Environmental Engineering Reference
In-Depth Information
1
√
t
,
gV
0
ʽ
K
s
16
D
sp
=
ˀ
(6.6)
1
/
2
w
where
V
0
is the volume of spilled oil,
ʽ
w
is water kinematic viscosity,
ˁ
w
and
ˁ
o
are
the water and oil density,
K
s
is the empirical constant.
The process by which wind-driven breaking and non-breaking waves split the
surface oil layer into droplets and then propel them into the water column is called
natural dispersion. The rate of oil entrainment from slick to the water column can be
scaled as [
16
]:
k
e
˃ʳ
H
s
ʻ
ow
=
L
ow
,
(6.7)
16
ʱ
where
ʻ
ow
is the entrainment rate,
k
e
is the coefficient evaluated from experiments,
usually 0.3-0.5,
is coefficient con-
cerning the mixing depth of the individual particles,
L
ow
is the vertical length-scale
parameter.
Based on the laboratory data, the intrusion depth
z
H
that the oil droplet may
penetrate is assumed as:
ʳ
is the dimensionless damping coefficient,
ʱ
1
0
H
s
.
z
H
=
.
5
+
0
.
35
·[
R
]
(6.8)
The horizontal diffusion coefficients are calculated from Smagorinsky formula,
while the vertical is obtained from the level 2.5 turbulence model [
3
].
6.2.3 Weathering Processes
The fate of oil at sea is governed by the external environmental conditions as well
as the several physicochemical properties of the oil. The change of physicochemi-
cal properties that spilled oil undergoes is collectively known as “weathering”. The
main weathering mechanisms which determine the fate of the oil slick are evap-
oration, emulsification, dissolution, oil-beaching, sedimentation, photo-oxidation,
and biodegradation. Although the individual processes may act simultaneously, their
behaviour importance varies with time. The last three ones will be predominant after
the first week, but they are ignored for our short-term forecasting.
Evaporation, which usually accounts for about 30% of spill oil mass loss, is a
significant process changing oil mass and physical chemical properties during the first
hours of an oil spill. The rate of evaporation is determined by the physicochemical
properties of the oil as well as sea water temperatures, winds and other processes
such as spreading and emulsification. The most frequently used equation to predict
evaporation loss is proposed by [
15
]:
ln
BT
T
exp
A
1
T
s
BT
G
K
a
t
h
BT
0
T
F
E
=
−
+
,
(6.9)
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