Mountain river in dry season flows at speed 4m/s and can drag and roll rocks up to Mo=10kg?

Question

In rainy season the speed of the stream increases to 10m/s.
What is the mass M of the biggest rock, which the river can drag and roll in rainy season?

Answer 1

Assume, per steve, that pressure or Fdrag/area increases by (v2/v1)^2, and that a rock of any size just rolls when Fdrag/M = k. Define eta = the just-rolling rock's new/old radius ratio. The new/old Fdrag/M ratio (which we want to keep the same) = (v2/v1)^2 * eta^2 / eta^3 = (v2/v1)^2 / eta. So radius increases by 2.5^2, area by 2.5^4, and volume and maximum movable mass by 2.5^6 = 244, to 2440 kg.

Answer 2

The only conclusion one can reach with the provided info is that the dynamic pressure of the water varies approximately with the square of the velocity.
The ability of a given velocity to move a rock of a particular mass is heavily dependent on its volume, shape and flow parameters such as Reynolds #, surface friction, etc.........