Geoscience Reference
In-Depth Information
(a)
(b)
Udden-
Wentworth
Oblate (disc, tabular)
Equant
(spherical)
Phi (
φ)
mm
μ
m
−
0.8
7
−
128
64
32
16
8
4
2
1
0.5
Cobbles
6
Bladed
Prolate
(roller,
rod-like)
0.6
−
5
Pebbles
d
I
d
L
4
−
−
3
0.4
−
2
Gravel
V. coarse
Coarse
Medium
Fine
V. fine
1
−
0.2
d
S
- shortest axis
d
I
- intermediate axis
d
L
- longest axis
0
1
2
3
4
5
6
7
8
9
500
250
125
63
31
15.6
7.8
3.9
2.0
0.2 0.4 0.6 0.8
d
S
d
I
(c)
Silt
Clay
Very poorly
sorted
Very well
sorted
Well
sorted
Moderately
sorted
Poorly
sorted
1 3 5 7
Fig. 1.4
(a) The Udden-Wentworth scheme, which is widely used to describe grain-size categories. (b) Classification scheme used
to describe particle form based on the ratio between the short, intermediate and longest grain axes. (Adapted from Graham 1982.)
(c) Visual comparison chart used for describing the degree of sediment sorting within a sediment deposit. (Adapted from Graham 1982.)
a wide area and can be defined as 'pollution
arising from land-use activities (urban and
rural) that are dispersed across a catchment or
subcatchment, and do not arise as a process of
industrial effluent, municipal sewage effluent,
deep mine or farm effluent discharge' (Novotny
2003). Examples include direct atmospheric
deposition, urban runoff and sediments (i.e.
from the road network), agricultural runoff and
sediments (i.e. from soil erosion), the reworking
of floodplain sediments (i.e. by bank erosion)
and background geology.
are typically made and are used to calculate a
mean diameter. For smaller particles, grain size
is typically determined by grading the samples
through a set of sieves (see McManus 1988). A
number of schemes have been devised to describe
and measure grain size, but one of the most
widely used is the Udden-Wentworth scheme
(Fig. 1.4a).
Descriptions of sediment shape are somewhat
more complex and may be taken to comprise
elements of a particle's form, roundness and
texture. Roundness is usually described on the
basis of comparisons with visual identification
charts. Form is also usually quantified by describ-
ing grains in terms of one of four standard classes:
oblate, equant, bladed or prolate, which reflect
the relationship between the short, intermediate
and long axes of grains (Fig. 1.4b). Other useful
schemes combine elements of both roundness
and sphericity in visual comparison charts (e.g.
Powers 1982). Particle sorting describes the range
of grain sizes that occur within a sedimentary
1.2.4 Particle description and classification
Regardless of origin, individual sedimentary
particles are typically described in terms of their
grain size and shape. Grain size is an important
parameter both from a descriptive perspective
and in relation to understanding sediment trans-
port and deposition (see section 1.3). For larger
particles, measurements of three orthogonal axes
