Geoscience Reference
In-Depth Information
After the floc diameter is calculated using Eq. (11.5), the floc settling velocity can
be determined using Eq. (11.6). Eqs. (11.5) and (11.6) consider the effects of sediment
concentration and flow shear on flocculation. It can be seen that the settling velocity
increases as either
C
or
G
increases. It seems that these two equations are applicable
only for low sediment concentrations and low shears.
The formula proposed by Peng (1989) based on the Yangtze Estuary mud consid-
ers the influences of sediment size, sediment concentration, salinity, and turbulence
intensity on flocculation. Zhang (1999) modified the Peng formula for a study in the
Yellow River mouth as
ω
sf
ω
C
0.03
sa
I
0.22
d
0.58
50
sd
=
0.274
af
(
C
)
(11.7)
UhS
e
where
I
is the turbulence intensity, defined as
I
with
S
e
being the energy
slope;
a
is a coefficient between 1 and 1.5 to be calibrated using observed data; and
f
=
/ν
(
C
)
is a function of sediment concentration
C
:
C
0.48
m
−
3
C
≤
15 kg
·
(
)
=
f
C
(11.8)
m
−
3
0.48
[
15
/(
C
−
14
)
]
C
>
15 kg
·
The exponent of salinity in Eq. (11.7) is 0.03, which is very small, as compared
with that in the Yue formula (11.4). Eq. (11.7) introduces a monotonous relation
between floc settling velocity and turbulence intensity. This does not agree with
the observations by Owen (1970), McConnachie (1991), and Haralampides
et al
.
(2003). The exponent of
I
should be a variable rather than a constant, and there
should be a threshold value of
I
at which the exponent of
I
turns from positive to
negative.
Eqs. (11.4), (11.6), and (11.7) were obtained under certain conditions, and thus,
their applicability should be restricted somehow. For more general applications, the
following formula was suggested by Wu and Wang (2004c):
ω
sf
ω
sd
=
K
d
K
s
K
sa
K
t
(11.9)
where
K
d
,
K
s
,
K
sa
, and
K
t
are the correction factors accounting for the influences of
sediment size, sediment concentration, salinity, and turbulence intensity, respectively.
Note that the effect of temperature is considered through
ω
sd
.
Following Migniot (1968), Qian (1980), Huang (1981), and Dixit
et al
. (1982), one
can evaluate the correction factor
K
d
as
d
r
d
50
n
d
K
d
=
,
d
50
≤
d
r
(11.10)
Eq. (11.10) is only applied to the range of
d
50
≤
d
r
. For
d
50
>
d
r
,
K
d
is set as 1.0.
This means that no flocculation occurs for coarse sediments.
