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.

 

[r13]

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]

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]

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]

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

 

[LittleInch]

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]

Noted. Glad we are not scientist