About the size-velocity data for river particles
Rivers and streams carry small solid particles of rock and mineral downhill, either suspended
in the water column ("suspended load") or bounced, rolled or slid along the river bed ("bed load").
Solid particles are classified according to their mean diameter from smallest to largest as clay, silt, sand, pebble,
cobble and boulder.
During low velocity flow, only very small particles (clay and silt) can be transported by
the river, whereas during high velocity flow, much larger particles may be transported. There are two steps in
transporting a sediment particle in suspension. First, the velocity must be high enough to entrain or lift the
particle from the river bed. Entrainment velocities are usually very high for both clay sized particles (which
are "sticky") and large particles (which are heavy). Second, the velocity must be high enough to keep
the particle in suspension. For very small particles, the speed necessary for a particle to remain in suspension
is often lower than the entrainment velocity.
The data show the speed necessary to carry particles in suspension, once they have been entrained.
There is a near perfect quadratic fit to the data. It would be a good exercise to discover if the perfect
quadratic fit holds if the units of measurement were changed.
As we can correctly (?) assume that with zero current speed, the diameter of an object moved
is also zero, we could add (0,0) to the data set. If we also assume that as current speed increases,
the size of the object moved increases (in the neighborhood of (0,0)), then we could fit the data with a power
function of the form: diameter = speed^2. If there is a minimum current speed k needed
to overcome frictional forces, then the function could be of the form: diameter = (speed-k)^2.
The data are important for many reasons; for example, predicting sediment transport due to
changes in the watershed. Clearcutting forests and building impermeable surfaces (parking lots, houses, etc.)
reduces the storage capacity of a watershed. Dikes along rivers prevent spillage into natural flood
plains during periods of high discharge. These modifications of the natural environment result in increased
river volumes and speeds, resulting in the transport of more and larger particles that may alter the riverbed ecology.
Many species of salmon have very specific requirements of sediment sizes on the river bed for successful reproduction.
From Nielsen, A. (1950) Oikos, 2,176-96 as reported in Ecology for Environmental Sciences, Anderson