Environmental Data Analysis BC ENV 3017

Hudson River sediment transport


A river can transport sediments and other particles. The size of the particles that can be transported depends on the flow velocity (Fig). The flow velocity needs to be high enough so that vertical components are of the order of the settling velocity:

Settling velocity or fall velocity or terminal velocity (w) of a (sediment) particle is the rate at which the sediment settles in still fluid. It is diagnostic of grain size, but is also sensitive to the shape (roundness and sphericity) and density of the grains as well as to the viscosity and density of the fluid. It integrates all of these into a key transport parameter.

For dilute suspensions, Stokes' Law predicts the settling velocity of small spheres in fluid, either air or water. Stokes' Law finds many applications in the natural sciences, and is given by:

w=\frac{2(\rho_p-\rho_f)gr^2}{9\mu}
here w is the settling velocity, ρ is density (the subscripts p and f indicate particle and fluid respectively), g is the acceleration due to gravity, r is the radius of the particle and μ is the dynamic viscosity of the fluid. This law is valid for low Reynolds numbers (slow flow, non-turbulent).

Classification of grain sizes (Fig)

Relationship between transport and flow velocity (Fig)

Flow velocity distribition in rivers (Fig)

Flow velocity tends to be lower in the shallow parts of the river and therefore coarser particles do not make it there and the finer particles settle out. We would expect a relationship between depth of a stream and grainsize of the sediments, with the finer sediments concentrated in  shallow parts of the river.

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