Environmental Engineering Reference
In-Depth Information
where:
d
84
=
diameter for which 84% of the sample is finer than
d
84
diameter for which 16% of the sample is finer than
d
16
(d) The standard deviation (
σ
s
) is described by:
d
16
=
0
.
5
d
84
d
50
d
16
σ
s
=
d
50
+
(3.6)
where:
d
84
=
diameter for which 84% of the sample is finer than
d
84
d
16
=
diameter for which 16% of the sample is finer than
d
16
diameter d for which 50% of the sample is finer than
d
50
(e) The geometric standard deviation (
σ
g
)isgivenby:
d
50
=
d
84
d
16
σ
g
=
(3.7)
where:
d
84
=
diameter for which 84% of the sample is finer than
d
84
d
16
=
diameter for which 16% of the sample is finer than
d
16
3.4 SHAPE
In addition to the diameter the shape of the sediment particle is also an
important aspect. A flat particle will have a smaller fall velocity than a
rounded particle and it will be more difficult to transport a flat particle
as bed load. The shape of a particle can be characterized by sphericity,
roundness or shape factor.
•
Sphericity is the ratio between the surface area of a sphere and the
surface area of a particle at equal volume.
•
Roundness is the ratio of the average radius of the curvature of the edges
and the radius of a circle inscribed in the maximum-projected area of
the particle.
Sphericity and roundness are not used in sediment theories.
The shape factor is a useful definition that can be used to describe a
particle. The shape factor (s.f.) is defined by:
c
√
ab
s
.
f
.
=
(3.8)
Here
a
,
b
and
c
are the dimensions of a particle in the direction of three
axes. For spheres the shape factor is 1. For natural sand the shape factor
(s.f.) is approximately 0.7 and for gravel it is approximately 0.9.
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