Environmental Engineering Reference
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Fig. 5.9 Simulation of a circular oil spill advected for 72h with Ęł = 0 . 5% with no diffusivity
( left ), with D = 10 m 2
/ s( center ) and with D = 100 m 2
/ s. Black circle indicates the initial location
of particles whereas the red dots the final distribution. The drifter trajectory is depicted with the
blue line where the blue cross is the starting position and the blue circle the final one
The effects of varying diffusivity can be illustrated as shown in Fig. 5.9 . Determining
the appropriate diffusion coefficient for an oil spill model is a challenge as it depends
directly on howwell the hydrodynamic model captures the structure of velocity field,
which may vary in both time and space. If we assume spatial variability in diffusion
is a primary concern, the approach of [ 39 ] allows development of a spatial map of
estimated diffusivity based on multi-year simulations of particle transport.
5.8.4 Oil Spill Shape
Although aerial or satellite imagery can sometimes be used to obtain a visual approx-
imation of an oil spill's initial shape, there will generally be some uncertainty in the
shape and/or extent of coverage for the early stages of spill. There is an open ques-
tion as to how uncertainties in the initial shape and size of an oil spill affect the
forecast. Unfortunately, the overall effect of the initial shape is also likely influenced
by the selected diffusivity, with higher diffusivities being less affected by the initial
shape. A simple test for the influence of the shape and diffusivity can be carried
out with representative initial shapes. If a and b are defined as the major and minor
semi-axis of an initial elliptical distribution of oil particles, insight can be provided
with study cases: (i) a
=
b a circle, (ii) a
=
2 b a prolate ellipse, and (iii) a
=
b
/
2
an oblate ellipse. For each shape, a set of diffusivities over the range 0-100 m 2
s
has been run using the same simulation setup as in Fig. 5.9 . The RSME results are
provided in Table 5.1 , with typical particle clouds after 72h of simulation shown in
Fig. 5.10 . These results indicate that the model tested is relatively insensitive to the
initial shape, and a circular spill is a reasonable initial condition when more detailed
data is not available.
/
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