

r13 (Civil/Environmental) · Answer 20 · 2020-09-23 23:11:10

A rough estimate of the flow velocity required to move the rock.

V = √[H/(ρ_wA)]
H = (W_rock - U)Tanø

A = Horizontal project area of the rock

IRstuff (Aerospace) · Answer 21 · 2020-09-24 05:28:25

Isn't this all moot? The water flow required to move such a boulder will cause widespread destruction and mayhem, and the positioning of this, or any other, will be irrelevant. The water flow that caused this is in no way normal flow, particularly given the high water mark on the boulder.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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DanielRonn (Civil/Environmental) · Answer 22 · 2020-09-24 07:46:08

Hi. Thanks for all your answers. As LittleInch says, more details are needed. I have attached a Google maps image and some more photos here where the location of the boulder is. I appreciate all info and evaluations on wither the river could in fact move the boulder to its present location. Some questions..

- 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.

https://files.engineering.com/getfile.aspx?folder=4ec844ab-4944-44ff-9126-5

DanielRonn (Civil/Environmental) · Answer 23 · 2020-09-24 07:47:16

Here is the Google Maps overview.

https://files.engineering.com/getfile.aspx?folder=4d093b89-5b68-4216-adff-a

DanielRonn (Civil/Environmental) · Answer 24 · 2020-09-24 07:49:17

What we now from geologists, is that the glacier could not have placed the boulder at its present location, as the overburden under it (the riverbed rocks) accumulated after the glacier was gone.

LittleInch (Petroleum) · Answer 25 · 2020-09-24 08:57:35

It is a bit odd isn't it as to why that single boulder seems to have survived and other haven't, but it seems to be off the main line and has become streamlined and hence any other boulders aren't impacting it directly in mega flood.

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.

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

r13 (Civil/Environmental) · Answer 26 · 2020-09-24 09:20:29

V - velocity (m/s)
H - buoyant force (N)
ρ_w - density of water (kg/m³)
A - horizontal project area of the rock (m²)
W_rock- weight of the rock (N)
U - net uplift (N), U = unit weight of water x ~~depth of water~~ volume of rock
Tanø - ~~soil~~ friction coefficient

Rock 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

HTURKAK (Structural) · Answer 27 · 2020-09-24 10:32:36

Dear DanielRonn ,

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.

LittleInch (Petroleum) · Answer 28 · 2020-09-24 10:42:44

And don't forget, you're looking at the boulder as it is now after a long time of being battered.

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

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

DanielRonn (Civil/Environmental) · Answer 29 · 2020-09-24 11:34:23

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

DanielRonn (Civil/Environmental)

(OP)

24 Sep 20 11:34

Here are some more photos...

DanielRonn (Civil/Environmental) · Answer 30 · 2020-09-24 11:36:12

The other boulders in the photo above marked with red arrows, are about half the size of the large boulder.

DanielRonn (Civil/Environmental) · Answer 31 · 2020-09-24 11:41:11

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

DanielRonn (Civil/Environmental)

(OP)

24 Sep 20 11:41

And the overview image again...

DanielRonn (Civil/Environmental) · Answer 32 · 2020-09-24 11:49:43

Retired13.. thanks for a good formula!! Some questions...

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?

r13 (Civil/Environmental) · Answer 33 · 2020-09-24 12:09:47

Daniel,

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.

DanielRonn (Civil/Environmental) · Answer 34 · 2020-09-24 12:23:10

retired13... THANKS!!!

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.

r13 (Civil/Environmental) · Answer 35 · 2020-09-24 13:23:07

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:

LittleInch (Petroleum) · Answer 36 · 2020-09-24 13:35:09

No but then neither is this boulder a flat smooth surface sliding on another flat smooth surface.

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.

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

r13 (Civil/Environmental) · Answer 37 · 2020-09-24 13:44:35

Note the the calculated velocity occurs near the river bed, during flood, the velocity above will be much greater. Also, the width/shape of the flow channel will affect the velocity. So the calculated velocity only provides a rough idea.

DanielRonn (Civil/Environmental) · Answer 38 · 2020-09-24 15:00:26

ok.. so the surface of the river will move much faster than 2.9? It would be great with a video of a river that moves in this speed at the bottom.. smile

LittleInch (Petroleum) · Answer 39 · 2020-09-24 15:48:50

scroll / jump through to about 4mins 50.

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)

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

r13 (Civil/Environmental) · Answer 40 · 2020-09-24 15:55:42

This is a profile of open channel flow. The velocity at the bottom is impacted by the roughness of the flow bed, that drags the flow.

DanielRonn (Civil/Environmental) · Answer 41 · 2020-09-25 09:39:59

Thanks all.. RETIRED13, some input on your very helpful calcs..

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.

r13 (Civil/Environmental) · Answer 42 · 2020-09-25 15:04:56

Daniel,

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.

chicopee (Mechanical) · Answer 43 · 2020-09-26 14:44:49

Find fissures in the rock and sledge hammer in steel wedges to break up the rock.

LittleInch (Petroleum) · Answer 44 · 2020-09-26 15:54:16

R13 (previously retired13)

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.

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

r13 (Civil/Environmental) · Answer 45 · 2020-09-26 16:08:51

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

r13 (Civil/Environmental) 26 Sep 20 16:08

Noted. Glad we are not scientist :)

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