Geoscience Reference
In-Depth Information
Table 2.2
Sediment grade scale
Class
Size range (mm)
Class
Size range (mm)
Very large boulders
4,000-2,000
Very coarse sand
2-1
Large boulders
2,000-1,000
Coarse sand
1-0.5
Medium boulders
1,000-500
Medium sand
0.5-0.25
Small boulders
500-250
Fine sand
0.25-0.125
Very fine sand
0.125-0.062
Large cobbles
250-130
Coarse silt
0.062-0.031
Small cobbles
130-64
Medium silt
0.031-0.016
Fine silt
0.016-0.008
Very coarse gravel
64-32
Very fine silt
0.008-0.004
Coarse gravel
32-16
Coarse clay
0.004-0.002
Medium gravel
16-8
Medium clay
0.002-0.001
Fine gravel
8-4
Fine clay
0.001-0.0005
Very fine gravel
4-2
Very fine clay
0.0005-0.00024
The aforementioned boulders, cobbles, gravel, sand, silt, and clay are classi-
fied based on the grade scale listed in Table 2.2, which is commonly used in river
engineering. Each class may be further divided into several subclasses.
Shape factor
The shape of sediment particles in natural rivers is very irregular. It is often described
by the Corey shape factor:
c
√
ab
S
p
=
(2.9)
The Corey shape factor of naturally worn particles is usually about 0.7.
2.1.2.2 Bulk properties of sediment mixtures
Size distribution
Amixture that consists of sediment particles with non-uniform sizes can be represented
by a suitable number of size classes. Each size class, numbered as
k
, is defined by the
lower and upper bound diameters and represented by a characteristic diameter,
d
k
.If
the lower and upper bound diameters of size class
k
are
denote
d as
d
lk
and
d
uk
, the
characteristic diameter
may b
e determined using
d
k
d
lk
d
uk
,
d
k
=
=
(
d
lk
+
d
uk
)/
2,
d
lk
d
uk
or
d
k
3.
The fraction,
p
k
, of each size class is the ratio of its weight (volume or number) to
the total weight (volume or number) of the mixture, ranging from 0 to 1. It should be
noted that
p
k
is also often defined by percent, ranging from 0 to 100.
The size distribution (composition, gradation) of a sediment mixture can be mea-
sured by sieving analysis. It is often represented by the frequency histogram (pyramid)
and cumulative size frequency curve.The histogram is constructed by plotting the sizes
representing size class intervals on the abscissa and the actual percent (by weight, vol-
ume or number) of the total sample contained in each size class on the ordinate, as
=
(
d
lk
+
d
uk
+
)/
