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Data Set #004

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About the Data

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 J.M.

     
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Size of Objects Moved by Different Current Speeds
From Nielsen, A. (1950) Oikos, 2,176-96
as reported in Ecology for Environmental Sciences, Anderson J.M.

Diameter of objects moved (mm)

Speed of current (m/sec)

Classification of Objects

0.2

0.10

Mud

1.3

0.25

Sand

5

0.50

Gravel

11

0.75

Coarse Gravel

20

1.00

Pebbles

45

1.50

Small Stones

80

2.50

Large stones (fist sized)

180

3.50

Boulders

 

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