SETTLING VELOCITY AND GRAIN SIZE
Objectives



To determine the grain size distribution in a sediment sample,
To make a grain chart based on the different fractions of sediments separated in
the grain size distribution process analysis, and
To find out the relationship between sediment particle size and settling velocities
as the latter are the controlling factors for sediment transport.
Introduction
Beaches erode, and rivers carve their way through the earth. Part of why this happens
relates to grain size distributions, and differences in sediment velocities. Coastal
environments are only one place where knowing the grain size distribution is important.
As an example, some beaches are managed by man to keep up with natural erosional
processes. When renourishment (adding sediment back onto an eroded area – like a
beach) is used, both the grain size distribution and the settling velocity of the sediment
need to be known – of both the sand that is lost as well as the replacement sand. Sand
that doesn’t match in terms of grain size distribution can case shifts in biological
communities contained within, and sand that doesn’t match in-situ sediment velocities
will erode unevenly, causing even more of an erosional problem.
Grain Size Distribution
Several techniques are available to analyze the size of beach materials, and each
technique is restricted to a range of sediment sizes. Pebbles and coarser materials are
usually directly measured with calipers. However, this is not practical for sediments
smaller than about 8 mm (from pebbles to clay). Coarse sieves can also be used for
material up to about 75 mm (cobbles).
Sand-sized particles (medium gravels through coarse silt) are usually analyzed using
sieves. This requires an ordered stack of sieves of square-mesh woven-wire cloth.
Sieves are nested one on top of the other, so that the one with the largest mesh size is
on top, and the smallest mesh size is on the bottom. A receptacle must be placed
under the bottom sieve to catch any sediment that passes through the smallest sieve.
The range of sieve mesh sizes must span the range of sediment sizes to be sieved.
Typically, about 6 full-height sieves or 13 half-height sieves plus a bottom pan are used
in the analysis of a particular sediment.
A 15-minute shaking period can be used in this procedure. A larger sample requires a
longer shaking period. Similarly, a sample composed primarily of fine-grained material
requires a longer shaking period than a coarse-grained sample of equal weight.
Exercises
Part I
Each group will be given a container and one type of sediment – either well sorted or
poorly sorted. You will mix the sediment with water as instructed in class and then
make an observation at each of the following time intervals. Describe the sediment
accumulation in the bottom as well as the sediment still in suspension.
Time
Comments
1 sec
30 sec
1 min
10 min
30 min
1 hr
End of Lab
A) What change did you notice in the amount and appearance of the sediment on the
bottom of the jar as time passed?
______________________________________________________________________
______________________________________________________________________
___
B) What changes did you notice in the water as time passed?
______________________________________________________________________
______________________________________________________________________
____
C) How would you explain these changes?
______________________________________________________________________
______________________________________________________________________
____
Part II
Next, you will learn to apply the physical law, which describes the velocity, or speed, at
which spherical particles settle through the water column. Suppose we had some sand,
silt and clay particles that are all composed of quartz (density = 2.65 g/cm3). According
to Stoke's Law, the settling rate of particles is affected by the gravitational force exerted
on the particle, the density of the particle relative to the density of the medium, and the
viscosity (resistance to flow-settling) of the medium. These factors are used in Stoke's
Law to determine settling velocity of spherical particles of varying densities according to
the following formula.
Velocity (cm/sec) = G * (p - ) * d2
18
G = Gravitational acceleration (980 cm/sec2)
 = the specific gravity (density) of the medium (seawater at 20° C= 1.03 g/cm3
p = the specific gravity of the sediment particle = 2.65 g/cm3
 = the viscosity of the medium (seawater at 20° C) = 0.01391 g/cm*sec
This formula can be summarized as follows:
 cm 
Velocity 
 
 sec 
980
cm 
g
g 
  2.65 3 - 1.03

2
sec
cm
cm 3 

 d(cm) 2
g
18  0.01391
cm  sec
cm
g
 1.62
2
sec
cm 3  d(cm) 2

g
18  0.01391
cm  sec
980
= 6340.76 cm/sec x d(cm)2
To determine the settling rate of the given sediment sizes, simply square the particle
diameter and multiply that number by the constant (6340.76 cm/sec)
1) Using this equation, calculate the sinking velocity of each quartz particle considering
their diameters are as follows:
sand = 0.100 cm ___________________________________________________
silt = 0.005 cm _____________________________________________________
clay = 0.0025 cm ___________________________________________________
2) Now calculate how long it would take each particle to settle through 1000 m of water.
sand = ___________________________________________________________
silt = _____________________________________________________________
clay = ____________________________________________________________
3) What is the relationship between sediment size and settling velocity?
4) How does this relationship relate to the distance away from a river mouth or estuary
where you would expect each particle to settle?
Part III
Things that you need to know in order to complete this section are below.
--The retained total mass for each sieve is calculated by summing the Mass Retained
for the current sieve, and each sieve larger than it.
--The accumulated mass of sediment finer than every size, is calculated by subtracting
the Retained Accumulated Mass from the Total Sediment Mass.
--The fraction of sediment finer than every size is calculated by dividing the
Accumulated Mass of Finer Sediment by the Total Sediment Mass.
1. Make a plot using logarithmic paper with sediment size in mm on the x-axis and %
finer for each size on the y-axis. The calculations for the size distribution analysis will
be recorded in Table 1.
TOTAL SEDIMENT MASS: _______________
Table 1. Grain size distribution.
Sieve opening
size (mm)
Mass of
sieve (g)
Mass of
sediment
and sieve
Mass
retained in
sieve (g)
Retained
accumulated
mass (g)
Accumulated
mass of finer
sediment (g)
Fraction of
sediment
finer (%)
Reference: King, D., and C. J. Galvin. 2002. Coastal Sediment Properties. In:, King, D. Coastal
Engineering Manual, Part 3, Coastal Sediment Processes. Chapter 1, Engineer Manual 1110-201100,
U.S. Army Corps of Engineers, Washington, DC.