Re: 'competence' and 'capacity'
Thanks ET,
This shows that Geology is bigger than any one person - these precise terms were unfamiliar to me!
Competence: The quick answer is that a gas lubricated flow is much less capable of carrying coarse material than is a similar cross-section liquid flow, therefore large flows are required to transport large grains or boulders. OTOH, there is ample evidence for gas-lubricated flows on Earth moving VERY large boulders.
Capacity: Depends on the scale of the flow. Again, I suspect less than an equivalent X-section of liquid, but large scale of flows permits large scale of transport.