Geoscience Reference
In-Depth Information
The specific gravity of sediment,
G
, is the ratio of the specific weights of sediment
and water at a standard reference temperature that is commonly set at 4
◦
C. The specific
gravity of quartz particles is
=
γ
f
=
ρ
s
s
G
f
=
2.65
(2.6)
γ
ρ
Particle size and grade scale
Sediment particle size may be represented by nominal diameter, sieve diameter, and
fall diameter. The nominal diameter,
d
, is defined as the diameter of a sphere that has
the same volume as the given particle, i.e.,
3
6
V
π
d
=
(2.7)
where
V
is the volume of the sediment particle, and
π
is the circumference-diameter
ratio
(
≈
3.14159
)
. The SI units often used for sediment size are mm (millimeters) and
m (meters).
A sediment particle may be considered as an ellipsoid. Denote
a
,
b
, and
c
as its
diameters in the longest, the intermediate, and the shortest mutually perpendicular
axes, respectively. Thus, as an approximation, the particle volume may be estimated
as
V
6, and then substituting this formula into Eq. (2.7) yields the following
relation for the nominal diameter:
≈
π
abc
/
√
abc
3
d
≈
(2.8)
The sieve diameter is the length of the side of a square sieve opening through which
the given particle will just pass. It is approximately equal to the intermediate diameter
b
. The sieve diameter is slightly smaller than the nominal diameter. For naturally worn
sediment particles over the range of about 0.2 to 20 mm, the sieve diameter is approxi-
mately 0.9 times the nominal diameter on average (U. S. Interagency Committee, 1957;
Raudkivi, 1990).
The standard fall diameter is the diameter of a sphere that has a specific gravity
of 2.65 and has the same terminal settling velocity as the given particle in quiescent,
distilled water at a temperature of 24
◦
C.
Sediment size may be measured by calipers, by optical methods, by photographic
methods, by sieving, or by sedimentationmethods (Vanoni, 1975; Simons and Senturk,
1992). For coarse particles, such as boulders, cobbles, and coarse gravel, size may be
determined by direct measurement of the volume or the diameters
a
,
b
, and
c
in
the longest, the intermediate, and the shortest axes, which are usually converted to
the nominal diameter by Eq. (2.7) or (2.8). For fine gravel and sand, size may be
determined by sieving or visual accumulation tube. For silt and clay, size is measured
by hydraulic settling methods, such as the pipet method, bottom withdrawal method,
and hydrometer method. The fall diameter is often obtained by these methods for silt
and clay.
